STOP Covering Potted Plants Top Soil with Rocks – Busting the Myths!

There is a disturbing trend emerging where indoor plant enthusiasts are putting rocks on top of potted plants soil. Here are the problems, as we see it:

  • placing rocks on your topsoil goes against the principles of achieving a light and airy indoor plant soil.
  • Rocks can cause heat stress to your plants, depriving them of the necessary moisture.
  • Rocks may look aesthetically pleasing, but cause more long – term damage than good.
Putting Rocks on Top of Potted Plants

Today, at the Garden Bench Top we are going to debunk all the so-called benefits houseplants receive from covering their topsoil with rocks.

We’ll look at why we believe there are better methods for achieving your indoor plant dreams, rather than rocks on top of potted plants.

So if you are ready, assume your debating positions, and let’s get ready to rumble!

What are the problems with putting rocks on top of potted plants?

Let’s first address, what we perceive to be, the main problems with covering your topsoil with heavy rocks.

Heat Retention

rocks retain heat and kill pot plants
credit: Richie Crowley on Medium

In 2017, a paper from ScienceDirect investigated the thermal stability of rocks for energy storage and found rocks are excellent conductors of heat. They also concluded rocks are highly suited for thermal – energy storage.

This is great for conservatism and the future of energy, but not so good news for your pot plants.

If your plant is in a position that receives plenty of sunlight, you could be unknowingly exposing your plant to prolonged periods of heat.

Rock cover has the ability to absorb heat from the sunlight, and slowly release the energy well after the sun has passed. Basically, it has the effect of cooking your plant by reducing humidity and slow roasting your plant by drying it out.

To compound the issue, the constant heat will evaporate the water content in your soil quickly, essentially drying out the only water source for your plant.

Soil Compression

Another problem that can be caused by a layer of rock mulch in your pot plants is soil compression.

Compressed soil can be a serious issue for indoor plants, as it restricts the plants’ roots ability to absorb oxygen from the surrounding soil.

Soil in your outdoor garden has natural aerators, such as worms and insects that help to break down the soil, allowing the necessary oxygen to penetrate the soil and oxygenate the plant roots.

Compressed soil restricts a plants’ ability to absorb oxygen

However, indoor plants don’t have the luxury of these creepy crawlies. As such, they require a specially crafted soil mix that includes substrates that are naturally aerated and porous, like perlite and vermiculite.

In fact, we are so passionate about airy and light soil we have come up with our own Garden Bench Top Indoor Plant Soil recipe. Make sure you check it out.

You may not notice your soil compressing immediately, but over time, your soil will become compacted. Side note – if you do notice compression of your soil straightaway, your rocks are much too heavy! And you should remove them immediately.

Dense and compacted soil results in less oxygen reaching the roots of your houseplants, which eventually leads to your plant suffocating.

Debunking the Benefits of Covering Topsoil in Potted Plants with Rocks

If the above problems weren’t enough to discourage you from putting rocks on top of potted plants, then we are going to debunk the ‘benefits’ others would have you believe about rock mulch for indoor plants.

Fungus Gnats Prevention

Fungus gnats are a pain. They love to fall into your tea, and always seem to buzz around your face at the most inconvenient times.

Fungus gnats seek out moist soil to lay their egg, which is why you see a lot of them hovering around your indoor plants.

And people believe putting rocks to cover the topsoil of your plants will prevent the fungus gnats from laying their eggs. But, in reality, fungus gnats are determined little insects. Rocks and pebbles are just too big, and offer gaps and enough space for the fungus gnats to still reach the soil.

Instead of rocks and pebbles, we offer other solutions to getting rid of fungus gnats, such as covering your soil with fine sand. Using sand offers the same protection that rocks and pebbles do, without the gaps. It also has the added benefit of preventing the gnat larvae from surfacing.

Reducing Water Loss through Evaporation

rock mulching reduces water evaporation
Credit: Unsplash

Mulching is an effective technique used by gardeners to retain water in your soil for your plants. For the benefit of those just beginning their green thumb adventures, mulching involves building a layer of materials on top of your soil to reduce water evaporation.

Rocks are a popular option to use as mulch, and it is a practice we highly encourage – for your outdoor garden and plants.

When it comes to indoor plants, however, we believe the above-mentioned problems caused by rock mulch, far outweigh the benefits of any reduction in water loss through evaporation.

