There’s an old gardening myth that it’s best to put a layer of gravel or rocks at the bottom of a plant pot to improve drainage, but how true is it? Can the practice actually harm plants more than help them?
The main reason for wanting to improve drainage in pots is because most plants don’t like having ‘wet feet’, otherwise known as waterlogged roots, because this leads to root rot, which can kill a plant.
Pots, planters, tubs and containers designed to hold plants always have drainage holes in their bases to allow any excess water to drain out freely, preventing water accumulating at the bottom of the pot.
If pots drain because they have holes in them, then why the need to increase drainage? Well, it’s because the potting medium in which the plant grows is designed to retain moisture, to a certain degree at least…
To figure out what’s best for plants, lets look at the science!
Potting Mediums, Striking the Perfect Balance
Too much water and plant roots rot, not enough water and plants dry out. A good potting medium (potting mix) has to strike the perfect balance between sufficient moisture retention and good drainage for plants to thrive.
Since any decent quality potting mix must retain some moisture, it needs to contain material which will absorb and retain moisture, much like a sponge does. This wicking or absorbent property of any potting medium is the critical key to understanding the behaviour of water in pots.
The Science of Plant Pots and Perched Water Tables
Water naturally runs to the lowest point under the influence of gravity, and will all run out from a container with drainage holes in the base unless there is something else present to hold it there.
Absorbent materials, such as a wet sponge sat upright or a wet bath towel hung from the line, behave the same way. The water will move downward, some of it will drip away, and some of it will be retained. The top of a wet sponge or bath towel will dry the fastest, and the bottom portions will remain damp for the longest period of time.
Potting mediums, being absorbent materials, behave much like any other when wet.
To get into some basic physics, two opposing natural forces are at play within a wet potting medium in a pot.
- Gravity, which exerts a downward pull on the water, causing it to be drained away through the drainage holes.
- Capillary action, which exerts an upward pull on the water, causing it to be retained, saturating the potting medium.
Both these forces have limitations though:
- The capillary action can only wick the weight of the water upwards to a limited height against gravity, and no higher.
- The gravitational force can only exert a limited downward pull on the water against the upward pull of the capillary action, and no more.
At some point these two opposing forces balance each other out, and when this happens, a layer of water-saturated potting medium is formed at the bottom of the pot which cannot not drain away, this is termed the perched water table because the water is literally ‘perched’ there and cannot move.
It’s important to understand that the perched water table does not drain, the water stays there unless plant roots draw the water up, or it evaporates away when the potting mix dries out, in which case the plant won’t survive!
Also, be aware that all pots filled with any kind of potting mix, potting medium or growing medium, call it what you will, have a perched water table.
The size and shape of the pot makes no difference, it doesn’t matter if a pot is tall and narrow or wide and shallow, neither if it’s big or small, if the growing medium/potting mix is the same, the perched water table will always be the same height.
Different growing media will have different perched table heights, the more absorbent materials will have higher perched water table, and the less absorbent ones will have lower levels.
Understanding Capillary Action
In this section we’ll go a bit deeper into the science if you’re interested, if not, please skip to the next section. I like to teach from first principles, as I believe this way we can really come to a deeper level of understanding, but then again, I’ve got qualifications in the biological sciences, so I’m biased!
Gravity is self-explanatory, it’s the ever-present force on this planet which pulls everything down!
Capillary action is created by the cohesive and adhesive forces of liquids.
Cohesive forces are forces of attraction between molecules of the same type. For example, molecules of water are able cling to each other.
Adhesive forces are forces of attraction between molecules of different types. For example, molecules of water are able to cling to other materials.
Capillary action by definition is the tendency of a fluid to be raised (or suppressed in the case of mercury) in a narrow tube (capillary tube) due to the relative strength of cohesive and adhesive forces.
To explain how this further, we need to understand the nature of water.
Water (H2O) is considered a polar molecule because it has a negative charge on one side of the molecule and the positive charge on the other. Its bent V-shape which gives it a partial positive charge on the side of the hydrogen atoms and a partial negative charge on the side of the oxygen atom.
Polar molecules act like magnets with north and south poles, the (+) positive charged atoms and (-) negative charged atoms of these molecules are attracted to one another.
When the positive side on one water molecule comes near the negative side of another water molecule, they attract each other and form a hydrogen bond, and this creates the strong cohesive forces between water molecules, and this explains why water clings to itself.
Water molecules will exhibit strong adhesive forces that allow them cling to other materials if those materials are even more polar (have a stronger electrical charge) than water itself, as the attraction will be stronger than the attraction of water molecules to each other.
The upward motion of liquids against gravity, known as capillary action, is a combination of:
- The forces of attraction between water molecules and another material above the water’s surface which doesn’t already have water clinging to it already (adhesion), causing the water molecules to climb upwards a little.
- The forces of attraction between water molecules to each due to the hydrogen bonds they form with each other (cohesion), causing them to pull each other up.
To put it another way, capillary action is a combination of the effects of adhesive and cohesive forces displayed by water.
Now that we know why water moves upwards and creates perched water tables in growing media, we can now re-examine our opening question from a more scientific perspective!
The Effect of Placing Gravel at The Bottom of a Pot on the Perched Water Table
Would it make any difference if we placed a wet sponge upright in the sink, or on a layer of gravel in the same sink? Now that we understand how the forces of adhesion and cohesion within liquids create capillary action, leading to the formation of a perched water table at the bottom of an absorbent medium, we can see that it won’t have any effect on these forces in any way at all.
Remember, the downwards force is due to gravity, which we can’t increase, a lower layer of another material won’t change the adhesive forces between the growing medium and the water molecules, nor will it alter the cohesive hydrogen bonds between water.
So what effect will adding gravel at the bottom of a pot below the growing medium have?
It will reduce the volume of potting medium, and push the perched water table higher up into the pot, as shown in the diagram below.
