Perlite and vermiculite

By Brownie

SOMEONE recently asked me if perlite was a natural product, as it looks a bit like balls of Styrofoam.

Perlite is that snow-white, crispy, brittle rock-like stuff that you find mixed through some potting soils. You can buy it in bags to mix up and use at home. It can also be used in soil-less gardening such as with hydroponics.

Perlite is made by heating a specific type of mineral silica rock (funnily enough, called perlite) to a high temperature where it expands, like popcorn, by up to 20 times its volume.

The high silica content classifies the product as a form of natural glass. This means that the final expanded perlite has a neutral pH, is sterile and is chemically inert, so it won’t adversely affect the plants that grow in it.

This is perfect for trying to strike cuttings, as the cut ends won’t pick up fungus and moulds from any potting soils. The free-draining nature of the mix also means that cuttings aren’t sitting in water and going rotten.

Vermiculite is a similar product and, like perlite, it too comes from a natural source.

Vermiculite is the mineral name given to an ore-like rock that is similar in appearance to mica.

Mined in Australia, vermiculite is heated and expands just like perlite except vermiculite forms a light, flaky material that can range in colour from pale tan to dark brown.

Vermiculite has a higher water holding ability than perlite, so it isn’t usually solely used as a growing medium, as the plant roots may become water-logged.

It is mainly used as a soil mix, as a topping layer over seeds or as a clay soil improver.

Some gardeners like to mix the two together, in a ratio of one part vermiculite to three parts perlite, to create a rather good growing medium for striking cuttings and for hydroponic growing. You can also add this ratio to a good potting mixture to provide a good free-draining soil for container growing.

Main image: Vermiculite (left) and Perlite (right)