Is it truth or myth that you should put rocks in the bottom of a pot to improve drainage?
It's a frequent piece of advice given to container gardeners everywhere. But recently a member of our Grow with KARE Facebook page contradicted that. So Bobby and I did some research.
Turns out, she was right! It is a myth that rocks improve drainage in a pot. In fact, they do the opposite.
Several research studies from across the country have found that a layer of rocks in the bottom of a pot in fact do not improve drainage.
Visualize a sponge on end, soaked with water. Eventually the top of the springe will dry out but the bottom will remain wet. The same thing happens with your soil in a container.
The bottom most layer of soil will remain wet, regardless of whether there are rocks underneath it.
So putting a layer of rocks in only raises that wet soil layer.
While shallow-rooted plants will not suffer, those with root systems that extend to even the mid levels of the pot will be subject to wet conditions and potentially root rot.
So what do you do?
To improve soil drainage all pots should have sufficient holes in the bottom to allow water to escape.
For plants that prefer well drained soil, we recommend mixing vermiculite, horticultural grit or even poultry grit into the potting soil and compost.
What you might still want to use rocks is for weight in the bottom of a tall or deep pot to prevent tipping in the wind. But otherwise avoid this common mistake!