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Vermiculite

Also known as Exfoliated vermiculite

Growing mediumDepends
Expanded vermiculite flakesPublic domain

In short

Vermiculite is a mica-like mineral that expands into light, accordion-shaped flakes when heated. It holds a lot of water and nutrients, so it is used in seed-starting mixes and to raise water retention in potting mixes.

How it looks

Golden-brown to grey-brown, shiny flakes with a pearly lustre, often concertina-shaped, a few millimetres long. Soft and easily squashed. Heating the raw mineral in a kiln (exfoliation) makes the layered flakes expand.

Signs

Vermiculite-rich mixes stay moist for longer. Plants in mixes with a lot of vermiculite may need less frequent watering.

What favours it

It has a high cation-exchange capacity (about 100–150 meq/100 g), so it holds and slowly releases water, air and fertiliser, and it supplies some potassium and magnesium.

Prevention

  • Buy vermiculite from current, reputable horticultural suppliers; do not reuse old vermiculite from building insulation.
  • Dampen it before use to reduce dust.
  • Use it where extra moisture helps, such as seed starting, and balance it with perlite or bark for plants that need air at the roots.

Treatment

Start with the first step and move down only if the problem persists.

  1. If a vermiculite-heavy mix stays too wet, repot into a mix with more perlite, pumice or bark.

Pros and cons

Pros

  • Very high water-holding capacity.
  • High cation-exchange capacity helps hold and release nutrients.
  • Supplies some potassium and magnesium.
  • Light and sterile when new; good for germinating seeds.

Cons

  • Historical asbestos contamination: the Libby, Montana mine, which once supplied about 80% of world production and closed in 1990, produced vermiculite contaminated with tremolite asbestos; modern mines are tested, but old stocks may pose a risk.
  • Can keep mixes too wet for plants that need sharp drainage.
  • Mined, non-renewable resource.

Look-alikes

Perlite
Perlite is white, rounded and gritty, drains freely and holds little water.

Typical uses

Seed-starting mixes, cuttings and general potting mixes; less suitable in quantity for succulents and cacti.

Sources

  1. Vermiculite — Wikipedia
  2. Growing Media — Texas A&M AgriLife Extension, Ornamental Production

Written in our own words from these sources. Check a product label before using any treatment.