What is a Perched Water Table?

The term perched water table is being used more and more in gardening circles. What is it and how do you control it?

It’s a phenomenon found in pots and containers and can end up rotting plant roots. It should be understood by every gardener.

What Happens When a Pot of Soil is Watered?

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 acting on the water. Gravity tries to pull it down through the soil and out the bottom. At the same time, a second force called capillary action pulls water back up the pot.

If you have ever watered a pot by setting it in a tray of water you have witnessed capillary action as it draws water up the pot right to the top. Is top watering or bottom watering better?

At the top of the pot, gravity is stronger than capillary action and in this region of the pot the soil particles get wet but the spaces between the particles, the pore spaces, are drained of water. However, near the bottom of the pot, both forces are of equal strength resulting in pore spaces that are full of water. This region is called the perched water table or saturation zone. It is fully saturated with water.

The Amount of Water is Important

If you only add a small amount of water there will not be enough to create a perched water table.

As you add more water you reach a point where nothing has drained out the bottom of the pot and the perched water table is fully formed.

As you add more water there is no space left in the pot and the excess runs out the bottom. Even though water is dripping out the bottom, the perched water table remains in the pot.

Still Not Convinced?

If you are not convinced that a perched water table exists, have a look at this video where I show you a way to demonstrate it with your own pots.

The Size of The Perched Water Table

How high is the perched water table and what determines its height?

The height is determined by soil properties including particle size and charge characteristics. Small particles have small pore spaces making it easier for capillary action to move water higher, resulting in a higher saturation zone at the bottom of the pot.

Soil particles also have an electrical charge which attracts water. You might know this as the EC properties of soil. Organic matter, compost, clay, peat moss, and coir all have a higher level of charge than sand and silt. A higher charge increases capillary action resulting in a higher perched water table.

The size and shape of the pot do not affect the height of the perched water table.

The above diagram shows the perched water table (in blue) for 4 different pots. The left one is a standard pot. The second one is tall and skinny and has the exact same saturation zone height as the first one. A wide pot also has the same height. The pot on the right is small but even it has the same height.

A gardener can reduce the relative size of the perched water table by using taller pots. Small pots make the problem worse.

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 the pot (width or height) does not affect the height of the saturation zone.

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

A bottom layer of stones moves the perched water table higher in the pot. It has been assumed that the height remains the same but recent science suggests that the height changes depending on the type of drainage material used. The latest data suggest that the height is less above drainage material than when no drainage material used, but this has not been measured.

What is clear is that the added drainage material reduces the total amount of soil in a pot. In turn this may mean less unsaturated soil for root development.

Is a Perched Water Table a Problem?

It occurs in all pots and containers and plants grow well in most of them. Why is that?

If you only add small amounts of water with each watering 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 a small root system that stays above the perched water table is also not 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 it was not harmed by the perched water table.

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.

How Do You Change the Size of the Perched Water Table?

The height is a function of particle size and charge characteristics. If you increase the particle size or decrease the charge, drainage increases and the height of the perched water table decreases.

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 of the bottom of the pot.

Cactus soil contains lots of sand and stones, which are relatively large and have a low charge resulting in a very small perched water table.

You can decrease the perched water table in any soil by increasing the average particle 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.

Since drainage material in the bottom of a pot reduces the total amount of water in the soil I suspect it also decreases the size of the perched water table. IN that case this may be a good way to reduce the perched water table, but it may still not be a good enough reason to use drainage material.