Does Gravel in Pots and Containers Increase Drainage?

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Robert Pavlis

Plant roots don’t do well sitting in wet soil, so drainage is key to healthy plants. Many books and web sites recommend the addition of stones, pebbles, or pot chards in the bottom of pots to increase drainage. This makes so much common sense that many gardeners believe it.

It is a myth. Adding stones or other bulky material in the bottom of a pot does NOT increase drainage. Scientists have known this for 100 years or more.

In this post I will explain why this does not work, I’ll discuss some other common myths related to drainage and I’ll show you how to increase drainage in ways that work.

Does Gravel in Pots and Containers Increase Drainage?
Does Gravel in Pots and Containers Increase Drainage? source: North Dakota State University

What Happens When a Pot of Soil is Watered?

Before discussing the effect of adding stones, let’s understand what happens when a pot of soil (or soilless mix) is watered. Water is added to the top of the soil, and some runs out of the bottom of the pot. The pot appears to have good drainage, but that is a myth.

Microbe Science for Gardeners Book, by Robert Pavlis

When a small amount of water is added to the pot, a film of water coats the outside of soil particles. Not enough water has been added to either sit in the bottom of the pot, or run out of the pot.

When more water is added, it still does not run out of the bottom, but it does start accumulating in the bottom of the pot. The soil in the bottom of the pot gets saturated; all air spaces get filled with water.

If even more water is added, the soil reaches a point where it is fully saturated, and can’t hold any more. Now the excess runs out the bottom.

What about that saturated layer of soil? Does the water run out from it? No. The bottom layer of soil stays saturated. When enough water is added to the pot, there is always a saturated zone at the bottom of the pot. This is a key point that is not understood by most gardeners.

Perched water table in a single pot, source:
The saturated zone (perched water table) in a pot with no stones, source: Deep Green Permaculture

The Science Behind The Saturation Zone

There are two forces acting on water in the soil. Gravity pulls it down – that seems obvious.

The second force is capillary action, and this force pulls water back up into the soil. If you have ever watered a pot by placing it in a tray of water, you have witnessed capillary action. Even though gravity is holding the water in the tray, water still moves up the soil.

Capillary action is not very strong and so water only moves up a fixed amount depending on the type of soil. A smaller pore size increases capillary action. Charged particles, like clay and compost, have a stronger capillary action than non-charged ones (sand and silt).

Back to the saturation zone.

Initially water just coats the outside of particles with a film of water. As the amount of water increases, gravity has more of an effect and starts pulling water to the bottom of the pot. At the same time capillary action pulls it up, preventing it from running out the bottom of the pot. The result is a saturated zone at the bottom of the pot. In this zone, all of the pores are filled with water – that is why it is called saturated. There is no air, which is bad for plant roots.

As even more water is added, the capillary forces can’t overcome the gravity, and excess water runs out the bottom of the pot. At some point the two forces are again in equilibrium; no more water runs out and the saturation zone remains.

There are a couple of key points to understand.

  • The height of the saturation zone depends on the characteristics of the soil.
  • The saturation zone exists in the bottom of all pots, provided enough water has been added.
  • The size of pot (width or height) does not affect the height of the saturation zone.

A Less Sciency Explanation

Here is another way to think of this issue. Water does not easily move between layers that have different particle sizes. This was shown many years ago in a simple experiment. A container had a lower level of sand (large particles), and an upper level of silt loam (smaller particles) Water was dripped onto the top. You can see in the picture that water (the black areas) moved down by gravity until it reach the sand layer (a). At that point, water started moving sideways instead of moving down (b). As more water was added, the loam became saturated and capillary forces could no longer overcome gravity and it started moving into the sand (c).

Water moving between silt and sand, showing the saturation zone
Water moving between silt and sand, showing the saturation zone

The movement of water in this experiment is no different than water moving in a pot of soil. The silt loam is the soil, and rocks are the sand. Water does not easily move from soil into the rock layer (picture b). Particle size is one of the parameters that are at play here.

YouTube video

What Happens When You Add Stones to the Bottom of a Pot?

Many sources claim that adding stones to the bottom of a pot increases drainage. To do that, the stones would have to overcome gravity and the capillary forces.

Stones do not change gravity!

Stones will not change the characteristics of the pores in soil and so they won’t change the capillary action of soil.

Once you understand how water moves through the soil, and the effect of these two forces, you quickly see that rocks at the bottom of the pot can’t reduce the saturation zone, so they don’t increase drainage.

The understanding of this has led to a new myth; adding stones to a pot “decreases drainage” which results in a saturation zone. This is not correct. As already mentioned, the saturation zone exists with and without the stones, and in fact the height of this zone is the same in both cases. Stones do not decrease drainage. They do not create the saturation zone.

Perched water table, with and without stones, source: Deep Green Permaculture
Perched water table, with and without stones, source: Deep Green Permaculture

What is a Perched Water Table?

A lot of gardening sources that understand this issue are using the term “perched water table”, as in “stones added to pots cause a perched water table”.

The term “perched water table” is used in place of my term “saturation zone”.

I have already discussed the first issue with this – stones don’t cause a perched water table, or a saturation zone.

The second issue is the use of the term. In geology, a perched water table is an area in soil were water collects above a solid formation, such has hard clay, or rock. This is not exactly what happens in a pot, and so the term is being used incorrectly when gardeners use it to talk about the wet layer in pots.