Adding gravel a the the bottom of a pot will create two potentially serious problem:
- Pushing the saturated water table layer upwards, closer to the plant roots actually increases the risk of root rot, as the roots will stay wetter, longer.
- Reducing the volume of growing medium available to the plant roots will reduce root growth space and overall root volume, as well as available moisture, thereby decreasing the plant’s drought tolerance and potential maximum growth size.
There is no benefit to be gained by adding a layer of gravel or rocks to a pot when we examine the matter from scientific first principles!
The Correct Way to Increase Drainage in Pots and Containers
If the same potting medium is used, irrespective of the size or shape of the pot, the perched water table always stays the same height because it is determined by the wicking ability of the potting medium, since gravity doesn’t change.
The way to increase drainage of the perched water table is to add materials throughout all of the potting medium to increasing the air spaces in the mix and reduce capillary action.
Some plants require extremely well draining potting mixes in containers. A lot of orchids for example are epiphytes (plants don’t grow in soil but obtain moisture and nutrients from the air and rain and usually grow on the surface of another plant), and many grow in trees. Growers of Cymbidium orchids use an orchid mix which is composed mainly of coarse 20mm (3/4″) composted pine bark pieces. This mixture contains huge air spaces and drains extremely well, barely retaining moisture in the bark pieces, so there is no perched water table.
Cactus and succulent growing mediums for pots are a coarse, open mixes made with some organic matter to retain a little moisture, and plenty of gritty material such as crushed quartz or other crushed rock, which act like a sandy soil and lets water pass almost straight through.
Perlite and vermiculte are materials which are used as soil amendments, and both are minerals that are made more porous by expanding them with heat, much like popcorn. Because they have large air spaces within them, they are used to increase the drainage and aeration in potting mixes. Perlite mainly increases drainage, while vermiculite will also hold some moisture and help retain nutrients too. Mixing either of these amendment materials right though a potting mix will increase aeration, improve drainage and reduce the height of the perched water table.
Hydroponic systems also use perlite as a potting medium, or ‘clay balls’ which are in fact clay coated pumice balls which are very porous and weigh almost nothing. These growing media have large air spaces both inside and between the particles, so they drain extremely well, but hold enough water to keep the roots moist.
We can see that the common practice in horticulture to increase drainage in pots and containers is to alter the composition of the potting medium to increase the air spaces within it, and not by making changes to the the space beneath the pot.
How Did The Tradition of Putting Gravel At the Bottom of Pots Originate?
The only way that gravel at the bottom of the pot will increase drainage is if the pot has insufficient drainage, either due to not having enough drainage holes, or by having blocked drainage holes.
This is speculation on my behalf, but I suspect that the reason gardeners traditionally used gravel in the bottom of pots is probably because pots were traditionally made of terracotta clay rather than plastic, and these pots only have a large single drainage hole in the base of the pot. With these pots, it was a traditional practice (and still is) to sit a very loosely fitting stone over the hole to stop the potting mix falling out. If this single hole became blocked, water would pool at the bottom of the pot and drain out very slowly, leading to waterlogging. The problem would be even worse in glazed terracotta pots, which don’t seep moisture from their sides and stay wet longer.
Adding gravel into the bottom of terracotta pots creates a small water holding area for the excess water that would normally drain out on its own if the gravel wasn’t there to collect into if the single drainage hole become blocked. That’s my educated guess, and like most traditions, people eventually forget the reason why something was done in the first place, and just keep doing the same thing simply out of habit.
With plastic pots, there are always plenty of drainage holes, and many are designed to increase drainage through the use of domed bases with drainage holes at the edges to take excess water away faster.
Do You Still Believe Gravel in Pots Improves Drainage?
If you’ve read this far, and still aren’t convinced that putting gravel at the bottom of a put just pushes the perched water table up, but doesn’t improve drainage, then I have two diagrams and two direct quotations from authoritative sources to illustrate the point:
From the North Willamette Research and Extension Center, Oregon State University, Physical Properties of Container Media:
“For a given media, the perched water table remains the same regardless of plant height. It is therefore unwise to use the same media in large containers as small.“
From the University of California, Agriculture and Natural Resources, Master Gardeners of Monterey Bay:
From the University of Illinois Extension, Urban Programs Resource Network – Successful Container Gardens, Choosing a Container for Planting – Drainage Is Critical to Plant Health:
“Skip the gravel inside the bottom of individual or pot liners – It is a myth that a layer of gravel (inside the bottom of an individual pot) beneath the soil improves container drainage.
Instead of extra water draining immediately into the gravel, the water “perches” or gathers in the soil just above the gravel. The water gathers until no air space is left.
Once all the available soil air space fills up, then excess water drains into the gravel below. So gravel in the bottom does little to keep soil above it from being saturated by overwatering.”
From the University of Tennessee, Institute of Agriculture, Agricultural Extension Service – PB1618, Growing Media for Greenhouse Production, we see that reducing the depth of the pot actually increases water retention and reduces drainage, so putting gravel at in the bottom of a pot actually has the opposite effect:
“Media Column Height/Containers – Another factor relating media to air/water relations in the root zone is the size of the growing container. With media in containers, the amount of air and water held in a given media is a function of the height of the column of media. The taller the column,the smaller the ratio of water to air spaces.
This is most important in plug production where the small cells drain very poorly or not at all, resulting in poor root zone aeration…
In all containers, there will be a certain amount of saturated media at the bottom of the container after drainage. This is due to what is called a perched water table. The saturation zone is a larger part of the total volume of the growing media in a very short container, such as a plug cell.
A good way to illustrate the effect of container height is to use a sponge. A sponge of the dimensions 2@ x 4.25@ x 8.5@ (72.25 cubic inches or 1,184 milliliters) represents the media in a container.
