Potting mix is the care factor people think about least and change most rarely, which is odd given that root rot is the leading cause of houseplant death and the mix is what causes it.
Bagged houseplant compost on its own holds too much water for most of what is sold as a houseplant. It is designed for bedding plants that are watered daily outdoors, not for aroids in a pot indoors.
What soil has to do
Three things at once, and they pull against each other:
- Hold water, so the plant can drink between waterings
- Hold air, because roots need oxygen and drown without it
- Anchor the plant so it stays upright
Most failures come from a mix that does the first well and the second badly.
The ingredients
Coco coir
Coconut fibre. Holds water well, stays open rather than compacting, and is a renewable alternative to peat. The base of most modern mixes.
Peat
Holds water well and is still the base of most bagged compost. It compacts over time and turns water-repellent when it dries out completely. Harvesting it also destroys peat bogs, which is why most growers are moving away from it.
Perlite
Puffed volcanic glass, the white specks. Adds air and drainage without holding water. The cheapest and most useful thing you can add to a bag of compost.
Orchid bark
Chunky pine bark. Creates large air pockets, which is exactly what aroid roots want. Breaks down over a year or two, which is part of why mixes need refreshing.
Pumice
Like perlite but heavier, so it does not float to the top over time. Better for tall plants that need ballast.
Worm castings
A mild slow-release nutrient source. A handful improves most mixes. Not a drainage ingredient.
Horticultural charcoal
Keeps a mix sweet and absorbs impurities. Optional, and it does less than its reputation suggests.
Three mixes that cover almost everything
Aroid mix
Equal parts orchid bark, perlite and coco coir.
For Monstera, Philodendron, Alocasia, Anthurium, Syngonium, Scindapsus, Epipremnum and anything else epiphytic or semi-epiphytic. These plants grow on and around trees, not in dense soil, and they rot in compost.
General houseplant mix
Two parts potting compost to one part perlite.
For Spider Plants, Pilea, Peperomia, Tradescantia, Dracaena, Ferns, Calathea and most of what is not an aroid or a succulent. Add extra coir for the moisture-lovers like Calathea and ferns.
Cactus and succulent mix
One part potting compost to two parts grit, coarse sand or pumice.
For Jade, Ponytail Palm, ZZ Plant, Snake Plant and every succulent. Far more open than people expect, and that openness is the point.
Drainage holes are not optional
A decorative pot with no hole will kill almost any houseplant, however careful you are. Water collects at the bottom where you cannot see it, and the roots sitting in it die.
The usual workaround is to keep the plant in a plastic nursery pot and drop that inside the decorative one, lifting it out to water. That works well, provided you actually empty the outer pot afterwards.
A layer of gravel at the bottom of a pot does not improve drainage. It raises the saturated zone rather than lowering it, so it makes the problem slightly worse.
When to refresh it
Potting mix breaks down. Bark and coir decompose, the structure collapses, and a mix that drained freely two years ago holds water like a sponge.
Signs it is time:
- Water sits on the surface instead of soaking in
- The soil stays wet for far longer than it used to
- The mix has shrunk away from the sides of the pot
- White crust on the surface, which is fertiliser salt build-up
Refresh every two years for fast growers, three for slow ones. You do not always need a bigger pot, just fresh mix.