Should You Use Rocks for Drainage in Pots?

If you’ve been growing plants in pots for any length of time, you’ve probably heard that putting rocks in the bottom of your planters can improve soil drainage and prevent root rot. But does this technique actually work?

Despite the pervasive advice to use gravel, pebbles, pottery shards, and rocks in plant containers to prevent soggy soil, studies have actually proven that this practice doesn’t improve drainage at all. Not only that, but that layer of gravel in your pots may actually hurt your plants by resulting in even soggier soil. Here's why you shouldn't put rocks in the bottom of your containers, plus a few better ways to help plant pots drain well.

Why Rocks for Drainage in Pots Doesn't Work

In theory, adding gravel to pots for drainage seems to make sense. If you’ve ever watched rain water puddling outdoors, you know that water runs through gravel quickly, but it often pools in soil and becomes mud. When applied to potted plants, it’s logical to think that water would move faster through more porous materials, like gravel. But this doesn’t happen if the water first has to percolate through a layer of soil.

Water actually doesn’t move easily between two different materials, like coarse gravel and more finely textured potting mix. When you water plants, water will move through the potting mix just fine, but when it encounters a different layer, like gravel or rocks, it stops moving.

To understand why this occurs, imagine placing a porous sponge on top of some gravel. If you begin to pour water on top of that sponge, the dry sponge will absorb the water, pulling it upwards against gravity. Only after the sponge is completely saturated and it can’t absorb any more water will it allow water to trickle into the gravel beneath it.

In plant pots, potting mix acts like a dry sponge and it pulls water upwards against gravity, causing water to linger just above the gravel line in what is known as a “perched water table.” The potting mix will only release water to the gravel beneath it when it is so waterlogged that it can’t absorb any more moisture. So, if you add gravel or other materials to the base of your pots, it actually causes water to linger around plant roots even longer, increasing the likelihood that root rot will develop.

How to Improve Soil Drainage in Pots

Although gravel placed in the bottom of plant pots won’t improve drainage, there are other things you can do to help ensure your plants don’t sit in soggy soil, including:

Choose the right pots.

A planter with adequate drainage holes is essential if you want to grow healthy plants. Even if planters don’t have holes at the bottom, you can drill your own holes with an electric drill and a masonry bit. If you’re concerned about soil slipping out of the holes and making a mess, cover the drainage holes with a paper coffee filter or a small bit of screen mesh.

Amend your potting mix.

Potting mixes usually contain materials like coarse sand, vermiculite, and perlite, which help them drain properly. However, if you’d like to boost the drainage potential of bagged soils, you can mix more of any of these amendments into your potting mix. Adding extra compost to potting mixes will also improve drainage.

Repot your plants.

Over time, potting mixes become compacted, which can slow down water drainage. So, if you’ve noticed that your pots aren’t draining well, it may be time to repot your plants. In general, most tropical houseplants will benefit from being repotted every 12 to 18 months.

Good Ways to Use Gravel with Potted Plants

While gravel, pebbles, and rocks won’t do much for planter drainage, there are still a few other ways to put rocks to use for your houseplants or container garden.

Make a pebble tray.

Tropical houseplants often struggle indoors due to low humidity levels, but if you have some extra gravel on hand, you can make a simple pebble tray to boost humidity. All you need to do is add some pebbles to a flat tray or plate and then pour some water into the tray so the water level is just below the top of the rocks. Then, place a plant pot on top of the pebble tray. The water in the tray will add moisture to the air around your plant as it evaporates.

Keep critters out of your potted plants.

Cats and squirrels can make a nuisance of themselves by digging in potted plants. Keep these creatures out of your plants by adding a layer of pebbles on top of your potting soil. The rocks will make digging in the soil less pleasant so the animals will go elsewhere to dig.