Should You Add Perlite to Houseplant Soil?

If you look at the soil your houseplant originally came in or a specialized houseplant mix from a nursery, you’ll probably notice small white rocks that are very lightweight and easy to crush. These dots are not a fertilizer or signs of a growth problem – that’s perlite.

With so much talk about the uses of perlite in the houseplant community, you may be wondering whether it’s an essential part of a houseplant soil mix or just a ‘nice-to-have’ addition. Let’s find out.

The Long Answer

Perlite is made from volcanic glass, mined worldwide from Armenia to California. In its original form, it has a relatively high water content. But, as soon as heat is applied, this moisture escapes, causing the material to expand. You can think of it like exploding popcorn kernels.

This expanded volcanic glass is bright white and incredibly light compared to the original material. It is mostly silicon dioxide, with some aluminum oxide and potassium oxide. Although it has several applications – mostly in the construction industry – it has become an invaluable resource in container gardening.

In container gardening, perlite is used as a soil amendment. It can also be used in garden beds but is often mixed with potting soil and other materials to provide the perfect soil conditions for the chosen plants.

What Are the Benefits of Using Perlite?

A material’s composition doesn’t explain why it’s important. Perlite’s primary benefits come from its ability to help manage soil conditions in containers. 

Better Drainage

Arguably the most important benefit of perlite is its impact on drainage.

Drainage is vital when growing in containers, especially indoors. Most plants don’t like their roots to sit in water for long periods and need excess moisture to drain away from the soil quickly. If the moisture hangs around the roots too long, the plant will likely develop root rot.

Drainage holes are a vital part of this process as the water needs somewhere to go, but the drainage levels in the soil are also important. This amendment aids drainage by increasing the space between particles, allowing water to flow freely. It also doesn’t hold onto much moisture, counteracting the effects of other amendments like peat moss or compost.

Perlite can’t compensate for the lack of drainage holes in a container (you must choose the right container for your plant!) Nonetheless, it is a great soil amendment to improve regular potting soil that typically doesn’t drain well enough for indoor plants. It is also vital for plants in low-light areas where moisture evaporation is much slower than usual.

Increased Aeration

The increased spaces between soil particles also improve aeration, which is essential for root health.

Like humans, roots can essentially ‘suffocate’ when deprived of air and oxygen. This occurs when your potting soil has become too dense or compacted due to decomposition, overwatering, or underwatering. Without air, the roots struggle to function and can no longer transport water and nutrients to parts of the plant that need it.

When you add perlite to the soil, the air is able to flow more freely around the roots. These large particles limit any compaction around the roots that can kill them off. As the material is lightweight, it also does this without making the containers too heavy.

No Breakdown

Perlite is an inorganic material that does not break down over time. This is different from other common soil amendments like peat moss. These materials decompose and alter soil health and structure over time, potentially leading to the proliferation of disease.

Although organic matter is still needed in soil mixes, using inorganic material can slow the breakdown of the entire soil mix. This limits the need for repotting due to inadequate soil conditions, keeping your houseplants happier for longer.

Remember that inorganic does not mean that perlite is toxic or made of chemicals. In chemistry, inorganic only means that it doesn’t contain carbon. It is still a suitable material for organic gardening.

No Impact on pH Levels

Another benefit that doesn’t have too much of an impact on houseplants but is still helpful to be aware of is the effect on pH.

Some houseplants are fussy about the soil pH they grow in, with many preferring slightly acidic soil with a pH below 7. Some soil amendments like compost or peat moss can change this pH level when used in large amounts.

But perlite is neutral and doesn’t impact soil pH levels, crossing that off your list of things to worry about when making your own soil mixes.

Most houseplants aren’t too bothered about slight fluctuations in soil pH, adapting well to soils marginally out of their preferred pH range. But for those plants that are fussy, perlite is a great help.

Should You Add Perlite to Houseplant Soil?

There are some very minor downsides to using perlite. For example, it is quite dusty and not great for your lungs, so it’s best to wear a mask if you’re prone to respiratory issues. It also floats in water, with loose particles rising to the top of the pot after watering. Fortunately, thorough mixing should prevent this from becoming a problem.

Perlite is a mined mineral that is subsequently heat-treated. As a result, while we’re not having shortages, it’s possible that we can run out in the future, as mined resources cannot be rapidly replenished. This is always a factor to consider!

Considering the few downsides and many benefits, it’s clear to see that you should add perlite to houseplant soil. The amount will depend on the other components in the soil mix, but a little extra perlite won’t hurt and can only improve the health of your plants.

How Much Perlite Should You Add to Houseplant Soil?

The ideal amount of perlite to use depends on your plant’s needs and the other components in your potting mix. Plants more prone to root rot will benefit from more drainage materials, while those that prefer moist soil can do with less.

The soil mix I use for most houseplants is a combination of two parts potting mix, one part perlite and one part coconut coir to improve moisture retention. For plants more prone to root rot, particularly epiphytic plants, increase the potting soil and perlite ratio to 1:1.

Also, consider the conditions your plant is placed in. Adding more perlite will improve conditions in low-light areas where evaporation is slow. When repotting, you can slightly adjust the amounts based on your plants’ performance to determine the perfect ratio.

It’s best to moisten your perlite before you add it to the mixture to limit issues with dust. Mix it well for even distribution to avoid compacting or uneven drainage throughout the container.

Perlite Alternatives

If you don’t have perlite around but want the same benefits, you can try a few alternatives. However, none of the alternatives quite match up to the real thing, so it’s important to understand the differences between them:

• Pumice: Similar in function but heavier and more costly. Contains more nutrients.
• Vermiculite: Improves aeration but retains more moisture.
• Grit: Improves drainage and aeration but is very heavy, weighing down containers when used in large amounts.
• Sand: Similar in use, ideal for succulent plants that prefer gritty textured soils.
• Rice Hulls: When available, these are a good short-term replacement for perlite, but they decompose in the soil over the season. Rice hulls may also still contain some rice seeds.