It would be much more efficient to use substrate in your indoor plant soil that possesses water retention properties (like perlite and vermiculite).

Plus, some indoor plants, like succulents, require a soil that is dry to prevent problems like root rot.

Makes your Plants Look Good

This is probably the only benefit that we can agree on.

Some decorative rocks offer great contrast against the leaves of potted plants and can make them really pop. For example, light colored rocks paired with succulents really do make the florets stand out, when compared with regular potting soil.

That being said, we still believe there are other ways to make your indoor plants stand out. Instead of putting rocks on top of potted plants, try injecting some contrasting colors and textures with different styles of pots Or get creative with the way you choose to grow your plants, like hanging baskets or trained vines along walls.

Or maybe try pairing different varieties of plants together to bring varying textures and colors of leaves and branches.

There are an infinite number of ways to make your plants interesting, other than putting rocks on the soil which comes with associated risks.

Prevents Splashing and Soil Loss when Watering

To be honest, we’re not entirely sure about the accuracy of this benefit.

In fact, in our experience, we found rocks on top of the soil increased splashing. We have lost count of the number of times water has ended up on the wall or kitchen bench instead of in the plant pot. All because the water hit a rock at the wrong angle.

We do admit rocks do help to prevent soil loss or displacement while watering your pot plants. However, we have adjusted our watering technique to use less powerful water cans. We tend to favor spouts that have several smaller outlets and create a shower effect while watering (as opposed to one powerful thick stream of water).

You also have the option of using a technique that is called bottom watering. We love using this method for plants that have sensitive leaves and do not like to get wet, like African violets.

As the name of the technique sounds, the goal is to water your plant from the bottom up. No – this doesn’t mean turning your plant upside down or installing some fancy plumbing into your planters.

Watering from the bottom is easy:

  1. Use the finger soil test method described HERE in our article to test the soils’ moisture levels.
  2. When your plants need a top-up of water, find a large container that will comfortably fit your planter, and fill it halfway with distilled water.
  3. Place your indoor plant into the water and leave it for 10 minutes.
  4. Check that the soil has absorbed enough water (if the top is still dry, leave for a further 10 minutes and repeat the soil moisture test).
  5. When your plant has absorbed enough water, pull it up slowly out of the water and allow the excess water to drain away.

For a visual lesson on watering from the bottom up, check out Planterina’s video tutorial:

Less Weeds

Yes, it is true, rock mulch is an effective form of control for preventing weeds from growing.

We even use rocks to keep those pesky weeds at bay. But, we only do so outdoors in our backyards and gardens.

If your houseplants have an uncontrollable weed problem, we would recommend trying a bit of more proactive maintenance with your plant inspections. Pull out any weeds that appear in your pots immediately. Regularly inspect the top layer of soil of your plants and remove any new shoots. After a few weeks of consistent weeding, your problem should be resolved.

If, however, you are time poor and don’t have the necessary headspace for consistent weeding, try changing out your potting mix with fresh indoor potting soil. This will get rid of the seeds and hidden weeds, leaving you to enjoy some low – fuss indoor plants.

Frequently Asked Questions

In this section we tackle all those questions that seem to come while you are researching particular topics.

On a side note – if we don’t answer your question below, please reach out to us via our contact page and we;’ll be sure to respond as soon as we can. We’ll even feature your question in this FAQ section so other fellow gardeners can benefit.

Can I Mix Rocks into my Pot Plants Soil?

In general, we do not recommend mixing rocks into your indoor plants’ soil.

It may seem like the logical thing to do, especially with us continually talking about aerated soil. However, it actually works to your plants’ detriment.

Rocks are hard, and do not have the ability to absorb any water or nutrients. In fact, all they do is take up space in your planter, which could be otherwise taken up with useful materials like perlite and pumice.

How can I Improve Drainage for my Indoor Plants?

Drainage is a key component in your indoor plants’ soil. Getting this aspect right is something that is often overlooked by beginner gardeners. However, if you do take it seriously, it can save you a whole lot of problems and headaches in the future.

To achieve a good amount of drainage for your houseplants, you need to choose the right indoor plant soil. One that consists of the right mediums and substrates to retain enough moisture for your plant, but also allow any excess water to drain away. We recommend using a combination of peat moss, coconut coir, perlite and vermiculite to keep your plants well hydrated and root rot free.