Perlite
Also known as Expanded perlite, Sponge rock
In short
Perlite is a volcanic glass that is heated until it pops like popcorn into light, white granules. It is added to potting mixes to open them up, improve drainage and stop compaction, and is also used on its own for rooting cuttings.
How it looks
Bright white, very light, irregular granules with a rough, foamy surface, usually a few millimetres across. They crush easily between the fingers and float on water. Raw perlite is a grey volcanic glass; the white form is made by heating it to about 850–900 °C so that trapped water turns to steam and expands it.
Signs
White granules rising to the surface and washing to the top of the pot after watering is common. Fine perlite dust is released when bags are handled dry. Some growers link perlite to fluoride damage on sensitive plants because it can contain fluorine.
What favours it
Chemically it is mostly silicon dioxide (about 70–75%). It is inert and holds little water itself, so it is used to add air rather than moisture.
Prevention
- Dampen perlite before handling, or wear a dust mask and eye protection, to avoid breathing silica dust.
- Rinse perlite or float it in water to wash off dust and salts, especially before use with carnivorous plants.
- Mix it evenly through the pot rather than layering it at the bottom.
Treatment
Start with the first step and move down only if the problem persists.
- If perlite floats to the surface, top-dress with a thin layer of mix or bark; it does not harm the plant.
- Where a heavier mineral is preferred, pumice or rice hulls can replace part of the perlite.
Pros and cons
Pros
- Very light and cheap; the most popular aerating additive in container mixes.
- Inert, stable and sterile when bought new; it does not decompose.
- Improves drainage and prevents compaction.
- Works on its own as a rooting medium for cuttings.
Cons
- Dust contains silica and should not be inhaled.
- Floats and migrates to the top of pots.
- Holds little water or nutrients.
- May contain fluorine that can harm fluoride-sensitive plants, and some batches carry salt.
- Non-renewable: it is mined and needs high-temperature heating to expand.
Look-alikes
- Pumice
- Pumice is heavier, often grey or cream, does not crush as easily and floats less readily once wet.
- Vermiculite
- Vermiculite is golden-brown and flaky and holds much more water.
- Mealybugs or fungal mould
- White perlite granules on the soil surface are hard and gritty; mealybugs are soft insects on the plant and mould is fuzzy.
Typical uses
Almost all container plants; common in houseplant, aroid, succulent and seed mixes and in carnivorous-plant mixes as a sand substitute.
Related
Sources
- Perlite — Wikipedia
- Growing Media — Texas A&M AgriLife Extension, Ornamental Production
- Beginner’s Series — Part 2: Media Mania Revisited — American Orchid Society
- Peat, Sand, Perlite — International Carnivorous Plant Society
- About Pumice: The Ideal Soil Amendment for Succulents — Debra Lee Baldwin
Written in our own words from these sources. Check a product label before using any treatment.