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.
I’ve seen it suggested to use packing peanuts. This reduces to weight.
That is a common suggestion and it does reduce weight. But the effect for plants is the same as using rocks in the bottom. The video explains.
I put a coffee filter over the holes in the bottom of my pots to keep the soil from coming through the holes. Is this a bad idea also?
A coffee filter is okay – personally, I use a small piece of old window screen to keep the insects out of the pot.
What about adding wood mulch to the bottom of the pots? Some pot are so deep there is more depth then
needed for the roots. I think the deep pots use too much potting soil.
The problem will still be similar to the problem with stones or anything else, unfortunately. Plus, as the wood chips break down they’ll be using lots of nitrogen, which the plants need for growth.
I saw your answer to “Sherr” above. It makes sense for the larger pots without holes to do as you suggested. But what about smaller pots? I have several that are from 4 to 8 inches or so, and even though I’m starting with a smaller plant, it would be difficult to put another small pot within the slightly larger one.
Julie: Any and every container needs to have drainage holes or your plants will have wet feet. Holes also aid in oxygen exchange for the roots. Use a drill to make 2 holes in the bottom. If it’s a very small pot, 1 should suffice.
But, if you don’t have rocks, then eventually the water will come to the holes at the bottom of the pot and would it NOT be the same story? There too water will want to stay in the soil and not cross the different horizontal interface, right? How does water “know” the difference between rocks and the pot material with holes in it?
Kris:
At that point, gravity does its job and pulls water through the hole in the pot because all of the air space in the soil is used. More here: https://web.extension.illinois.edu/containergardening/choosing_drainage.cfm
So, how does this relate to french drains and dry creek beds in landscaping?
Annette: Great question. It does not affect using stones for drainage elsewhere. In flower pots, tension is placed on the water by gravity pulling it down, the soil trying to absorb the water, and the plants that are pulling it up into their roots. This is what creates the surface tension on any material other than soil that’s in the bottom of the pot.
What if you live in an area where the heat can dry out planters in hours, resulting in the need to water two or more times a day? Wouldn’t stones and other means if retaining water be important? Maybe you should use different methods for different regions depending on rainfall.
so no old sticks and branches either?
Definitely not.
What about filling the bottom of a very large pot with empty plastic water bottles to save on the amount of soil needed. I’ve seen this method many times.
Bonnie: Plastic water bottles will have the same effect as adding stones, so no, it’s not recommended.
Iam putting wood charcoal. Self made wooden charcoal.
If you’re making your own wood charcoal (biochar) I would grind it up and add it to the mix instead of adding it by itself on the bottom. It’s excellent for holding water which is exactly what you don’t want at the bottom of the pot, but could be of benefit in the mix itself.
What about for roses? Should I use stones in the pot?
The advice holds true for any plant – do not use stones in the pot.
What about coffee filters over the holes to keep the soil in the pot?
Maureen: When potting soil is wet, it expands and there is little leakage from the hole in the container (unless perhaps you have a DIY container and your hole is too large). But if you’re concerned about this, the recommendation is to use a small piece of window screening or something similar to keep the soil in and allow the water to drain freely. Coffee filters will retain water and are not recommended.
But what if your pots don’t have holes at the bottom, what should we do then?
Susan: If you’re using a container without holes, it’s a best practice to insert a pot with holes into that one, so that it is holding the actual pot. Having said that, depending on the plant, and if the pot is large, you might get away with creating a kind of false bottom with plenty of air capacity so that the soil at least eventually drains. But bear in mind that the water that drains has nowhere to go and will stay in the pot indefinitely.
I think you fudged your results. No one would fill the container that full of rocks. The reason normal people put rocks in a pot is to just cover the drainage hole to keep the soil from flowing out. Just a few stones to cover the hole. You projected a whole different scenario to meet your objective. Not scientific at all.
Martin: The video does indeed use more stones than you would normally use to illustrate the point. Study after study has confirmed the same results – water is very slow to move from soil through stone.
But what if we do not use as much stone as you did. Let us say in a 12-inch high container,
we fill 1 inch, then the water would collect below the plant’s root ball.
Hector: A plant’s root ball should be able extend all the way to the bottom the container for optimum health. I’ve seen this many times – not so much with annuals, but most definitely with perennials. But even marigold roots will fill a large pot in one season if they’re allowed to. It’s a matter pf physics – if the water can’t pass over the stone quickly, the water above the stone will waterlog the soil, which is not a healthy environment for the plant’s roots.
I put dry leaves at the bottom of my pot so that I don’t use as much soil and so that pot isn’t as heavy. Is this still recommended? Thanks
Debra: Adding leaves to the bottom of the pot is definitely not recommended for two reasons. First, the leaves may clump together and block drainage, since there won’t be bacterial and fungi action to degrade them as there is in open garden soil. Second, there’s a very high probability of introducing diseases to your plants like fungal infections that are carried on the leaves.
So what should be done with the new ceramic pots that have no drainage holes and unable to drain for indoor plants?
Sherr: I assume you’re thinking of the very large pots without drainage holes? Many of them are meant to contain a pot with holes, which is how I use them. That way you have an attractive container for a pot that might be a little beat up looking. This can also save lots of money on potting soil as you only need to fill the inner pot which could be a of a considerably shorter depth.
I have used a diamond drill to make holes in the bottom of the container. Easy