Logic would have you believe that adding gravel or pot chards in the bottom of a pot will increase drainage, but that is a myth that scientists have understood for more than 100 years. In this post I will explain why gravel does not work and show you how to increase drainage in ways that work.

What Happens When You Water a Pot?
When you water a pot containing either soil or potting media, the water fills the pot and excess water flows out the bottom of the pot. You would expect that all of the soil is equally wet, but that is not what happens. There are two forces at play. Gravity pulls water down while capillary action pulls water up. The end result is a top layer of soil that is wet with most of the larger pores between soil particles are full of air. Below this is a fully saturated layer commonly called a perched water table. This perched water table has all of the pore space full of water and is more fully described in What is a Perched Water Table.

Movement of Water
What happens when you layer one type of soil above another? You might expect that water easily moves from one layer to another but that is not what happens. Water has difficulty moving from one soil to another when the particle size of the soils are different. 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, image (a). At that point, water started moving sideways instead of moving down (b). As more water was added, the silt loam became saturated and capillary forces could no longer overcome gravity and the water started moving into the sand layer (c).

The movement of water in this experiment is no different than water moving in a pot. Water in a pot full of soil does not easily leave the pot because the soil and air have different particle sizes. This experiment also illustrates why you should never layer different types of soil in the garden. Always mix them together.
Adding Gravel to the Pot
The particle size of gravel is very different from that of the potting media. Therefore water does not easily move from one into the other. Instead, a perched water table is formed at the bottom of the soil level, right above the gravel. Stones do not change the characteristics of the pore spaces in soil and so they won’t change the capillary action of soil.
Once you understand how water moves through soil and the effect of these two forces, you quickly realize that rocks at the bottom of the pot can’t reduce the perched water 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 also a myth. 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 increase or decrease drainage. They do not create the perched water table.

Still Not Convinced?
If you still doubt the presence of a perched water table, try the simple experiment in this video. I hope it will convince you.

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 with or without gravel, so it’s not a problem.
If roots fill the pot and use the excess water quickly, it is also not a problem. Nor is it a problem for plants that like a wet root zone. Plants with small root systems that stay above the perched water table, as is the case with tall containers, also don’t have a problem. Plants in small pots that dry out quickly grow just fine.
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 they are not harmed by the perched water table.
The claim that stones in the bottom of a pot rots plant 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 media and out the drainage holes” is also incorrect. A gravel layer does not change the flow of water in soil.
Why are Stones a Problem?
Gravel in a pot causes two problems.
- 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 and that is rarely a good thing for plants
How Do You Increase Drainage?
Drainage in soil is a function of pore size (which is largely determined 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, perlite or vermiculite 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.
The reason I put rocks in the bottom (or shards) is to keep the hole from clogging up.
I would just like to say how much I am enjoying reading your posts!
A very refreshing mindset 🙂
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.
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.
Robert:
As for drainage in houseplant pots, aren’t we talking about shards, not chards?
Rae Wade
yes – fixed
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
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.
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?
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.
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.
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.
I put a few rocks or pot shards in the bottom of pots so that the soil doesn’t fall out of the hole.
It won’t fall out even without a chard.