You must have come across some houseplants with rocks on the soil surface. This is especially common among cacti and succulents, giving the pot an aesthetic appeal. However, do rocks serve a different purpose in planters besides aesthetics?
You can put rocks on the soil surface in a planter if you aim to slow down moisture loss through evaporation. However, mixing the rocks with potting soil or placing them at the bottom of the planter can lead to poor root growth and waterlogging.
In the rest of the article, I will explain the science behind the pros and cons of using rocks and planters and how to properly use them. I’ll also provide a list of alternatives that work much better than rocks. Read on!
The Benefits of Adding Rocks to Planters
There are three main reasons why many gardeners put rocks in planters:
- Enhance moisture retention
- Regulate soil temperature
- Improve drainage
Among these three, there’s scientific evidence that supports the first two and disproves the last one.
I’ll explain these in more detail below:
Rocks Enhance Moisture Retention
Placing rocks on top of the soil, right around the base of the plant can slow down moisture loss through evaporation. One important thing to note here, however, is that this benefit works only with porous potting mixes containing sand, which is commonly found in cactus and succulent mixes.
So even if cactus and succulent lovers don’t know the science behind it and have simply acquired the practice from fellow gardeners, there’s actually a benefit to placing rocks on the soil’s surface.
Nonporous rocks don’t absorb and retain water. Instead, the water just passes through the sides of the rocks and goes directly into the soil every time you water your plant.
However, they should be small enough to allow maximum infiltration into the soil and prevent runoff. I recommend using small rocks, pebbles, or gravel, measuring about 1/3 inch (0.85 cm) in diameter.
The main reason rocks enhance moisture retention is due to their high heat capacity of 2000 J/(kg x K). That means it takes 2000 Joules of energy to raise or drop the temperature of a 1-kg (2.2 lb) rock by 1 °C (34 °F). This is about half as much energy it takes to heat up the same volume/weight of water.
Since the rock layer shields the soil, the sun’s heat cannot reach the soil directly. A 1-inch (2.5 cm) layer of rock mulch will allow water to seep through to your plant’s roots and remain in the soil longer than it would without using a rock mulch layer.
Rocks Help Regulate Soil Temperature
By extension, an average rock’s high heat capacity also contributes to its ability to regulate soil temperature.
Even when compared to the smaller size and weight of rocks commonly used on the surface of the soil in planters, it can still take much sun energy and longer exposure to adequately heat the rocks enough to affect the soil.
This is significantly different from horticultural sand, which has a low heat capacity. Due to the smaller surface area of each particle, sand can heat up more quickly, making it suitable for germinating warm-weather vegetable seeds.
Rocks protect plant roots not only from the summer heat but also in winter because it takes as much energy to reduce the temperature of rocks. Nonporous rocks also don’t hold on to water that can freeze and thaw and split the rocks as it expands. Therefore, they can last longer than porous rocks.
In addition, since a layer of rock mulch can keep water in the soil for longer—and water has a higher heat capacity than rocks—the plant’s roots remain safe belowground in winter. Of course, this is even more effective if you use lukewarm water every time you water your plants.
Pro-tip: Smaller rocks like pebbles have a heat capacity of around 1100 J/(kg x K), which is a bit lower than loam, silt, and clay at 1300 J/(kg x K). So, to be clear, adding a rocky layer above the soil in planters for moisture retention and temperature regulation only works for coarse potting mixes like cactus and succulent mixes.
Other Benefits: Aesthetics, Support, and Protection
When properly used, rocks can present these additional benefits to your planter:
Improved Aesthetics
Rocks come in different colors and shapes. They’re also easy to clean, making them a convenient addition to your soil’s surface.
Added Weight to the Planter
Some planters topple over when the plant becomes top-heavy. A 1-inch (2.5 cm) layer of rocks on the soil surface can go a long way in keeping your planter steady.
Protection From Soil Erosion
Not everybody has the time and patience to water potted plants gradually or to switch to a watering can with a narrow spout. Luckily, keeping rocks on the soil will prevent the soil from splashing out of the pot every time you water your plant.
The soil can also remain at a safe depth over the plant’s roots.
Protection Against Pests
Pests like fungus gnats and mites won’t be attracted to the organic matter in your soil because the surface is covered by dry rocks. As a result, they won’t find it attractive to lay eggs in your pots.
Keeping Perlite Dust Within the Pot
Perlite is a common ingredient in cactus and succulent mixes. However, perlite dust may contain crystalline silica, which is carcinogenic to humans and can also cause respiratory problems.
Debunking the Myth that Rocks Improve Drainage
Pots are like a microenvironment that replicates the natural soil environment of houseplants in their native habitat. However, pots have limited space that cannot perfectly mimic the various soil layers present in nature, and they don’t necessarily have to.
