What is perlite?
Perlite is a lightweight granular material that's white in colour. It looks and feels like little bits of polystyrene but is actually made from expanded volcanic glass, heated to 1000°C until it ‘pops’ (like popcorn) to many times its original size. It's lightweight, sterile, and easy to handle, and is long-lasting. It's neither alkaline nor acidic.
Perlite's expanded nature makes it extremely porous, so it can absorb water, but it also improves drainage, so is ideal to mix into compost to ensure water drains freely. Perlite is particularly useful in plant propagation, including taking cuttings and sowing seeds. Other materials that perform similar functions are vermiculite, grit and sharp sand.
How to use perlite
Perlite helps aid drainage at the same time as retain water. Therefore you can use it in a number of ways:
Use perlite to aid drainage
Succulents and other plants sensitive to moist soil benefit from having perlite added to the potting compost. The perlite will trap air in the compost and encourage water to drain through, ensuring the plant's roots will never sit in damp soil. Mix perlite with compost at a ratio of around 1:4. Do this before planting to ensure an even mix.
Use perlite when taking cuttings
Perlite can aid water retention in compost, which can help improve the chances of your cuttings taking root. To do this you need a compost that's specifically designed for cuttings (with a finer texture and lower nutrient content than general use multi-purpose compost). Mix in perlite at a ratio of 50:50, fill the pots, water thoroughly and then allow the compost mix to drain for several hours before inserting your cuttings.
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Cuttings can also be rooted in perlite on its own. Moisten the perlite and fill a polythene bag around a third full. Prepare softwood or semi-ripe cuttings by cutting just below a leaf joint and removing the leaves on the lower half to two-thirds of the cutting. Insert the bare lower part of the shoot into the perlite, fill the bag with air and seal the top. After several weeks, roots should start to form. Once roots are well developed, the cutting can be taken out and potted up in compost.
Use perlite in seed sowing
Mix perlite into seed and cuttings compost at a ratio of 50:50 to create a moist environment perfect for delicate seedling roots. A fine layer of perlite is also excellent for covering seed that needs light to germinate because the perlite lets light through whilst still keeping the seed moist and aiding germination.
Perlite or vermiculite?
Vermiculite is a naturally occurring mineral that is also heated to very high temperatures to make it expand. Sold as bags of brown-gold flakes at the garden centre, it can absorb up to four times its weight in water.
Vermiculite is better for water and nutrient retention than perlite and is best used for plants that need more moisture to grow. Vermiculite also protects seedlings against damping-off and other fungal diseases.
While it does also help retain some water, perlite is primarily used to aerate compost. It excellent for creating a free-draining potting compost for plants that need good drainage, such as cacti and succulents. It can also help create an airy compost for seedlings.
You can use vermiculite and perlite together – mixing a little perlite into a seed sowing mix will ensure plenty of oxygen gets to the roots, while a topping of vermiculite will lock the moisture in.
Taking care when using perlite
Perlite is dusty, so take care to avoid inhaling the dust. Dampen with water before using it. If using a whole bag of perlite in one go, pour a couple of litres of water into the bag, seal the top and shake it, then let it soak for quarter of an hour before using.