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Pumice

Also known as Pumice stone

Growing mediumDepends
Pumice stones, one cut to show the bubbly interior Photo: Mauro Cateb · CC BY-SA 3.0

In short

Pumice is a light, frothy volcanic rock full of gas bubbles. Crushed into small grains it is a popular, long-lasting mineral additive for succulents, cacti and bonsai because it drains freely yet holds some water in its pores.

How it looks

Light-coloured grains or stones, usually white, cream or grey, sometimes greenish-brown or darker, with a rough, bubbly texture. Porosity can be 64–85% by volume. Dry pumice is light enough to float, though it eventually waterlogs and sinks. Horticultural grades are commonly about 3–10 mm (1/8 to 3/8 inch).

Signs

Mixes with enough pumice drain quickly and dry evenly; too little leaves a heavy, slow-drying mix. Because it is mineral, it does not break down, so a mix rich in pumice keeps its structure between repottings.

What favours it

Pumice forms when gas-rich lava is blasted from a volcano and cools quickly, freezing the bubbles in place. It is chemically about pH-neutral.

Prevention

  • Rinse dusty pumice before use.
  • Use grain sizes suited to the pot: finer grains for small containers, larger grains for beds.
  • Match the proportion to the plant: more pumice for cacti and other water-storing succulents, less for thin-leaved plants.

Treatment

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

  1. To speed drying in a heavy succulent mix, repot into a mix with a larger share of pumice or similar grit.

Pros and cons

Pros

  • Does not decompose or compact.
  • Heavier than perlite, so it is less likely to float to the surface.
  • Holds some water in its pores; one succulent grower reports it staying moist about twice as long as bark.
  • pH-neutral and suitable for many plants.

Cons

  • Mined, non-renewable material that can be hard to find or expensive in regions without local deposits.
  • Heavier than perlite, adding weight to large pots.
  • Holds no nutrients of its own, so plants need feeding.

Look-alikes

Perlite
Perlite is bright white, lighter and crumbles between the fingers.
Lava rock (scoria)
Scoria is dark red, brown or black, heavier and sinks in water.

Typical uses

Cacti, succulents, bonsai and other plants that need fast drainage; also used as a general mix additive.

Sources

  1. Pumice — Wikipedia
  2. About Pumice: The Ideal Soil Amendment for Succulents — Debra Lee Baldwin
  3. Bonsai cultivation and care — Wikipedia

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