What is vermiculite?
Vermiculite is a naturally occurring mineral (magnesium-aluminum-iron silicate) that is mined in various countries around the world, including the USA and South Africa. It gets its name from the Latin 'vermiculare', which means to breed worms, and the English suffix 'ite' which mean mineral or rock. When vermiculite is heated to very high temperatures, it expands into long strands resembling small worms.
Vermiculite is lightweight, non-toxic and sterile and does not deteriorate over time. It has a neutral pH. It's used in the building industry, the car industry and in packaging. For use in horticulture, it's sold as lightweight, absorbent brown-gold flakes.
Vermiculite can absorb three or four times its weight in water and also attracts various plant nutrients such as potassium, magnesium and calcium. These qualities make it useful for propagation and for potting up plants that do best in moist compost, including house plants.
Vermiculite is sold in bags at the garden centre or online. Biological controls are often supplied in a small bag of vermiculite to make them easier to use. It is also used as an anti-caking agent in fertilisers.
How to use vermiculite
Vermiculite helps to aerate soil while simultaneously retaining water and nutrients, which it then releases over time. Vermiculite is therefore useful in seed sowing and propagation. It can also be added to house plant compost. You may find different grades of vermiculite flakes available – use the finest for seed sowing and the larger sizes for potting on, cuttings and house plants.
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Seeds of all sizes can be sown directly into 100 per cent vermiculite. Alternatively, you can use a 50:50 mix of vermiculite and seed sowing compost. You can also sow just using sowing compost, then cover the seed tray with vermiculite after sowing. Covering a seed tray with vermiculite helps to keep the compost moist and also helps to ward off the fungus that causes damping off. It also helps to insulate against temperature and humidity changes in the greenhouse.
Pricking out and cuttings
A mixture that contains around 30 per cent vermiculite is ideal for pricking out and potting on seedlings. It's also a good mixture for cuttings.
As vermiculite holds water and then drains like a sponge, it's great for adding to the compost of house plants that always seem to need watering, such as the maidenhair fern (Adiantum raddianum) and peace lily. The plant roots can take up moisture when they need it. As vermiculite takes a long time to dry out, you'll need to water less. Vermiculite also aerates the compost, which helps to stop it compacting or crusting over.
A 50:50 mix of house plant compost and vermiculite is ideal.
Vermiculite or perlite?
Perlite is expanded volcanic rock and looks like polystyrene granules. While it does help retain some water, it's primarily used to aerate compost. It's 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.
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.
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.
Is vermiculite safe to use?
Yes. It's mined around the world according to the strict industry protocols. As with any gardening medium, it's best to wear gloves when handling vermiculite. Store in a cool, dry place away from weedkillers and other garden chemicals.
Many years ago there was an isolated case of a mine where asbestos had developed alongside the vermiculite and contaminated it. These asbestos minerals are called Libby Amphibole Asbestos (LAA). This mine, located in Libby, Montana, USA was closed in 1990.