Gardening glossary
Perlite
Perlite is a naturally occurring volcanic glass that contains trapped water. When manufacturers heat it to around 870 degrees Celsius, the water turns to steam and the rock puffs up into the lightweight white granules you see in nearly every bag of potting mix.
What perlite does in soil: it creates tiny air pockets. Roots need oxygen as much as they need water, and dense, peat-heavy mixes can compact over time, suffocating roots and inviting rot. Mixing in perlite at 10-30% by volume keeps the medium loose, drains excess water quickly, and lets roots breathe.
When to lean on perlite: - Succulents, cacti, and snake plants: 30-50% perlite by volume — they hate wet feet. - Aroids like monstera and philodendron: 20-30% perlite with bark and coco coir. - Seed-starting mix: 25% perlite for quick drainage and easy root penetration. - Cuttings: pure perlite or a 50/50 perlite-and-vermiculite mix roots cleanly.
A few practical notes. Perlite is dusty when dry — wet the bag lightly before scooping to avoid breathing in fine particles. It floats, so it tends to migrate to the surface of pots over time; this is cosmetic, not a problem. It is pH-neutral, sterile, and chemically inert, meaning it doesn't add nutrients but also won't shift soil chemistry.
People sometimes confuse perlite with vermiculite. Both are volcanic-origin amendments, but perlite drains while vermiculite holds water. If you want a drier, faster-draining mix (succulents, herbs), choose perlite. If you want moisture retention (seed germination, ferns), choose vermiculite or blend the two.