When you grow plants in the ground, the roots can only access moisture available in the root zone. The excess moisture can escape deep into the ground, beyond the reach of the roots. The water then pools or gathers at the water table, creating a saturated zone underneath and an unsaturated zone above, where it’s safe for plant roots to grow.
In pots with regular potting mix, a certain amount of water collects at the bottom and doesn’t exit the pot even after the excess water drains out of the drainage holes. This wet region becomes the perched water table.
The amount of water that remains at the bottom of the pot depends on the downward pull from gravitational force and the upward pull from capillary force balancing each other out.
Adding a rocky layer at the bottom of the pot will reduce the soil volume and raise this water table, moving it closer to the root zone. The rocky layer will be dry but the soil layer immediately above it will remain wet for longer, exposing the roots to unwanted extra moisture.
Therefore, you should not put coarse materials at the bottom of your planter if you aim to improve drainage. Contrary to popular belief, rocks or coarse materials at the bottom of pots don’t improve drainage but instead disrupt root growth and increase the risk of root rot.
You shouldn’t mix rocks in the soil either. They can take up much space in the pot, reducing the surface roots can grow into. In addition, rocks don’t hold onto nutrients, so there’s no real value in mixing them with the soil.
Best Practices for Using Rocks in Planters
Now that you know the pros and cons of adding rocks to planters, it’s time to understand the best practices to maximize the benefits and avoid the disadvantages. Here are some things to keep in mind:
Stick to the Surface Level
Place the rocks only on the surface of the soil. A 1-inch (2.5 cm) layer should be enough to serve the purpose of temperature regulation and moisture retention.
Use Nonporous or Smooth Rocks
Porous rocks can hold moisture like a sponge and attract mold and pests. Moreover, porous rocks are susceptible to damage from the winter freeze-and-thaw cycles.
Know When to Use Rocks
Use rocks only over fine sand, cactus mix, or succulent mix. On the other hand, loam, silt, and clay are excellent at holding moisture. Because of the high water content, these soil types can also maintain relatively stable temperatures.
7 Alternatives to Rocks in Pots
Rocks are beneficial to potted plants in certain situations. However, they can cause some issues with drainage, so you may not want to use them in all of your plants. Plenty of alternatives can offer you many of the same benefits for your plants without messing up the drainage in your pots.
It’s essential to know how to add them to your pots or potting soil to maximize their benefits.
For instance, perlite or sand significantly improves drainage when mixed evenly with the potting soil. Similarly, sphagnum moss, coco peat, and vermiculite must be evenly distributed in the potting mix to boost its moisture retention capacity.
Mulching materials like newspaper and wood mulch can be thinly layered on top of the soil. They work similarly to rocks in that they slow down moisture evaporation.
On the other hand, you can put a coffee filter at the bottom of the pot, covering the drainage holes to prevent the soil from leaking out.
Below, I’ll discuss these materials in more detail and explain why they’re excellent alternatives to rocks:
1. Perlite
Perlite is one of the best materials you can add to your potting soil for several reasons. It is a mineral that occurs in the earth naturally and looks like small white flakes. It has plenty of functions and can greatly benefit your plants.
Perlite is absorbent, so it can pull some excess moisture out of the soil. If you accidentally overwater your plants, perlite can help to dry out the dirt faster, preventing root rot. Adding perlite to the ground can create tiny air gaps that benefit aeration.
These air gaps are also suitable for drainage and are much better than using rocks. It’s also healthier for the plant’s roots since it won’t block them from spreading out.
Adding perlite to your potted plants is excellent for helping them grow. The material is perfect for adding more drainage and aeration and keeps your plants healthy. However, it doesn’t add any nutrient content to the soil. It’s only for changing the texture and quality of your potting soil.
2. Sand
Not all plants like sand very much because the material heats up rather quickly, especially outdoors. However, most warm-weather seeds started in pots indoors can use the extra heat to improve their chances of germinating.
A mature, top-heavy potted houseplant can also benefit from the added air pockets and weight the sand brings to the planter, so it doesn’t fall over as easily. You’ll want to choose coarse sand since the fine grains won’t help much with aeration.
3. Vermiculite
New gardeners often confuse vermiculite with perlite, thinking they have the same function. Although similar, they’re not interchangeable. A simple fact to go by when planning your potting mix is that perlite is for drainage, whereas vermiculite is for moisture retention.
Moisture Retention
To explain more clearly, perlite also has a moisture retention capacity, allowing it to hold 3-4 times its weight in moisture. However, vermiculite trumps perlite in this category, being able to hold up to 16 times its weight in water.
Drainage
Vermiculite is porous, providing your potting mix with improved drainage and aeration when mixed evenly into the soil. Despite this, vermiculite can compact in your potting soil over time. In contrast, perlite is more porous and lightweight and does not alter its shape in the potting mix.