When fully saturated, the sponge holds 950 ml; that is, the total porosity is 80 percent.
Holding the sponge so it is 2 inches high results in about 50 ml water draining out, resulting in a volume air space of 4.2percent.
If it then is held so it is 4.25 inches high, another 125 ml drains out, resulting in a volume air space of 14.8 percent.
If the sponge is then held so it is 8.5 inches high, another 375 ml drains out, resulting in a volume air space of 46.5 percent.
Starting with the same volume of media (sponge), the effect of container height (sponge height) on media air space is dramatic. We can conclude that the choice of containers is important in managing water/air relations in the root zone, especially of plugs.”
Hopefully that’s convincing! If not, then see for yourself. I’ll show you how to test this, all you need is some empty soft drink bottles. This is a proper controlled experiment, so please don’t go changing the experiment design parameters on whim!
A Simple Experiment to Test Whether Adding Gravel at the Bottom of a Pot Improves Drainage
Here is a simple experiment that can be set up to determine whether adding gravel at the bottom of a pot improves drainage or not. This would make a great classroom experiment.
- Make some clear plastic pots by cutting the tops of clear plastic soft drink bottles so the perched water table can be viewed through the sides.
- Make the same number of decent sized holes (around 6mm or 1/4″) in the bottom of each clear pot.
- In the first pot, place only potting mix in it, fill to only within 2.5cm (1″) of the pot, leaving a gap from the surface of the potting mix to the top of the pot to make watering easier. When filling the pots, just tap the sides gently to settle the potting mix slightly, don’t compress it down.
- In all the other pots, add increasing amounts of gravel at the bottom, then fill with potting mix to within 2.5cm (1″) of the pot. Tap the sides gently to settle the potting mix slightly, don’t compress it down.
- Water each pot with the same volume of water.
- Wait till all pots drain well, this will depend on the type of potting medium used and the volume of the containers.
- Observe the perched water table.
If the physics is true, then the perched water table, the wet bottom layer of the potting medium will be the same thickness in every pot, and the gravel will simply push it up higher in the pot because it’s pushing all the potting mix up higher in the pot.
That said, now lets play some mind games!
The Permaculture Design Approach, Turning Drainage Problems Into Solutions!
If we look at the Permaculture Attitudinal design principle – “Everything Works Both Ways”, we see it states that whether we see something as positive or negative, as a ‘problem’ or as a useful resource, depends on our attitude.
So how can we turn the problems created by adding gravel at the bottom of pots into solutions? This is a real exercise in lateral thinking, or more accurately, Permaculture holistic solutions thinking.
If we do a Permaculture functional analysis of the outcomes our outputs, we see that the technique reduces soil volume and raises the saturated perched water table.
One of the problems gardeners encounter often is unknowingly planting a tiny plant into an overly large pot. Small plants don’t have enough roots to take up huge quantities of water, and in large pots the potting mix stays too wet for too long, causing root rot once again. The growing medium wont be as saturated as the perched water table, but it will still be wet enough for way too long to be detrimental to the plant. There is wisdom in the gardening advice to plant up to the next size pot when repotting, and increase pot size gradually rather than plant into the biggest pot available at the outset.
A shallow rooted plant in a tall narrow pot will have similar issues, there will be too much overly wet potting mix which the roots will never be able to reach, and if the potting mix stays too wet for too long it will break down much faster, and sink down, dropping the level of the plant in the pot. Filling the bottom of the pot with coarse scoria, which is light in weight, will eliminate the unusable space in a tall, narrow pot and effectively reduce pot size to a more suitable volume.
The only kind of plants which love a saturated growing medium are marginal aquatic plants, and there are plenty of useful edible ones such as watercress, taro, kangkong and water chestnuts for example. With these plants it’s much better to remove the drainage altogether and saturate all of the growing medium though, or sit the pots in a saucer of water.
There are always exceptions to the rules, as we’ve discussed in this section, but in general, it’s best not to place gravel, stones, pebbles, scoria, terracotta pot shards or any other materials at the bottoms of pots below the growing medium.
Give plants as much space to spread their roots out, relative to what they can use or need. The more moisture retentive growing medium/potting mix available, the less often a plant will need to be watered, as long as the pot is not too big. Nearly all plants prefer a natural wet-dry cycle, as that’s what they experience in nature.
Most people will place a stone or pebble over drainage holes in pots, especially the large central ones at the base of terracotta pots, to prevent the potting mix falling out and making a mess. The point is not to block the hole, but to simply create a loose-fitting barrier to prevent the loss of growing medium while still allowing water to freely drain out.
As a final thought worth pondering, it’s curious how gardening has as its foundations the applied sciences of horticulture and agriculture, yet it’s filled with so much dogma and myths, very strange indeed…
- To learn how to improve drainage in pots, please see article – How to Improve Drainage in Plant Pots, The Proper Way to Do It!
- If you need to put a material at the bottom of a deep pot to fill space, please see article – The Best Ways to Fill Space in Tall Plant Pots That Are Too Deep
Hi Angelo: INTERESTING, and not all intuitive
It might need a better explanation, although you do well as always.
It prompted me to look around. http://oregonstate.edu/dept/nursery-weeds/feature_articles/physical_properties/physical_properties.html
Paul
Hi Paul, thanks for your comment, I had a look at the links you suggested and only included the Oregon State University one, it was the only one which provides any real scientific explanation, albeit a very limited and simplified one. The others were without any real explanations, so I didn’t include them as a I only link to authoritative sources, so I hope that’s okay. Hope it’s not too cheeky of me to say that my explanation is the only one from first principles and goes to a greater depth scientifically though. You’re right, some things in gardening are definitely not intuitive, and if the science is there, we should use it!
