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 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 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 from University of California, Agriculture and Natural Resources, Master Gardeners of Monterey Bay.
Hopefully that’s convincing!If not, I’ll show you how to test this, all you need is some empty soft drink bottles.
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 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…