When you look at the soil your houseplant came in or a nursery-made mix just for houseplants, you may notice small white rocks that are very light and easy to crush. These dots are not a fertilizer or signs of a growth problem – that’s perlite.
Perlite is talked about a lot in the houseplant community. You may be wondering if it’s an important part of houseplant soil or just a “nice-to-have” extra. Let’s find out.
Perlite has many benefits for houseplants, particularly drainage and aeration, which improve root health. It also doesn’t break down to compact the soil and has a neutral pH. It was a few downsides, mostly minor inconveniences, making it a worthwhile addition to houseplant soil.
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. Even though it can be used for many things, mostly in construction, it has become very useful for container gardening.
In container gardening, perlite is used as a soil amendment. You can also use it in garden beds, but most of the time it is mixed with potting soil and other things to make the perfect soil for the plants you choose.
When caring for houseplants and gardens, you want to provide the best growing environment possible One tip often suggested is topping off the soil surface with a layer of perlite. But is this really beneficial or could it cause problems? Let’s take an in-depth look at the pros and cons of using perlite as a soil top dressing
What is Perlite?
First let’s start with what exactly perlite is. Perlite is a type of volcanic glass that has been heated rapidly to cause it to expand into lightweight porous particles. It’s whitish-gray in color and looks somewhat like Styrofoam beads.
Perlite is commonly used as an amendment mixed directly into potting soils and garden soils. It improves drainage and aeration. Mixing perlite into soil mixtures is generally recommended, but spreading it on top of the soil is a different story.
Why Do People Put Perlite on Top of Soil?
There are a few reasons gardeners and houseplant enthusiasts have taken to topping off their potted plants and garden beds with perlite:
-
It acts as a reflective mulch to increase light exposure. The light color bounces sunlight back up to the plant leaves.
-
It creates a dry top layer that discourages fungus gnats from laying eggs in damp soil. Fungus gnats are a common houseplant pest.
-
It provides an insulating layer to retain moisture and prevent rapid evaporation from the soil surface.
-
It gives a clean, finished look to potted plants.
-
When wet, it turns a darker color which can indicate when watering is needed.
These potential benefits make using perlite as a top dressing seem appealing. But how well does it actually work for these goals?
Assessing the Pros of Perlite as a Topping Mulch
Let’s examine the positives of using perlite on top of soil and see if the claims hold up:
-
Reflective mulch – Perlite will indeed bounce back more sunlight than dark soil or potting mix. However, other lightweight options like sand or gravel also provide reflectivity. And perlite loses this ability as it becomes dirty and algae-coated over time.
-
Discouraging fungus gnats – A dry perlite layer provides no habitat for fungus gnat larvae, so it may help reduce their numbers. But moisture still penetrates from underneath, allowing some eggs and larvae to survive. It’s not a complete control solution.
-
Moisture retention – The perlite layer does insulate and slow down evaporation from the soil surface. But if you water deeply and allow pots to dry out between waterings, this thin mulch provides minimal benefit.
-
Visual appeal – Fresh, clean perlite does give a tidy, finished look to potted plants. But its pristine appearance diminishes over time.
-
Watering indicator – The color change does signal when the soil is becoming dry. However, checking soil moisture by touch or weight is just as effective.
The positives exist, but perlite has limited effectiveness for these purposes. There are likely better options available. It’s more important to assess the drawbacks.
Looking at the Downsides of Top-Dressing with Perlite
While using perlite as a soil topping offers some benefits, it also comes with these disadvantages:
-
It floats up and splashes when watering, making a messy clean-up required after each watering.
-
Wind or brushing against plants also scatters the lightweight particles around easily.
-
After watering or rainfall, the perlite mixes with the soil, leaving little protective layer over time.
-
Without rinsing or replacement, the perlite develops a green or brown tint from algae and dirt, losing its clean appearance.
-
It provides perfect habitat for algae and liverworts to grow if moisture penetrates from below.
-
Perlite contains no nutrients to support plant growth or feed soil microbes like organic mulches.
-
It must be reapplied routinely to maintain an effective layer after disturbance or decomposition.
-
If perlite gets mixed into the soil, it can alter the soil structure and drainage.
These negative points mean maintaining a perlite top dressing requires vigilance. The benefits are temporary without repeated reapplication.
What Are Some Better Alternatives to Perlite Topping?
Rather than regularly reapplying perlite, you can explore other more effective options:
-
Choose a mulch like bark, leaves, straw, or hulls that enriches soil as it decomposes over time.
-
Use an organic compost top dressing to feed plants and beneficial microbes while retaining moisture.
-
Try a sand topper for reflective properties without blowing away.
-
Allow established mosses to grow to create a living soil cover.
-
Use systemic mosquito bits or other biological controls in the soil to prevent fungus gnats without a top layer.