I have Asian ceramic fishbowl planters for houseplants without holes. Your article vindicated my suspicion that when garden ctr used stones at bottom before adding fresh mix and new plant, my plant did not do well due to rotting roots. I dont over water and my plants thrive without holes and stones. This article is so well written and explanatory.
Thanks Angelo, myth buster. I really liked that you went into detail, it becomes very clear.
Thanks! You’re welcome!
Excellent article, well written! Would be great to add an explanation of why the water doesn’t cross into the gravel layer.
Thanks Ben! The water doesnt flow down into the gravel layer below because the water is ‘perched’ and can’t move, it’s held up there in the perched water table by capillary action against the force of gravity.
But doesn’t some water fall down because of gravity and fill the spaces in between the rock layer?
No more than would drain out normally and pool on the ground beneath the pot. It just drains the same amount of water a bit quicker because its not sitting in the wet puddle, but when the drained water on the ground evaporates its the same. The only thing holding it up the water is the potting mix, and teh bottom part of the potting mix stays really wet, that’s the part that’s not healthy for plant roots. If we put gravel at the bottom, we have less space for potting mix, and just end up pushing the soggy, waterlogged bottom of the potting mix closer to the plant roots.
Last notable comment in the article mentioned a stone on the drainage hole to prevent soil leaching from the pot. A better choice is placing a coffee filter in the pot to contain the soil.
Thanks Elizabeth, that’s an excellent idea!
Wonderful informative and well written article-thanks??
Thank you for this enlightening article. I have wondered for years why nurseries dont have gravel or the like in the bottom of their pots. But no happy as I will now be obsessed with correcting my pots. ?
You are completely incorrect in your article. you said it yourself, the perched water table will always be the same height. So if you have a pot, like in your illustration, and the pwt is at the bottom 3 inches of the pot (for example), if you replace those 3 inches of soil with a very course medium like clay pebbles (which has much lower capillary action, then more water at that level will drain out of the drain holes in the pot. The pwt will NOT be “higher up in the pot”.
Thanks for your comment Will, the article is in fact correct, you can try it out by making some clear pots out of soft drink bottles and testing it out, it’s been documented many times by others and I’ve explained the science of it and how it works. My apologies for not being clearer in my explanation, so I appreciate you bringing the point up.
I should have explained that the perched water table will always be the same height in the potting medium, no matter what you sit under it. If you pour water from the top, the medium will act like a sponge and prevent a certain amount draining out of the bottom, that is the perched water table. The more absorbent the potting medium, the taller the perched water table will be.
The simple way to understand the perched water table phenomenon is as follows. The absorbency of the potting medium pulls the water upward and stops the water draining out, no matter what is underneath. The only thing that forces the water down is gravity, and the downward force of gravity always remains the same no matter what sits underneath. If there is no other downward force, the perched water table remains the same. You could force more water to drain out by adding another downward force, such as tying a rope to the top of the pot and spinning it really fast to create centripetal force, much like what happens in a laboratory centrifuge. I really should do some videos to demonstrate things like this!
Angelo, I’ve wondered the world over WHY oh why must we put gravel in the bottom of our pots and I am happy you completely busted this myth. Science wins again.
I also had the same question that Will brings up and thank you for answering regarding the perched water table. No more gravel in my pots (and I’ll be repotting soon!)
I save the plastic mesh bags that some garlic seed cloves (or onion sets, etc) come in and cut out a batch of squares for the bottom holes in my pots.
Thanks for your comment Prissy, there is so much unsubstantiated dogma in gardening, and so much bad cut-and pasted information on the internet proliferating it. It’s good to put the science back into gardening, and give people factually correct information to help them garden more successfully! 🙂
What you put under the potting medium matters. There are three forces at play, not two. Capillary action of the material above the boundary, gravity, and the capillary action of the material below the boundary. The material underneath could pull the material down with capillary action (as in the example of felt given by Tom Wolfe). What you are implying without stating explicitly is gravel will have minimal capillary attraction.
Thanks for your comment. Yes, that’s correct, the unstated assumption is that gravel wicks less than the potting medium, which is the case.
There is no good reason for ever putting anything more absorbent than the potting medium inside the bottom of the pot, that’s not how plants are grown, so when we pot up plants correctly, we only have two forces at play in a proper pot, gravity and the wicking action of the growing medium.
Furthermore, even good wicking materials designed for the purpose will wick much less than potting mix, and are used to wick moisture into pots, not out of them! That’s how capillary watering trays used in the horticulture industry work to supply seedling pots and punnets with water from below.
With capillary watering trays, the potting medium wicks the moisture away from the wicking material, which in turn draws more water from a water reservoir to creat a constant supply of water which flows from reservoir to capillary mat to the potting medium. You can see how this works, and also build one, in my article How to Build a Self Watering Seedling Tray.
Hope this helps 🙂
I think Will is saying you can add clay pebbles to the potting mix and the perched water table wont rise- however, adding clay pebbles to the potting mix is essentially just creating a new potting mix of which it will have a different perched water table level nevertheless, the perched water table will be consistent to with said potting medium. Thats what I got from reading your article at least!
He may not have read all the way through. Likely if he did the experiment with a standard potting medium and then his potting medium with the clay pebbles, his potting medium will have a lower perched water table, but thats simply because he made a different potting medium. This wouldnt be the case if he put ALL the clay pebbles at the bottom of the pot, as the potting mix would still be standard and have the same perched water table.
This is morning trying to learn haha. Am I getting this right?
Hi Brandon,
Thanks, that’s spot on, adding any amendment through the potting mix creates a whole new potting mix, with different drainage characteristics and a different perched water table height.
The problem with using clay balls in the potting mix is that they are large and significantly reduce the actual root space for the plant.
I suspect I’ve discovered the origins of all the ‘potting medium voodoo’ in the gardening world, it’s from people trying to work out ways of planting indoor plants in planters with no drainage holes! Planters are meant to hold pots with drainage holes, they work like a drip saucer even with a pot in them, but much deeper, so it’s important to elevate the bottom of the pot to stop plants drowning!