Nutrient-Holding Capacity
An added benefit of using vermiculite instead of rocks or perlite is that this mineral is capable of holding on to cations or positively charged ions in the soil, such as potassium, magnesium, and calcium. These nutrients go back to the soil and into the plant’s roots as the vermiculite slowly releases water.
4. Sphagnum Moss or Coco Peat
You might also consider using sphagnum moss when growing acid-loving plants like blueberries or azaleas. Specifically, the Canadian sphagnum peat moss will make the soil more acidic, so not every plant will love it. If your plant prefers an alkaline pH, you may need to avoid using this alternative.
Many gardeners use sphagnum moss as a growing medium, but it also works very well with mature, moisture-loving plants.
This natural material holds water like a sponge and will suck moisture from the soil, which also helps manage drainage. As the soil gradually dries up, the moss slowly releases the moisture back for the plant’s roots to access. It works well for plants that like humidity since it slowly releases moisture into the air.
Sphagnum moss is also extremely fibrous, which helps to keep some air pockets in the soil and allows for ample drainage. It can help the soil retain its structure too. That way, the dirt can’t compress and make it impossible for the plant’s roots to keep growing.
However, there are environmental concerns with the harvest and use of sphagnum moss. The process of harvesting this moss consequently releases large volumes of carbon emissions into the atmosphere, increasing global warming.
As an eco-friendly alternative, you can opt for coco peat because they work similarly by improving moisture retention and drainage. The only difference is that coco peat does not significantly alter soil acidity because it has a pH of around 6.5, whereas pure sphagnum peat moss is between 3.0 and 4.5.
5. Newspaper
Many gardeners use rocks on top of their soil to stop water from evaporating from it so quickly. You might be surprised to learn that newspapers can do the same thing.
Start by cutting the newspaper into a shape that fits your planter. Three layers of newspaper are required to prevent moisture from evaporating from the pot. Topping your soil with too many layers will reduce air circulation.
It’s also suitable for trapping water inside porous pots, like unglazed clay. To do this, you’ll want to line the inside of the planter with newspaper before filling it with soil and adding your plant.
The newspaper will help keep more moisture inside. Plus, it’ll eventually break down and combine with the earth.
Some people may be concerned that the toxic materials present in newspapers can leach into the soil. The good news is that around 90% of newspapers in the US use soy ink, which is eco-friendly and safe for pots. However, I still recommend to avoid using newspaper on edible plants.
6. Wood Mulch
Wood mulch is an excellent alternative to rocks if you’re trying to retain moisture and regulate soil temperature for outdoor potted plants.
As the mulch breaks down, it returns nutrients to the soil that your plants can use to grow. You can set mulch about 2 inches (5 cm) on top of the dirt’s surface to trap moisture, but you don’t want to place it too close to the base of your plants. Many gardeners find that mulch looks nice too.
Compost, sawdust, and straw can also work as excellent alternatives to wood mulch.
7. Coffee Filters
Coffee filters are a good alternative when you have a large drainage hole on your planter and need to stop dirt from flowing out each time you water your plants. It’s better than using rocks at the bottom of the planter because they won’t block the drainage hole.
Water can easily pass through the drainage holes, but your soil won’t escape. It’s much more efficient than using rocks. Plus, you can easily combine this method with others on this list and get excellent results.
Coffee filters are perfect for cleaning your space without lowering drainage in your potted plants. However, they can’t offer you as many benefits as rocks, or other materials, can.
Common Related Questions
Do I Need Rocks in My Planter?
You don’t need rocks in your planter. Although beneficial for plants that grow in sandy soil, rocks are not necessary. Using the right potting mix with a good balance of drainage and moisture retention is enough to keep your plant healthy.
In addition, plants that grow in loam, silt, or clay don’t benefit from the high heat capacity of a rock mulch. These soil types can hold enough moisture and have a better ability than rocks to regulate the temperature around the roots.
What Do You Put at the Bottom of the Planter?
You can put a mesh pad or coffee filter at the bottom of the planter. These materials allow water to pass through and exit the pot while preventing the potting soil from leaking out of the drainage holes every time you water your plants.
You should avoid putting rocks, styrofoam, sand, and any other coarse materials at the bottom of your pot. They don’t improve drainage and instead cause the root zone to be unnecessarily moist for longer.
Key Takeaways
Rocks can be a beneficial addition to planters when used correctly. They can aid in moisture retention and temperature regulation when placed on the soil surface.
Avoid mixing rocks with the potting mix or placing them at the bottom of the pot because they’ll disrupt root development and won’t help with soil drainage. To improve drainage, go for the trusted alternatives like sand and perlite, which you can mix evenly with the soil.
