Why You Should Never Add Stones to the Bottom of Your Flower Pots

Science has proven that adding stones to the bottom of your flower pots can actually harm the plants. So why are we still doing it?

Just like me, you’ve probably heard for your entire life – at least your gardening life – that you should add small stones, gravel, pieces of broken ceramic pots, or other filler to the bottom of your containers before planting. The reasoning is that it provides better drainage for your plants as it helps the water drain faster through the pot.

It sounds reasonable, right? But it turns out that stones in the bottom of the pot actually slow drainage and can harm your plants. Scientists have proven it.

Actually, they busted this gardening myth more than 100 years ago and many times since then. But somehow, this important fact hasn’t entered the collective conscience of home gardeners – we’re still throwing all kinds of things into the bottom of our flower pots. I was guilty of this too until I stumbled across the research on the Garden Professors page on Facebook.

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Studies show that in a pot, water does not quickly cross from soil to stone

Scientists have demonstrated that water does not easily move between a fine textured material like soil and a coarser material like stones. More specifically, water is very slow to cross the area between the two materials. So the coarse materials at the bottom of the container slow water drainage, making the soil more likely to become waterlogged and potentially drown your plant’s roots.

“The fact is, when water moving through a soil reaches a horizontal or vertical interface between different soil types, it stops moving.”

Dr. Linda Chalker-Scott, Ph.D., Washington State University Extension Urban Horticulturist in The Garden Professors Blog

Why this happens: gravity

Rather than water freely moving down through the the soil and across the stones, water resists crossing the barrier between them. That’s because the water in the upper layer is being pulled upwards by the dry soil and the plants. This offsets the effects of gravity, which would otherwise pull the water through the stones. The water builds up in the top layer until it’s oversaturated, and only then is water in the soil released into the layer below. The oversaturation of the soil poses a threat to your plant’s roots and may be why your container-grown petunias didn’t survive the season from time to time.

“It is a myth that a layer of gravel (inside the bottom of an individual pot) beneath the soil improves container drainage. Instead of extra water draining immediately into the gravel, the water “perches” or gathers in the soil just above the gravel. The water gathers until no air space is left. Once all the available soil air space fills up, then excess water drains into the gravel below. So gravel in the bottom does little to keep soil above it from being saturated by overwatering.”

Successful Container Gardens, University of Illinois Extension

So the next time you pot up your plants, just add soilless potting mix and nothing else, And of course, make sure there are good drainage holes in the pot.

Read more: The Myth of Drainage Material in Container Plantings, Washington State University Extension, or download the pdf.

To see for yourself why this happens, check out this video from Joe Gardener.