Hello,
Great article. Could you comment on the use of a gravel layer (probably clay pebbles) in combination with a fabric wick reaching from the very bottom of the pot, through the pebbles and into the solid. Will the wick lower the PWT? Will the wick also allow water to travel upward as needed without having the roots be overly moist. Thanks! frank
Hi Frank, wicking materials are more absorbent than the potting medium and will raise the water higher than the perched water table (PWT) in the bottom of the potting medium. Having a layer of gravel makes no difference to the system when using a wick.
Since wicks lift liquids upwards, they will pick up water from the PWT and carry it up higher into the pot to increase moisture to the roots around, effectively raising the PWT, but not evenly around the pot, only around the wick, which is why some wider wicking pots use multiple wicks or wind them around the inside of the pot for more even moisture distribution.
This is the principle by which wicking pots work, as explained in my article DIY Self-Watering Pots and Mini Wicking
Beds.
Dear Angelo,
I do not agree at all with your explanation and your graphics on gravel drainage.
Not always 1 + 1 + 1 = 3 this means that you have automatically raised the humidity level from the bottom of the pot without gravel to a humidity level above the gravel. That is not right.
Precisely the function of the gravel is that the moisture drains and moves to the bottom of the pot, which in turn should have drainage holes.
If you want, I will leave you my contact and I will explain it to you with a practical video.
I am the Technical Director of Dmmen Orange in Spain and Portugal and I have 20 years of experience in pot cultivation with various drainage systems and gravel is one of the most recommended for fans of ornamental plants.
Hi Javier, thanks for your comment. You’re welcome to disagree! That’s how people find common ground for understanding! ?
There are only two forces at play on water in a pot of growing medium. The upward-acting wicking force of capillary action and the downward-acting force of gravity. When any excess water drains away due to gravity and the two forces reach equilibrium or balance, a certain amount of water will be retained at the bottom of the potting medium, this is known as the perched water table.
If you use the same potting medium, irrespective of the size or shape of the pot, the perched water table always stays the same height because it is determined by the wicking ability of the potting medium, as gravity doesn’t change. The way to increase drainage of the perched water table is to add materials throughout all of the potting medium which reduce capillary action by increasing the air spaces in the mix, which is why we sometimes add potting medium amendment materials such as perlite so potting mix drain better.
From your experience stated I am guessing you’ve probably grown lots of plants in containers with very different requirement. Cymbidium orchids for example are grown in an orchid mix which is composed mainly of 20mm composted pine bark pieces. This mixture drains extremely well and holds little moisture. Cactus and succulent mix contains a little organic matter but plenty of coarse gritty material which acts like a sandy soil and lets water pass almost straight through. Hydroponic systems use potting media such as perlite, which is an expanded mineral filled with air, or ‘clay balls’ which are in fact clay coated pumice balls which are very porous, to drain extremely well but hold a slight amount of moisture. We can see that it’s a common practice in horticulture to increase drainage in a pot by altering the potting medium, and not the space beneath the pot.
The only way that gravel at the bottom of the pot will increase drainage is if the pot has insufficient drainage, not enough drainage holes for example. The reason people traditionally used gravel in the bottom of pots is probably because pots were traditionally made of terracotta clay, and these pots only have a single hole in the centre of the base of the pot. It was traditional to put a stone over the hole to stop the potting mix falling out, so if this single hole became blocked, water would pool at the bottom of the pot and drain out slowly.
The simple way to test this is to make some clear plastic pots by cutting the tops of clear plastic soft drink bottles and making the same number of holes in each, so the perched water table can be viewed. In the first pot, place only potting mix, and in the rest, put increasing amounts of gravel beneath the potting mix. Water each with the same volume of water, wait till they drain, and then observe the perched water table. If the physics is true, then the perched water table will be the same height from the bottom of the potting in every pot, and the gravel will simply locate it higher up in the pot because it’s pushing up the potting mix. I might have to set this up when I have time and photograph it.
Thanks for your question, to help make my article clearer I’ve updated it to address your excellent points and included two diagrams to illustrate the point, one from the North Willamette Research and Extension Center, Oregon State University and another from University of California, Agriculture and Natural Resources, Master Gardeners of Monterey Bay.
This article is really interesting. I am an engineer with solid background in porous media and capillary / gravity driven flows. I think the statements in the article are correct. However, I think the problem is even more complicated than the article describes. The volume of the PWT should be very important considering watering frequency, type / size of plant and climate. Gravel would allow you to adjust the volume. Another way to do it would be to change the shape of the container, but gravel might be a better way to optimize the PWT volume for a specific plant.
Hi Matt, thanks for your comment. My background is in the biological sciences, and we both understand how things like this works from the fundamental principles of physics. Unfortunately, horticulture, being an applied science (I work as a horticulturist) is rife with prescriptive, procedural advice and learning, where people don’t fully understand why they’re doing what they’re doing, it’s often a case of ‘when X happens, do Y, and if that doesn’t work, then do Z’. When words of advice from this kind of procedural thinking gets passed around into the non-professional gardening world, and gets changed with every whisper from one person to another, then gets cut-and-pasted ad infinitum across the internet, we end up where we are now having to correct the misinformation!
Can I please ask you to better explain or qualify your statement, it’s unclear what you’re suggesting, and people will misread what you’re saying and draw the wrong conclusions.
I’ve mentioned how the container shape affects the PWT in the article. The watering frequency, plant transpiration rates, season, climate, composition of potting mix are all variables in a system with fixed properties, they may alter the PWT quantitatively (volume) as you suggest, but it doesn’t change the dynamics of the system, the PWT is still there and operates the same way, and changing the composition of the mix is the variable I’m suggesting we modify, as we can change that but don’t have necessarily control of the other variables.
