Putting rocks in the bottom of plant pots used to be commonplace. It was thought that a layer of rocks in the bottom of a pot would help with drainage.
Current thinking holds that this is not necessary, and for the most part, this is true.
It’s good to put a rock or shard of pottery over the drainage hole in the bottom of a pot to keep soil from falling out or blocking it.
However, most of the time, a layer of pebbles is not necessary.
Why Put Gravel In The Bottom Of A Pot?
Gravel in the bottom of the pot can cause water retention.
Putting gravel in the bottom of a plant pot is based on the notion that you might overwater.
With gravel in the bottom of the pot, excess water would (theoretically) drain out faster thanks to the force of gravity.
This is true, but it doesn’t consider the fact that the soil in the pot will exert an opposing force by wicking the moisture up into the soil.
This phenomenon is known as the “perched water table” because the opposing forces of wicking vs. gravity can cause a layer of water to “perch” in the middle, making an excellent environment for fungal and bacterial growth.
For this reason, it can be better just to have soil at the bottom of the pot so that extra water can just drain out of the pot and away without getting caught up in a renegade physics experiment. [source]
Plants that like dry, rocky soil may like rocks in the bottom of the pot.
Succulents and cacti like sharply draining, gravelly, sandy soil that allows water to pour through. Unfortunately, this sort of soil retains very little moisture.
Furthermore, some succulents and cacti have very shallow roots and don’t need deep soil.
When this is the case, a layer of gravel at the bottom of the pot, under sharply draining cactus mix, won’t hurt anything.
If your pot is deep for your plant, those shallow roots will never make it to the bottom of the pot.
For this reason, your cactus mix will be wasted, so you might as well put a layer of rocks in the bottom.
Can Plants With Deep Roots Survive In A Pot With Rocks In The Bottom?
The rocks in the bottom won’t interfere with the growth of plants with deep roots, but they can make repotting difficult.
This is because deep roots will just grow into the rocks and become entangled with them.
You may have to cut them out when you repot the plant.
When Is A Layer Of Rocks Necessary In The Bottom Of A Plant Pot?
In pots without drainage holes, rocks in the bottom can help with drainage in that they may hold the soil and roots of the plant above excess water that collects in the rocks.
This is a very bad idea for most plants because that can lead to root rot.
However, it can be helpful if you force bulbs indoors.
A layer of rock in the bottom of a clear glass container with the bulb held in place by peat moss or something similar can work very well.
Extra water will settle into the rocks, and as it evaporates, it will keep the moss moist for the bulb. So in the glass, you can keep an eye on it and avoid overwatering.
It’s also best to remember that soggy peat moss will rot the bulb.
Try These Simple Soil Retention Alternatives
For many container gardeners, the value of a layer of pebbles to keep the soil from falling through the drainage holes is a significant benefit.
Luckily, there are quite a few alternatives to pebbles for this purpose.
Here are a few you can try:
- Put a coffee filter in the bottom of the pot.
- Put a paper towel in the bottom of the pot.
- Fold/cut a plastic net produce bag and place it in the bottom of the pot.
- Cut a piece of old screen or landscape fabric to fit over the drainage hole.
- Cut a piece of sturdy Styrofoam tray (e.g., well-washed meat or produce packaging) to fit the bottom of the pot loosely with space for water to run out.
Some people use Styrofoam packing peanuts for this purpose. However, this can cause annoying problems when it comes time to repot.
Plants quite merrily send their roots growing through and into soft, porous bits of loose Styrofoam, and it can be very difficult to disengage them.