Draining for cultivation in a container

by Rock Giguère

The two very important elements for growing annual plants successfully in a container are, having a pot with drainage holes for root aeration and usage of a quality potting soil that ensures good air penetration and good water drainage. A soil with good structure is loose and friable. We therefore do not need other means to ensure the proper drainage of our plantings and thus prevent our plants from rotting.

Addition of a layer of gravel in the bottom of the container: a myth

The addition of a layer of gravel or a concave shard of sandstone pot in the bottom of a container, supposedly to increase the drainage capacity, is a myth that doesn’t want to die.

Dr. Linda Chalker-Scott, Associate Professor in the Horticulture Research Department of Washington State University, has written an article entitled “The Myth of Drainage Material in Container Plantings” to attest to the futility of this gesture.

Today, Dr. Chalker-Scott reminds us, garden centers sell high-quality organic potting soil for the preparation of our flower pots.

The water passes through good soil faster than a bed of gravel. It has even been proven that water flows less quickly through a thick layer of filter media in the bottom of the pot. The roots suffocate if the soil is saturated with water, because water then takes the place of oxygen in the soil pores. Soil specialists have demonstrated this for more than 100 years, she says, but the drainage bed myth refuses to die …

The container’s drainage holes

Most pots sold in garden centers have drainage holes. The ideal number to ensure good drainage depends on the container’s diameter. One hole is enough for a pot of 15 cm in diameter and less. Additional holes need to be added for larger pots. For a wooden half-barrel, for example, it would be necessary to drill about nine drainage holes. A 2 cm (¾ “) wick should be used.

Another myth: a coffee filter on the drainage holes

Some “experts” recommend placing equipment such as a coffee filter over the drainage hole before adding the planting soil to keep it inside the container. Other sources, a little more advised, recommend this, hoping that the filter breaks down over time so as not to block the ground permanently. It is true that the soil will slide through the holes for a short time but after a short period it will stay inside. Moreover, the saucers under the container will collect the earth that escapes while the soil agglomerates.