Thanks
Excellent Article, in one article you have addressed many phenomenons.
I also appreciate your efforts in providing significant explanations in the comments section.
Well, golly! I just wanted to know if I should jam some gravel in the bottom of the pots of my fledgling pothus ivy plants. Clearly, the answer is “no.” Not sure if I needed to read an exhaustive treatise to get the answer, but, well, it was fascinating. No gravel – none of the time!
You wouldn’t believe the emails and questions I get defending the practice, even after all that evidence I’ve presented lol! 🙂
Very good article! I was wondering recently why, two of my plants had different results, although they were the same size, planted in identical pots, using the same compost and watering almost equally. One of them had one part potting soil (very rich in compost) mixed with one part clay balls on the bottom of the pot and then slowly decreasing the content of clay balls towards the surface. The plant in the pot which had only potting soil I noticed it retained too much water on the bottom and was hardly drying, which in the end led to root rot.
Now, your article makes me curious to see how perched water table differs from let’s say compost versus sand. You stated that sand should improve it but I wonder if that is enough as I am planning on using mainly sand from now on. Food for the thought!
Hi Johnny, thanks for your question! You’ve preempted my next article on which explains the proper way to increase drainage in pots, which I’ll publish soon, but your solution is half-way there.
The secret to improving pot drainage is to alter the composition of the potting medium, by mixing something with a larger particle size all the way through it, and there are several different amendment materials for this purpose.
Sand will improve soil, as it has larger particles than clay or silt, but it’s way smaller than potting medium particles and will clog it up instead, filling the air spaces. A better solution is perlite, a heat-expanded mineral used in hydroponic systems, from experience, it works really well.
Hi, I also altered the composition all the way through the pot, the expanded clay balls are 5-12mm diameter and they retain little moisture, similar to perlite. The reason why I chose expanded clay balls is because they don’t float as easily as perlite. When I mixed them with the compost, it was 50% clay balls and 50% soil on the bottom of the pot, then slightly decreased the clay balls, somewhere around 40% clay balls and 60% soil in the middle of the pot and on the surface it was 30% clay balls and 70% soil. The reason why I wanted to alter the composition this way is because the surface is completely drying up in one day when it gets full sun if I use equal parts of clay balls and compost.
This weekend I took out of the pot a lemon tree which was planted in 70% expanded clay balls mixed with 15% coco peat and 15% compost all the way through the pot. The roots look beautiful, I’ve never seen better roots before.
I also tried to mix sifted1-2mm sand particles with 3-4mm sand particles and I measured the water retained by the mix. It seems this mix could also be useful as it retained 10-15% of it’s weight in water. The drainage and aeration should also be okay as there are no particles smaller than 1mm in the mix.
You have a nice blog, read a few articles till now and looking forward to your next ones!
Thanks for sharing that, I use a mixture of the clay balls and potting medium in my hydroponic setup, but the reverse, with all clay balls at the top which reduces evaporation, but more potting medium mixed lower down. I grow aquatic edible plants that way, water chestnuts and taro, and about half a dozen others.
My nest article “How to Improve Drainage in Plant Pots, The Proper Way to Do It” is coming soon!
Hi Angelo,
Thank you for your extensive research and scientific explanation! I would say I have never completely read an article with this much interest.
Would you have any articles or information (based on scientific research) that regards indoor potting without drainage holes? I am very curious to this topic as I have many pots without drainage holes and would also like to reuse glass jars that I have collected.
Much love and kind regards,
Stijn
Hi Angelo
Like many others, I’ve read this article with great interest. I am in the middle of clearing a number of my pots of old compost and refreshing and refiling them in preparation for re-planting them. I’m a new and still inexperienced gardener (lockdown started me!) and have up to now used gravel and broken crockery in the bottoms of pots, but I’ve been thoroughly persuaded by your article not to.
I was already aware that drainage could be improved by mixing compost (I’ve been using a multipurpose compost) with vermiculite, or perlite, or hydroleca (or similar) and was planning to do this. I’m not clear, though, which I should use, and in what proportions to compost. What are your thoughts on this?
I’d also like to ask you – if you have time – for recommendation as to what to do with used compost.
Thanks, Julie
One puts gravel in the bottom of a pot so that the potting soil does not clog the drain hole. I have seen this many, many times. Too hard to understand?
Thanks for your comment Ron, and yes, what you’ve stated is very difficult to understand because it’s not logical.
If the potting medium is blocking the drain holes, then you have bigger problems than drainage, because all potting mixes are designed to provide some moisture holding capacity, and also drain reasonably well.
For the potting medium to be able to block drain holes, it must have terrible drainage properties, and is best avoided! How can any plant possibly grow in such a potting medium? The short answer is, it cant.
When repotting aquatic plants, I use clay loam garden soil in pots which are lined on the inside with two layer of newspaper, and after planting the pots up, and putting 2cm of pebbles as mulch over the top to stop the water clouding, then watering in the plants (effectively creating a pot of mud), when I set them aside, even they drain eventually, albeit rather slowly. If your potting mix is clogging the drainage holes in the pot, use a better brand of product, or if you’re making your own, please change your formulation.
Hope this explanation helps.
I think you’ve missed the following simple solutions:
1) wicking bed: fill the bottom of the pot with fine gravel. Cover with felt. Drill a hole in the pot just below the gravel level. Problem solved. Water level never exceeds bottom of hole (below soil level), capillary action keeps the soil moist.
2) Terra Cotta pot style wicking bed (hole in bottom). Fill bottom with gravel above the level of the underlying tray. Cover gravel with felt. Problem solved. Water level never exceeds top of tray (below soil level).
Thanks Tom, great solution, on this site we have articles on Wicking Bed Construction, How to Build a Self-Watering Wicking Bed and DIY Self-Watering Pots and Mini Wicking Beds with diagrams which show what you’ve explained.
