Perlite

Perlite is one of those quiet workhorses you'll spot in seed and cutting compost without ever thinking much about it. It's the scatter of tiny, bright-white, polystyrene-like beads in a bag of mix. Despite looking artificial, it's a completely natural material: volcanic glass that's been heated until it pops and puffs up, a bit like popcorn, into light, chalky granules full of trapped air.

What it actually does

Perlite earns its place by improving two things at once: drainage and aeration. Each granule is hard, doesn't rot, and holds very little water itself. Stir a handful through compost and it creates lots of little air gaps, so excess water can run away instead of pooling around the roots. Plant roots need air as much as they need moisture — sitting in a cold, waterlogged mix is one of the quickest ways to kill a seedling, often through root rot or damping-off. By keeping the compost open and free-draining, perlite gives roots the oxygen they need and helps them grow strong.

It's worth knowing how perlite differs from vermiculite, the other white-ish additive you'll see. Vermiculite is spongy and holds onto water and nutrients, so it keeps a mix moist. Perlite does the opposite — it lets water through. Many growers use a little of each: vermiculite to retain moisture, perlite to stop things going soggy.

When to add it

Perlite comes into its own in two situations. The first is containers. Pot-grown plants can't drain into the wider soil, so in our damp UK climate they easily become waterlogged after a wet spell. Adding perlite to potting compost keeps the mix airy and free-draining, which matters for tomatoes, courgettes and anything in a grow bag or tub.

The second is cuttings and seeds. Cuttings root best in a very open, well-aerated mix, and perlite is ideal — some gardeners even root cuttings in pure perlite kept just moist. For raising seedlings it lightens heavy compost and reduces the risk of damping-off.

How much to use

There's no need to be precise. A rough guide is one part perlite to three or four parts compost for general potting, or up to half-and-half for cuttings and anything that demands sharp drainage. Tip it into a bucket or trug, mix thoroughly with your hands, and you're ready. One word of warning: perlite is dusty when dry, and the fine dust is best not breathed in. Dampen the bag slightly before you open it, or mix it outdoors on a still day. A single bag goes a long way and stores indefinitely, making it a cheap, reusable staple for any UK container grower.