However, the term is now well entrenched in gardening and horticulture writings and one could argue that the bottom of the pot is a “solid formation” trapping the water. For these reasons, I think its use is acceptable.

Is a Perched Water Table a Problem?

After telling readers not to use pebbles because it causes a perched water table, writers go on to warn them about the dangers of the wet zone, but is this really a problem?

That depends……

If you water by adding small amounts of water, you never create a wet zone, even above stones, so it is not a problem.

If roots fill the pot and use the excess water quickly, it is also not a problem. Nor are plants that like a wet root zone. Plants with small root systems that stay above the perched water table, as in large containers, also don’t have a problem. Plants in small pots that dry out quickly grow just fine even with a layer of stones.

Remember that all pots, even ones without stones, have a perched water table, and that not all plants in pots have rotten roots! If you have ever bought a root bound plant you can easily see that it was not harmed by the perched water table.

You can clearly see that a claim such as “adding stones to the bottom of a pot will rot your roots” is a myth or at least an exaggeration. I am not suggesting it is a good practice, but it does not always lead to rotten roots.

Statements such as, “Placing gravel at the bottom of the container can prevent the free flow of water through potting medium and out the drainage holes.” are also incorrect. The gravel does not change the flow of water in soil.

Why Are Stones in a Pot a Problem?

Since even pots with no stones have a perched water table, the problem is not the existence of a wet zone. The problem is two fold.

  • Stones reduce the amount of soil in the pot.
  • Stones raise the perched water table higher in the pot (see the above picture).

Therefore, roots have less soil to grow in, and they are closer to the wet zone. The amount of unsaturated soil decreases which means there is less total air in the soil and air is critical for proper root growth. Adding stones has the same effect as using smaller pots.

 

Still Not Convinced?

Try this experiment. Take a kitchen sponge and set it on the table in a vertical position. Slowly pour some water onto the top. The water will make its way to the bottom of the sponge and sit there. Very little will leak out onto the table unless you pour too much into the top. You can take the sponge and turn it over and the water will again move down the sponge due to gravity, and stop before it runs onto the table. In this experiment, the sponge is acting similar to soil and the table represents the gravel.

How Do You Increase Drainage?

Drainage in soil is a function of pore size (determined largely by particle size) and charge characteristics of the particles. If you increase pore size or decrease charge, drainage increases.

Some potting media has almost no perched water table. For example, orchids are potted in large chunks of bark or coconut husk. The pore size of these is so large, that the capillary action is almost zero, so gravity pulls all the water out the bottom of the pot.

Cactus soil contains lots of sand and stones, which create large pore spaces in the soil resulting in a very small perched water table.

You can increase the drainage of any soil by increasing the pore size. Adding sand to most potting soil will accomplish this. The key is to mix it in with the soil, not to lay it at the bottom of the pot.

Perlite and vermiculite will also work to increase drainage.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

31 thoughts on “Does Gravel in Pots and Containers Increase Drainage?”

  1. I’ve noticed when repotting my plants that eventually the soil does work itself in between the gravel, filling the empty space (my plants hardly ever get repotted, so the process of soil migration may take a long time) and eventually there is a soil/gravel mix at the bottom, with soil reaching the bottom of the pot. Though the gravel doesn’t impede drainage, a large amount results in a significant reduction in soil volume, which would require watering more often. In the case of outdoor raised beds, it would seem that adding gravel would result in a light rain saturating a gravelly soil more, because of the reduced soil volume.

    As to adding sand, I’ve experimented with filling plastic cups (with holes at the bottom) with different sand/soil mixtures, ranging from 100% soil to 50/50 sand/soil and have found that adding sand actually retains more water both immediately after watering and several hours later. (Measurements were made by weighing the cups.) I used the soil in which my potted plants were growing as well as commercial topsoil. You may want to reproduce this experiment with your own soil, as all “soil” is clearly not the same. I do remember your mentioning in your excellent book “Soil Science for Gardeners” that soil particles aggregate so that may have something to do with it, as sand may be filling in the space between larger peds.

    Reply
  2. Interesting article. I would think by now people would test the “theory” with widely available moisture meters by probing at different levels of the pot. Makes sense though. I’ve been skeptical about those rocks all along but would always put them because it’s a lot easier than continue to argue about it with my mother.

    Reply
  3. I cut a circle of landscape cloth for the bottom of all my houseplant pots. You have more room for soil and none of that soil can escape from the pot. It seems to work just fine–I have a clivia (re-)potted like that, that is 54 years old. One roll of landscape cloth will last until your grandchildren have grandchildren.
    Rae Wade
    GA Master Gardener

    Reply
  4. I put a few smalls rocks in the bottom of my pots to stop them falling over in the gale force winds we occasionally have here.
    Last year, I tried about 10 pots with rocks and anther 10 without. Every time we had a strong wind storm, only the rock-less pots fell over.

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    • I tried that Peta, but it didn’t work, would love to hear back if anyone has found a solution to stop pots blowing over?

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  5. I would have thought and maybe wrongly still believe that the use of traditional crocks, ie. broken porous terracotta encouraged some absorbed moisture to be retained in the soil, slowing a complete parching of the soil.

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  6. Robert,
    What about self watering pots which sit in a reservoir of water at the bottom one would imagine (perhaps?) that the perched water table at the bottom of the pot is too wet, especially in winter when watering demands are less.

    Reply
    • Assuming a pot is sitting in shallow water – the perched water table should be the same as if it were not sitting in the water, except that the water will never dry out when sitting in water.

      Reply

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