I prefer to use coarse scoria instead of fine gravel for greater water holding capacity, and geotextile fabric in place felt because it doesn’t break down easily, but that’s personal preference, what you’ve described will work fine either way.
Hi Angelo!
I’m writing an article which mentions yours, and I want to check how to refer to you correctly. What pronouns should I use for you? (for example, “he”, “she”, “they”)
Thank you 🙂
Hi, you can refer to me as ‘he’, that would be fine! Thanks! 🙂
Thank you! I’ve published the article now. I did some of the tests you suggested here, and got some surprising results – turns out gravel/rocks actually does LOWER the perched water table. It pulls water down out of the soil, because gravel itself has a capillary effect which adds to gravity. You can see all my results here! https://parvusplants.uk/article/the-myth-of-the-myth-of-using-rocks-for-drainage/
Hi parvusplants, it was great to see that you did an experiment to test this out, I encourage that! Having a science background, my training is in designing experiments and determining how to test the variables of concern. I must let you know after checking through what you did, I’ve spotted a mistake in your experimental design, and in your interpretation of the results.
You have accidentally eliminated the effect of gravity by using a non-porous cutting board underneath. By not allowing gravity to carry the water away, the effect is the same as sitting the sponges in a bowl. For this experiment, we need to model the performance of a pot with drainage holes, not one without.
The reason why the sponge sitting on a single layer of gravel drained better is because the gravel creates a tiny air space above the non-draining cutting board surface, allowing gravity to do its work, which is not possible with the sponge sitting on the wet surface. What you’ve compared is the advantage of slight drainage versus no drainage, and demonstrated that drainage is important in pots.
To accurately test the effect of capillary action versus gravity, you would need to sit both sponges on a wide mesh, one which won’t let the gravel fall through, with a gravel layer under one of the sponges. Thinking through this logically, you can pile a whole draw of cutlery under one sponge, but it wont make it drain more, why would it?
Only the force of gravity pulls water down, capillary action pulls water upwards against gravity. If the gravel was able to wick more than the sponge, it would pull more water upwards into the sponge. That’s how the principle of sub-irrigation works, and how all self-watering pots operate. See more here – https://deepgreenpermaculture.com/diy-instructions/diy-self-watering-pots-and-mini-wicking-beds/
Remember, the laws of physics don’t change, which is why they’re universal laws! I like exercises such as this because they’re practical applications of scientific principles that get people thinking about things, which is great. Hope this helps 🙂
The board was very slightly tilted in both conditions to allow the water to drain away as it moved out of the sponge. ? For the test using pots of soil, I placed them on top of empty pots so that the drainage hole was exposed and water could escape. I also have a science background and a physics degree, so I understand how the principles of science work. 😀
Capillary action is not defined as action specifically against gravity, it is defined simply as the action which moves water through small spaces – it works in *any* direction, in opposition to gravity or in addition to it. If you dip a piece of tissue into a container of water and let the tissue hang downwards over the edge fo the container, it’s still capillary action which causes the water to wick through it – even though it’s acting in the direction of gravity rather than against.
Thinking logically, a pile of cutlery underneath a sponge would also make it drain more! Probably only a little bit though. A pile of cutlery touching each other and the sponge creates small air gaps at the contact points, which induces capillary action downwards, in addition to gravity. Have you done that test yourself? 🙂
Interesting conversation! 🙂
The visible water pooling on the non-porous surface at the base of the sponge would suggest adhesion of water to that surface, and cohesion to the water in the sponge, which is what water is expected to do, increasing water retention.
Awesome that you have a physics background, then I can speak more specifically. I generalised slightly regarding capillary action to keep it accessible to readers. As I’ve mentioned in the article, capillary action is a combination of the forces of cohesion between water molecules and adhesion of water to other surfaces. Yes, it acts in every direction, but in such a specific scenario, in a pot of absorbent potting mix, which has a defined bottom and top surface, we’re primarily interested in the vertical wicking in opposition to gravity, and the point where the forces reach equilibrium to create a perched water table.
If you put both sponges on the dish rack, and pile cutlery under one sponge, they’ll dry at the same time because its the air gap prevents pooling and re-wicking at the base, increasing height doesn’t help.
I think I understand where you’re coming from here. Two pots have the same size drainage holes, and sit on flat surfaces. If there’s an air gap between bottom of the pot, and the surface it’s sitting on, which can simply be created by using pot feet under the pot to elevate it (no need for gravel IN the pot), then water dripping out of the pot can’t pool at its base and slow the water draining. This only has the potential to initially speed up the drainage rate, as both pots left for a sufficient time will drain the same volume of water, because the wicking is a function of the absorbency of the potting mix.
If you can indeed place a more absorbent layer below the potting mix, it would become saturated, and hold extra water, but the potting mix would still retain the same level of moisture, unless the water volume if measured and intentionally restricted. But that would be no different to using less water each time.
The issue with gravel is that it is used in horticulture as a drainage medium, and not a wicking medium, so therefore has a lower wicking ability than the potting mix. I use ‘clay balls’ in my hydroponic setups, these are specially made pea-sized scoria (porous volcanic rock) balls with an an absorbent clay coating, these are the most absorbent of artificial rocks or gravels available. We use these because they’re much less absorbent than potting mixes, the high water absorption of potting mixes make the root zone too wet in hydroponic setups.
Yes, there was some built-up water pooling when I took the photo, but with the board slightly tilted there was none left by the end of the draining stage of each trial. It was perhaps a slightly misleading photo to use!
