Should You Put Gravel or Rocks at the Bottom of Plant Pots for Drainage?

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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.

image

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.

  1. Gravity, which exerts a downward pull on the water, causing it to be drained away through the drainage holes.
  2. 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.

 

perched-water-table-plant-pot

 

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.

gravel-in-pots-perched-water-table

Adding gravel a the the bottom of a pot will create two potentially serious problem:

  1. 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.
  2. 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 drian 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.

 

Still Don’t Believe Me?

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:

  1. 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.

     

  2. From the University of California, Agriculture and Natural Resources, Master Gardeners of Monterey Bay.
  3. 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 linersIt 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.”

  4. 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. The dramatic effect of container height on air space is evident in Table 1, which shows the change in percent volume airspace of a 1:1 peat:vermiculite media in various growing containers.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 80percent. 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.

  1. 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.
  2. Make the same number of decent sized holes (around 6mm or 1/4″) in the bottom of each clear pot.
  3. 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.
  4. 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.
  5. Water each pot with the same volume of water.
  6. Wait till all pots drain well, this will depend on the type of potting medium used and the volume of the containers.
  7. 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 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…

 

 

 

About Angelo (admin)

Angelo Eliades is a presenter, trainer, writer, permaculture consultant, urban permaculture pioneer and food forest specialist.
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17 Responses to Should You Put Gravel or Rocks at the Bottom of Plant Pots for Drainage?

  1. Paul Taylor says:

    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

    Like

    • Angelo (admin) says:

      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!

      Like

  2. lisa Hall says:

    Thanks Angelo, myth buster. I really liked that you went into detail, it becomes very clear.

    Like

  3. Ben Hargy says:

    Excellent article, well written! Would be great to add an explanation of why the water doesn’t cross into the gravel layer.

    Like

    • Angelo (admin) says:

      Thanks Ben! The water doesn’t 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.

      Like

  4. Elizabeth Wall says:

    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.

    Like

  5. Lydia says:

    Wonderful informative and well written article-thanks‼️

    Like

  6. Barbara Johnson says:

    Thank you for this enlightening article. I have wondered for years why nurseries don’t have gravel or the like in the bottom of their pots. But no happy as I will now be obsessed with correcting my pots. 😆

    Like

  7. Will Gant says:

    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”.

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    • Angelo (admin) says:

      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!

      Like

  8. Frank says:

    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

    Like

    • Angelo (admin) says:

      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
      .

      Like

  9. 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 Dümmen 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.

    Like

    • Angelo (admin) says:

      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.

      Like

  10. Vinay Karade says:

    Excellent Article, in one article you have addressed many phenomenons.
    I also appreciate your efforts in providing significant explanations in the comments section.

    Like

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