A Story of Gravel, Drainage Layers and Public Safety

There are horticultural techniques that were so widespread that we didn’t even think about them: we just applied them.

One of these techniques is the use of a drainage layer at the bottom of our houseplant pots and outdoor containers.

Horticultural experts have been telling us for a long time that a drainage layer is useless, even harmful to plants. It has been a long time since we stopped applying it in the horticultural industry (when was the last time you saw a drainage layer at the bottom of a pot bought in a shop?). Now, if something is no longer done at the production level, it might be worth finding out why.

The Dreaded Drainage Layer…

In my case, the beginning of the end came in 1979. I had gotten into the habit of using a cheap drainage layer: the gravel from the parking lot of the apartment building where I lived.

It was very practical: I didn’t even have to run to the store! And since I didn’t have a car, bringing back a 50-pound bag of gravel by bus, with a transfer, and finishing it all off with a ten-minute walk was quite a feat.

I had all the equipment I needed: a shovel, a rake, a bucket and a dark coat with a hood. That way, when I went out in the dark to take my sample, I could hope that my neighbors wouldn’t see me (I’m sure they already found me strange enough, with my fluorescent lights illuminating my almost industrial production of houseplants day and night).

However, one evening, while I was looking for the sacrosanct gravel, I discovered that the parking lot was completely frozen! No matter how hard I scraped, all the gravel was firmly stuck in the ice.

What to Do?

Yet I had no time to lose: the baby was sleeping and so I had a few minutes to spare! I gathered up my courage and decided to only put soil in my pots until spring. To my surprise, I noticed that the plants without a drainage layer grew better than those with one. Since then, even though I have long had the impression that I was doing something forbidden, I no longer put a drainage layer in my pots.

It was also around this time that I had started giving lectures on indoor plants. Was I going to have to confess to my audience that I didn’t respect one of the most fundamental laws of horticulture? Of course not! I presented my lectures like any good self-respecting speaker, with the good old “do as I say, not as I do”.

(Besides, it took me years before I could publicly admit that, deep down, I was a laidback gardener and that I did almost nothing by standards, but always in the most expeditious way possible!)

Totally Useless, and Even Harmful

Then, several years later, I came across evidence that I wasn’t so mad after all! A study published in the journal Quill & Trowel in 1990 had come to the same conclusion as me, but in a scientific way. The drainage layer is, at the very least, completely useless and sometimes downright harmful.

What the study showed is that when there is too radical a difference between the size of the particles surrounding the soil, drainage is not favored; on the contrary, it slows down or stops. In fact, for perfect drainage, the particles must be of approximately the same size, or for extra-efficient drainage, there must be a gradual decrease in particle size. The largest at the top, the medium-sized ones in the middle and the smallest at the bottom. So completely the opposite of what tradition dictates. (Curiously, this study confirms Japanese tradition, where the large particles are always placed on the surface of bonsai pots, the medium-sized ones in the middle and the finest at the bottom!)

I Repeat, the Drainage Layer Is Useless

From the moment I had scientific proof that my potting method was perfectly acceptable, I began to admit my sin, even publicly. As I had then started to write, I slipped a word into my articles and books. However, it seems that no one reads or believes me, because I still come accross the old method in books, articles and even television programs. Sometimes I feel like I am preaching at the bottom of a layer of gravel!

Despite everything, it’s not just that I want to alleviate the suffering of a few million poorly drained small indoor plants. I’m more concerned about public safety.

After all, if I go and collect my gravel from a private parking lot where people drive slowly, just imagine how many gardeners collect theirs from the side of the road! So if you do as I do (in a dark coat in the evening), I think you’ll be relieved to no longer have to frequent these dangerous places. It’s so easy to save human lives sometimes!

---

Larry Hodgson published thousands of articles and 65 books in the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Fleurs, plantes et jardins in April 2001.