-
Improve watering practices like bottom-watering plants to leave the surface dry and unattractive to gnats.
-
Rely on sticky yellow traps to catch adult fungus gnats flying around rather than trying to disrupt larvae habitat.
These methods provide ecosystem benefits and often prove more sustainable long-term solutions.
When Perlite Topping May Work Best
While perlite as a top dressing has some disadvantages, there are a few situations where it could be useful:
-
In a controlled indoor growing situation with no wind or rain to displace it.
-
When growing seedlings or cuttings that need moisture retention. The layer can be removed later.
-
For houseplants prone to fungus gnats and soil mold if you stay on top of reapplying it.
-
As a temporary topper during the initial transition of aquatic plants to growing emersed.
-
For plants growing in open terrariums or Wardian cases where appearance is important.
So it can have niche applications, but use with consideration of the ongoing maintenance required.
How to Apply Perlite Top Dressing
If trying perlite as a soil topper, follow these tips:
-
Use horticultural or coarse perlite grades. Avoid super fine powdery versions.
-
Rinse the perlite first to remove any dust and prevent white residue on plants.
-
Apply no more than a 1⁄4 to 1⁄2 inch deep layer. Too much can inhibit water and air penetration.
-
Watch for any sign of reduced growth which could indicate excessive perlite is preventing moisture from wicking upwards.
-
Replenish as needed after watering or disturbances displaces the layer. White perlite against dark soil is most effective.
-
Consider mixing the spent perlite back into pots when repotting to recycle it rather than sending it to the landfill.
The hassle of maintaining the top dressing may outweigh the benefits of perlite for most situations. But it can serve a purpose in the right application.
While using perlite as a soil topper does offer some benefits like moisture retention and fungus gnat control, the disadvantages often make it high maintenance and temporary. Consider alternative options like mulching that enrich the soil and require less ongoing care. But for some niche uses like seedlings and houseplants, a perlite top layer may be helpful if properly managed. Get to know the pros and cons so you can make the right choice for your specific growing conditions and needs. With good information, you can determine whether perlite topping will benefit your plants or not.
Should You Add Perlite to Houseplant Soil?
There are some very minor downsides to using perlite. For example, there is a lot of dust, which is bad for your lungs, so if you have trouble breathing, it’s best to wear a mask. 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. It’s possible that we will run out in the future because mined resources can’t be quickly replaced. Right now, there aren’t any shortages. This is always a factor to consider!.
There aren’t many bad things about adding perlite to houseplant soil, and there are a lot of good things. A little extra perlite won’t hurt and can only make your plants healthier. The amount will depend on what else is in the soil mix.
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.
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 it when their roots stay wet for long periods of time. They need the soil to drain any extra water quickly. If the moisture hangs around the roots too long, the plant will likely develop root rot.
The drainage holes are very important because the water needs a place to go. But the levels of drainage in the soil are also very 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.
You must choose the right container for your plant because perlite can’t make up for containers that don’t have drainage holes. However, it is a great way to improve regular potting soil that 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.
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 have a hard time working and can’t get water and food to the parts of the plant that need them.
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.
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.
Soil mixes still need organic matter, but adding inorganic matter can make the whole mix break down more slowly. 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.
The effect on pH is another benefit that doesn’t have a big effect on houseplants but is still good to know about.
Some houseplants are picky about the pH of the soil they grow in. Many of them like 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 change the pH levels of the soil, so you don’t have to worry about that when you’re mixing your own soil.
Most houseplants don’t mind when the pH of the soil changes a little, and they can do well in soils that are a little outside of their ideal pH range. But for those plants that are fussy, perlite is a great help.
5 quick tips Perlite
FAQ
Should I put perlite on top of soil?
How to add perlite to existing soil?
Can you put perlite on top of soil for gnats?
How much perlite to add to top soil?
Should you add perlite to your garden’s soil?
Adding perlite to your garden’s soil can keep your plants happy in several ways. Besides improving soil drainage and aeration, “it helps to prevent soil compaction, allowing roots to grow freely and access oxygen more efficiently,” says O’Neill.
How do you use perlite in a potting mix?
By promoting better root structure, perlite indirectly influences the plant’s ability to uptake water and nutrients. Incorporating perlite into your soil mix is simple. A common ratio for potting mixes is one part perlite to two parts soil, but this can be adjusted depending on the needs of specific plants or the existing soil structure.
Can you put perlite in compost?
Because perlite helps with drainage and water retention, it’s a great addition to soil or compost used for houseplants — especially succulents and others that are moisture-sensitive. Before planting anything, mix perlite with compost at a ratio of around 1:4, then plant away.
Does perlite improve soil structure?
In the gardening world, perlite is used to improve soil structure by providing drainage and aeration. “Air for the soil” is what I like to call it, and it’s the next best thing to compost for circulating air between the roots for strong, healthy plant growth.