What are perlite and vermiculite actually for?

Both are inert, sterile, lightweight and pH-neutral, which is why they turn up everywhere. The difference is physical. Perlite is a rigid closed structure that traps air between the granules; vermiculite is a soft layered structure that soaks up water like a sponge. Swapping one for the other reverses the effect you were after.

Safety

Step by step

1. Use perlite where you want air and drainage.Cutting mixes, seed mixes for plants that rot easily, succulent and alpine composts, and anything in a pot that must never be waterlogged. A quarter to a third by volume is typical.
2. Use vermiculite where you want moisture held.Covering fine seed, which keeps them damp without excluding light; mixing into seed compost for plants that must not dry out; and rooting cuttings of plants that like it moist.
3. Cover seed with vermiculite rather than compost.A thin sprinkle lets light through, holds moisture around the seed, and does not cap or crust the way compost does. It is particularly good for slow germinators that would otherwise need watching daily.
4. Mix perlite into heavy peat-free composts.Many peat-free mixes hold too much water at the base of a pot. A third of perlite forked through opens the structure and reduces root rot noticeably.
5. Do not use either in the ground.They are container materials. Spread across a border they cost a great deal and disappear into the soil profile with negligible effect. Organic matter is what improves ground soil.
6. Damp the bag before opening it.Both raise a great deal of irritating dust. A splash of water into the bag, shaken, removes the whole problem.
7. Know the alternatives.Sharp horticultural grit and coarse sand do a similar job to perlite, cost less, and add useful weight to a tall pot. Coir and leaf mould hold water like vermiculite. Builders' sand does not substitute for either — its fine rounded grains fill the air spaces rather than creating them.
8. Reuse them.Both are inert and do not break down. Perlite in particular can be sieved out of spent compost and reused indefinitely.

Tips

• If you only buy one, buy perlite — poor drainage kills far more container plants than drying out does.
• Vermiculite over a tray of fine seed is one of the cheapest improvements to germination rates there is.
• Grit at around one part in three does most of what perlite does and stops a tall pot blowing over as well.

Common mistakes

• Using vermiculite to improve drainage — It does the opposite. It holds water, so it makes a wet mix wetter. Perlite or grit is what adds air.
• Using builders' sand as a substitute — Its fine rounded particles pack into the air spaces rather than propping them open, and it often carries salt or lime. Sharp horticultural sand or grit is the right material.
• Pouring dry perlite from a height — It creates a cloud of irritating silica dust. Wet the bag first.

Questions people ask