Should You Use Drainage in Your Pots?

It used to be very common to add drainage material, such as rocks, Leca, or clay pot shards in the bottom of pots for added drainage. Then it was claimed that this is a myth and it does not work. But then some new science this year upsets everything by showing drainage material may actually work.

Should a gardener use drainage material? What kind of material works best?

What is Drainage Material?

Gardeners use the term drainage material in two different ways. Items such as rocks, Leca, gravel, or clay pot shards can be added to the bottom of the pot. It is claimed that it helps water flow out the bottom of the pot.

A second type of drainage material is mixed into the soil or potting media. This can be sand, gravel, perlite, or vermiculite. Adding material that has a larger particle size than the soil adds large pore spaces which results in less water being held in the soil. The soil is considered to drain better.

The two types of drainage material could be used separately or combined. The potting media you buy usually contains some perlite to lighten the soil and make it more porous. Additional material could be added to the soil and at the bottom of the pot when you pot up a plant.

The rest of this article will deal only with the material in the bottom of the pot.

What Happens In the Bottom of a Pot of Soil?

The latest science shows that drainage material in the bottom of a pot increases drainage. That means there is less water in the soil sitting above stones, rocks, and clay pot shards compared to soil without drainage material.

Some claim that adding pebbles to the bottom of a pot causes a perched water table but that is not correct. A perched water table exists at the bottom of all pots, with or without drainage material. This saturated layer is moved higher up in the pot when drainage material is added to the bottom.

The claim that stones in the bottom of a pot rots plant roots is a myth or at least an exaggeration. Most roots will grow just fine in this layer provided they get what they need.

Most potted plants seem to form roots around the outside of the soil, right between the space between the soil and the pot. These roots continue to the bottom of the pot and eventually make a solid mass of roots along the bottom. At this later stage, the plant is considered to be pot-bound.

The Pros and Cons of Drainage Material

Should gardeners use drainage material? To answer this it is important to understand the pros and cons of using the material.

The Pros of Using Drainage Material

• Soil is drier which is better for roots that like such a condition.
• Drier soil is less likely to lead to root rot, especially for gardeners who water too much.
• Adds weight to the pot making it more stable.

The Cons of Using Drainage Material

• It reduces the amount of soil in the pot, which means less soil and space for root growth. It also moves the roots closer to the perched water table.
• Soil dries out more quickly, requiring more frequent watering.
• Reusing soil is more difficult since the drainage material gets mixed into the soil when you unpot a plant. Potting soil does not get old and can be reused.
• Changes the weight of the pot making hefting the pot, to determine if it is time to water, more difficult.
• Another product to buy.
• Complicates the potting process by adding more varialables such as type of material, particle size, etc.

Should You Use Drainage Material in the Bottom of the Pot?

If the plant is growing well – don’t use drainage material.

If the potting media is too wet and roots are rotting, have a close look at your watering habits. It is better to fix your watering problem than to change the potting media or the way you pot plants. Water only when needed.

Watering from the top or bottom can also make a difference. The danger with bottom watering is that the pot can sit too long in water.

If you are watering correctly and the roots are still rotting, then the plant may need a drier soil. You have two options: add drainage material in the bottom or change the porosity of the soil which is easier to do. Add some more perlite which I prefer over vermiculite. This will allow the soil to drain better and add more air pockets in the soil.

If you made the potting media very porous and you still have a problem, try adding some drainage material to the bottom of the pot.

I have grown many different kinds of potted plants and have never used drainage material.

What Drainage Material Works Best?

There is only one study, by Avery Rowe, that looked at the effect of drainage material on soil in a pot and it concluded that a thick layer (60 mm) of sand worked best. A thick layer of grit also worked well.

This table shows the volume of water, in milliliters, in the soil layer as well as the % change between only soil (i.e. no drainage layer) and soil with a drainage layer. The results from my analysis of the raw data from the study.

Material	CPB Soil (water in ml)	% change	CV Soil (water in ml)	% change	JL Soil (water in ml)	% change
Only soil	304		381		335
soil + 30 mm gravel	167	-45	334	-12	297	-11
soil + 60 mm gravel	188	-38	234	-39	297	-11
soil + 30 mm Leca	145	-52	313	-18	321	-4
soil + 60 mm Leca	137	-55	256	-33	309	-8
soil + 30 mm grit	156	-49	317	-17	258	-23
soil + 60 mm grit	111	-63	219	-43	282	-16
soil + 30 mm sand	158	-48	250	-34	256	-24
soil + 60 mm sand	106	-65	208	-45	215	-36

Pots Without Holes

I was asked about using drainage material in pots without holes. The advice is simple: don’t. It leads to all kinds of problems. Pots need to have holes.

Drilling holes in ceramic pots is easy.

If you must use a container without a hole, drainage material is better than nothing, but water very carefully.

Terrariums

Terrariums are an exception to the rule. Since these containers do not have drainage holes it is important for the excess water to go somewhere. A drainage layer provides a semi-hollow place for excess water to accumulate. You still need to water carefully so that there is very little water in the lower level.