You’re right that if there is an air gap between the cutlery (or gravel, or anything else) and the sponge/soil then it will have no effect on drainage. But there isn’t an air gap when the soil is sitting directly on top of the gravel in a pot, they are in direct contact. This contact allows the gravel to exert a capillary effect on the water that’s pooling at the bottom of the soil. It’s correct that gravel is less absorbent than soil, but it is also *more* absorbent than open air. So soil which is on top of gravel ends up with a lower perched water table than soil that is on top of open air (i.e. in a pot with drainage hole but no gravel layer).
Have you conducted these experiments yourself? I’d love to see how your results compare to mine.
With a pot sitting on the ground, the drainage is slowed a bit at the start compared to having an air gap, but overall volume drained is unchanged. Time is an important variable here to consider. Pooled water evaporates, and/or is absorbed by concrete or paved surfaces or soil if that’s what the pots are sitting on.
You’re correct, when potting mix sits on top of gravel in a pot, the fine particles fall into the gaps and fill the air space, eliminating that advantage. The best way to create an air gap to speed up drainage is to use pot feet, or elevate the pot in some way. I sit my pots in a tray of scoria when I construct humidity trays https://deepgreenpermaculture.com/2019/03/31/how-to-grow-pandan-in-temperate-climates/#how-to-make-a-humidity-tray The water pools below and make the air humid as it evaporates, but the scoria lifts the bottom of the pot above the water below –
If you were pouring say, exactly 100ml of water and no more into a pot when watering, and increased the amount of absorbent material in the pot, whether it was some form of significantly absorbent rock, or extra potting mix, then the same about of water will simply be distributed over a greater volume of potting mix, making the mix a bit less damp for a given amount of water. Using less water, say 90ml, without changing the potting mix would have the same effect though for much less effort and materials.
For some people, seeing is believing! As you’re well aware, all good science is repeatable, so when I get the time, I’ll have to repeat the experiments carried out in one of the studies I cited, the University of Tennessee, Institute of Agriculture, Agricultural Extension Service PB1618, Growing Media for Greenhouse Production. Thanks! 🙂
I think you misunderstood my point about the air gap. The lack of air gap between soil and gravel *is* the reason why the gravel is able to actively pull water out of the soil by capillary action – which is the part you don’t seem to believe can have an effect.
The UoT study is interesting but doesn’t specifically cover the points you made here about the effect of drainage layers on the perched water table. I look forward to seeing your results when you’ve tested your predictions ?
Hi Angelo, I wasn’t able to respond to your other comment so I’m writing a new one. Suppose we had an infinitely tall planter that was 99% filled with gravel and just the top as soil. Wouldn’t the water fall down through the gravel as if there was a hole at the bottom of a normal pot? Or would some amount of capillary action still take place? Further does capillary action take place when there is proper drainage in your planter?
If the potting mix holds a 5cm (2″) perched water table at the bottom due to its capillary action, no matter what’s beneath it, as that’s how much it wicks. o answer your question, in this hypothetical example, if we filled the pot so these was only 10cm of potting mix, then it would be saturated with water from the middle down.
That doesn’t change unless the porosity, and therefore the wicking ability of the potting mix is changed, making it a new, faster draining potting mix. To do this, see my article – How to Improve Drainage in Plant Pots, The Proper Way to Do It. If putting gravel at the bottom of pots did increase drainage, all the professional horticulturists working in plant production nurseries would just do that, placing different amounts of cheap gravel in pots, rather than creating a whole range of potting mix blends with expensive amendment materials such as perlite, vermiculite, peat, etc. Hope this explanation is helpful! 🙂
hello admin, My husband built a big cement planter for me which is very special planter used back home in India. I am very excited to move my holy plant (Tulasi plant ) into it. The planter has 3 drain holes. A friend told me to put a layer of gravel. I am not sure and when researching came across this article. Please advise if I should put a layer of gravel and will it promote growth of my plant.
Please note this planter will be moved indoors in winters.
Thank you very much in advance.
Hi Angelo, thank you for another excellent article! I came across this website last night and am eagerly working my way through all the wonderful information! It is so true that the gardening world is saturated with myths and I find it puzzling why more gardeners dont take a more scientific approach, or at least possess a little more skepticism and curiosity. I had never heard of the perched water table so thank you for adding this to my knowledge base!
It was interesting timing that I just found this article, because only a couple days ago I repotted my African Violet plantlets in separate clay pots (after rooting several together in a little tray). I bought little 3? terra cotta pots at the store, only to realize when I got home that there were no drainage holes! My first thought was to put gravel in the bottom to hold the excess water, but I figured this was probably just me being lazy and that the end result would be less than desirable, so I just drilled holes in the bottom. As I was repotting them, I noticed the roots were much smaller and more shallow than I had anticipated, so in the end I still used gravel in the bottom to limit the amount of potting soil (I admit I was kind of patting myself on the back when you mentioned the utility of this in tall pots lol), as I had been reading that AVs in particular do better in more shallow pots, but I was having a hard time finding any. When I first opened the article I was like oh no, maybe I made a huge mistake using gravel!, but I hope they will be ok since the roots should still be above the perched water table (I used 50% pearlite to help with drainage since my potting mix didnt seem to be cutting it).
I am curious, you talk about most plants needing a wet/dry cycle, and most of the advice I have seen is to let soil dry to a certain depth (1? below the surface, 3? below the surface, top half of the medium, etc.), but as far as Im aware there are only a few types (eg. succulents) with which you can (or should) let the medium completely dry. So what about the roots (of a non-succulent plant) at the bottom of its pot? Are they adapted in some way to not need a dry cycle? Or is the wet/dry cycle less about each individual root and more about how the whole plant perceives its environment (if that makes sense)? Thanks again and I look forward to reading more of your articles!
Very informative article. Helped me with my terracotta plant pots. Thank you so much.
I have expanded clay in all my pots 🙁 they are all plastic vases that came with no holes (and I drilled some but now I know it was not enough), or glazed terracotta. Apparently they are doing fine but when I repot them I’d be more careful. Hope they survive