Quick Answer
Horticultural pumice is porous volcanic rock that gives cactus mixes drainage, aeration and permanent structure in one material. We use it instead of perlite, which hides mealybugs, floats to the surface and crushes to dust. For cacti, use fine 1-3mm pumice at about 50% in seed mixes and 3-8mm at up to 60% of the mineral fraction for growing on, blended with sieved organic matter such as compost and worm castings. It never breaks down and can be sieved out and reused indefinitely.
If you hang around cactus growers long enough, you will hear one word more than any other: pumice. It is the backbone of nearly every serious mix, the answer to half the problems beginners have, and one of those materials that sounds exotic until you understand what it actually does. This guide covers what pumice is, why we use it in everything, how it compares to the alternatives, and exactly how much to use for cacti in the UK.
What is horticultural pumice?
Pumice is volcanic rock, cooled so quickly during an eruption that gas bubbles were frozen into it. The result is a light, hard stone that is riddled with tiny pores. Those pores are the whole point: they hold air and a little moisture at the same time, which is exactly the combination roots want.
In a pot, pumice does three jobs at once:
- Drainage: water runs through a pumice-rich mix quickly, so roots never sit in a bog.
- Aeration: the gaps and pores keep oxygen at the roots, and oxygen at the roots is what actually prevents rot.
- Structure: unlike organic matter, pumice never breaks down, compacts or slumps. A mix built on pumice behaves the same in year three as it did on day one.
Pumice vs perlite (or: why we do not use perlite)
Perlite gets recommended everywhere because it is cheap, and to be fair it drains fine. We do not use it, and it is not snobbery, it is three practical problems you only discover after living with the stuff:
- It is white. So are mealybugs and their eggs. A pot topped with perlite is a pot where you cannot spot the most common cactus pest until it is a proper infestation. This alone is disqualifying for a nursery.
- It floats. Every watering, the perlite migrates upwards until your carefully blended mix is a white crust on top and dense compost below, the exact opposite of the structure you wanted. It is honestly the worst possible grit component there is, just avoid :-).
- It is fragile. Perlite crushes to dust with handling and repotting, and the dust holds water in all the wrong ways. It is also not very healthy for humans and breathing it in over a long period can cause problems.
Pumice does everything perlite promises, without being a pest camouflage or floating to the top like cheap cereal. It costs a little more and it is worth every penny.
Pumice vs grit, sand and the fancy stuff
- Horticultural grit: the traditional UK choice, and still decent. It drains well but holds no moisture and no air inside the particle itself, it is just solid stone. Heavier pots too, which matters when you have hundreds of them. Grit and pumice blend well together if grit is what you have.
- Sharp sand: useful in small amounts for fine texture in seedling and loph mixes, useless as the main mineral because it packs tight.
- Akadama: lovely stuff from the bonsai world, holds moisture beautifully, but it slowly breaks down over a few years and costs a fortune in the UK. Nice to have, not necessary.
- Molar clay: the budget cult classic. It works and we get the appeal, just check it is pure fired clay with no additives, and expect it to hold more water than pumice. Molar will also break down over time and being Diatomaceous Earth, also not great for human lungs so avoid breathing it in.
- Lava rock (scoria): similar virtues to pumice, heavier and often sharper. Good in blends.
None of these are wrong. Pumice simply does the most jobs at once with the fewest downsides, which is why it is our default. The fine grit option we sell is perfect for all cacti in our experience, a uniform grit profile does wonders for roots and drainage.
The mixes we actually use
Substrate does not need to be complicated. Two mixes cover almost everything we grow:
Seed germination (everything, Trichos and Lophs alike)
- 50% sieved organic (John Innes No.2, or half compost half worm castings)
- 50% grit. fine grit or fine pumice, 1 to 3mm. Horticultural grit, lava rock and molar
Same mix for every cactus we germinate. No special globular recipes, no fuss. Full method in the germination guide.
Growing on (seedlings through to adults)
- 4 parts sieved organic (we like half compost, half worm castings). We still use peat based mixes which leans itself towards a more inorganic environment. If you are using peat free, we recommend 50% organic and 50% inorganic due to coco coir mixes drying out unfavourably fast.
- 4 parts pumice or grit (or a blend of both, whatever floats your fancy really)
- 2 parts sharp sand
Roughly 40% organic to 60% mineral. Trichos are hungry, forgiving plants and this carries them from seedling trays to big columns. For Lophophora we run the same idea, slightly leaner on the organic side. The full reasoning lives in Cactus Substrate 101.
What about worm castings?
Our favourite organic component, and criminally underrated for cacti. Castings feed gently without the nitrogen spike of hot composts, improve texture without turning to sludge, and bring a living biology that potting compost lacks. We swap up to half the organic fraction for castings in most mixes. You can find our worm castings here.
Which grade of pumice?
- 1 to 3mm (fine): the workhorse. Almost everything in a pot gets this. This is the grade we stock in bulk.
- 3 to 8mm (standard)
- Bigger chunks: useful as a drainage layer in very large pots and for top dressing, more decorative than essential.
Pumice FAQ
Do I need to wash it first?
A quick rinse to knock the dust off does no harm, especially for fine grades, but it is not a ritual. We use ours straight from the bag for most potting.
Can I reuse pumice?
Yes, indefinitely, and this is one of its best features. Sieve it out of old substrate, give it a rinse, and it goes round again. It never wears out.
Is pumice the same as lava rock?
Related but not identical. Both are volcanic, pumice is lighter and more porous, lava rock is denser and heavier. Both work, pumice is kinder on your back.
How much pumice for cacti?
As the mineral half of the mix: around 50% for seed mixes and up to 60% of the total for growing on, blended with grit or sand as you like. If in doubt, more mineral, not less. Nobody ever killed a Trichocereus with too much drainage.
The bottom line
Buy once, use forever, spot your mealybugs, and never fish white bits off the top of your pots again. Pumice is not fancy, it is just right. If you are starting from scratch, a bag of pumice, some John Innes No.2 and a bag of worm castings will carry your whole collection.