You’ve probably heard that you should put rocks, gravel, or styrofoam peanuts in the bottom of your plant containers to improve drainage or act as filler. But this gardening myth has been debunked. Here’s why you should skip putting anything besides potting soil in the bottom of your pots, plus some cool workarounds to save money on potting soil.
Potted plants are a quintessential design feature of desert gardens. It’s common to see annual flowers, small trees, and succulents of all sizes growing in containers as colorful, eye-catching focal points.
So it’s important that desert dwellers understand how to properly pot their plants. And one of the biggest mistakes that many gardeners make is putting fillers — rocks, gravel, styrofoam peanuts, plastic water bottles, and more — in the bottom of pots.
There are several reasons why this is commonly done.
Putting rocks or gravel in the bottom of pots supposedly increases drainage. Excellent drainage is critical, since overwatering kills more desert plants than underwatering.
Adding rocks and gravel is sometimes done to provide ballast. Tall pots, while architecturally beautiful, are not very stable and can get blown over by wind or knocked over by hungry wildlife.
Additionally, using large containers is highly recommended to keep plant roots cool, and that takes a lot of potting soil. No one likes spending money on soil just to fill the bottom of a big container. (It seems like a waste!)
These all seem like good reasons to add fillers to your pots. (In the past, I’ve added fillers for these reasons, too.) However, it’s now known that adding a layer of anything that isn’t potting soil is not only unnecessary, but actually impedes drainage.
Let’s take a look at how that happens.
How Water Moves: Gravity vs Capillary Action
When you water your plants, it seems logical to assume that the following happens…
Water runs through the soil due to gravity. Soil soaks up what water it can hold and any excess runs down through the soil and either settles in the spaces between the rocks or runs out the drainage hole. This keeps excess water away from your plant roots, preventing rot.
But what really happens is not that simple.
There’s an opposing force to gravity called capillary action. If you’ve ever put a sponge or paper towel in water and noticed how the water moves up, that’s capillary action at work.
Capillary action causes the potting soil to pull water back from the gravel, creating a wet zone where the soil meets the gravel known as a “perched water table.”
As you can see in the diagram below, adding a layer of gravel or other filler, moves the wet zone up, closer to your plant’s roots.
Additionally, according to horticulture expert Linda Chalker-Scott, PhD, “When water moving through a soil reaches a horizontal or vertical interface between different soil types, it stops moving.” This further contributes to the wet zone.
You can check out her blog, The Garden Professors, for images of this occurring in experiments she conducted.
What If You Water Shallowly?
Now, you may be thinking you can get around this by giving your plants small amounts of water at a time. It’s true, you can avoid creating a wet zone this way.
But that runs counter to the well-established advice that when you water, you should water deeply. This helps plants establish strong root systems and flushes out salts (i.e., minerals from soil, fertilizer, and hard water). So that won’t be best for your plants in the long run.
Ditch the Gravel — And What To Do Instead
Instead of putting gravel in the bottom of your pots, most simply, fill your pot with one type of potting soil or mix all the way down.
I know many of us have large or tall pots that need a lot of potting soil, so here are a few sneaky workarounds.
Don’t Fill to the Top
This first suggestion is so simple, I’m embarrassed I didn’t think of it sooner! There’s no good reason to fill your pot to the tippy top with soil other than that’s how it’s usually done.
For example, instead of adding a foot of “stuff” to the bottom of your pot, simply leave a foot gap between the top of your soil and the top of your pot. This works well if you have a tall pot with a tall plant such as tall succulents or small trees.
This will also provide the added benefit of keeping your plants’ roots shaded and cooler.
Add Gravel to Your Soil
Instead of adding a layer of gravel to the bottom of your pot, add it to your soil. Mix the gravel into your potting soil, creating a homogenous mixture. Gravel, when added to soil, creates spaces which increase drainage and aeration. This works well in large, outdoor planters. It also adds more weight to tall pots to help keep them upright.
Use a Cachepot
You can use a cachepot — a pot within a pot. You grow your plant in a smaller inner pot and set it inside a large decorative pot. If you have a particularly tall pot, you can place your potted plant on top of an upside down nursery pot, terra cotta pot, or whatever else you have around that’s the right height.
Use a False Bottom
Instead of filling a large or tall pot all the way to the bottom, create a false bottom.
You can buy pots with built-in false bottoms. I have a few of these by Veradek. If you look closely, you can see a faint horizontal line about halfway down. There’s an insert with drainage holes at that point. There are more drainage holes at the very bottom of the pot, so water doesn’t collect in the empty space in between.
It’s not hard to create a false bottom. I’ve used inexpensive plastic plates (Target sells 10″ plates for $.50), plant saucers, or paint bucket lids. Find one that fits snugly inside your pot and leave enough room above it for soil. Note, that if you do this, you’ll have to drill drainage holes.
You can also cut off the bottoms of sturdy nursery pots which already have holes.
If you aren’t into drilling holes, you can buy planter inserts designed specifically for that purpose. Bloem Up-a-Daisy planter inserts come in a variety of sizes.
If you want to avoid plastic or have an unusual size planter, some gardeners create false bottoms with plywood. I haven’t tried this, so I’m not sure how long they last.
How to Fill Drainage Holes
To keep soil from falling out of the bottom, you don’t need a layer of rocks, which doesn’t work all that well anyway.
You can use anything that will block soil from falling through, but will let water through and won’t stay soggy. I use coffee filters in smaller pots and pieces of landscape fabric cut to size in larger pots.
Deep Dive: Recommended Videos
If you want to know more about the science behind drainage and wet zones, and/or see some helpful visual demonstrations, there are loads of videos on YouTube.
I watched several hours of videos on the topic of perched water tables — so you don’t have to. 😉
These were my top picks, depending on your level of interest and how much time you wish to devote to the topic.
If you’ve got 5 minutes, watch:
How to Get the Best Drainage for Your Container – Why What You’ve Been Taught is all Wrong by JoeGardenerTV
If you’ve got 15 minutes, watch:
Stop Putting Gravel in the Bottom of Pots & Proof of the Perched Water Table by Garden Fundamentals
And if you want to really geek out on the science and have an hour to spend, watch this two-part series by Gardening in Canada:
Rocks in Pots. A Soil Scientist Explains Perched Water Tables & Quick Fixes — Part 1
How to Use Rocks in Plant Pots for Drainage — Part 2
Don’t be concerned that this is a Canadian soil science channel. Soil everywhere follows the same principles.
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Author Bio
Deane Alban is the creator of Southwest Gardener. She is a science writer with a bachelor’s degree in botany from the University of South Florida. Gardening is her lifelong passion. She’s been gardening in Tucson for 15 years.
