Cactus & Succulent Mix

Nothing kills a cactus faster than kindness. The instinct to water regularly, to use rich compost, to keep the pot sitting in a tray of moisture — every one of these habits is lethal to a plant that evolved in conditions of extreme mineral poverty, almost zero organic matter, and rainfall events that might arrive twice a year. The cactus and succulent mix exists as a deliberate act of restraint: a growing medium that replicates hostile conditions because hostility, for these plants, is home.

The Physiology of Drought

Cacti and most succulents store water in their cells — in their stems, leaves, and roots. This internal reservoir means they can survive extended dry periods, and it also means their roots are extraordinarily sensitive to sustained moisture. When a cactus root sits in wet soil for more than a few days, the fine root hairs die, the main roots begin to rot, and the rot moves upward through the plant. Because the damage begins underground and is invisible, most growers do not notice until the stem begins to soften or the plant topples over from a rotted root crown.

The only effective prevention is a mix so fast-draining that retained moisture is never an issue. Standard potting compost, even if watered infrequently, holds moisture at the root zone for too long. The mineral content of the cactus mix — grit and perlite together making up seventy per cent of the volume — means that water flows through within minutes, leaving only a brief film of moisture on the mix particles before draining fully away.

What Is in the Mix

Coarse Horticultural Grit (40%) — Angular, sharp grit (not rounded beach sand, which compacts) is the primary drainage agent. The irregular particle shapes create permanent air voids between grains that water cannot fill. This is the component most often omitted in commercial cactus mixes, which are frequently disappointingly soil-heavy. If buying pre-made cactus compost, adding extra grit is almost always worth doing.

Perlite (30%) — Volcanic glass that adds further drainage while being lighter than grit. It is particularly useful in terracotta pots because it slows the drying slightly compared to pure grit — enough to prevent the fine feeder roots from desiccating between waterings while still eliminating the risk of waterlogging.

Peat-Free Compost (20%) — The organic fraction. Cacti grow slowly but they do grow, and some nutritional base is necessary. At twenty per cent, compost provides a slow release of minerals and maintains a loose but workable structure. Any quality peat-free potting mix works here; there is no need for anything specialist.

Fine Gravel (10%) — Optional but useful. A handful of fine aquarium gravel or horticultural grit added to the base of the mix improves drainage at the pot bottom and provides a stable foundation for roots without the risk of the drainage hole becoming blocked by fine mix particles.

Potting and Container Choice

Terracotta is strongly preferred for cacti and succulents. The porous clay wicks moisture from the mix and ensures that the substrate dries thoroughly between waterings. Plastic pots retain moisture far longer and shift the risk toward overwatering. If plastic is the only option, reduce watering frequency accordingly and consider going up only one pot size at a time — a cactus in a large plastic pot with this mix will take weeks to dry out fully.

Pot size matters greatly. Cacti prefer being slightly rootbound — a pot only one to two centimetres wider than the root ball is ideal. Excess soil volume holds moisture that has no roots to absorb it, which creates a persistently damp zone at the pot edge that encourages rot.

When potting, cover the drainage hole with a piece of mesh or a broken shard of terracotta to prevent mix loss without blocking drainage. Fill loosely with the mix, settle with a tap, then top-dress with a shallow layer of grit or gravel. The top-dressing keeps moisture away from the stem base — the most common site of rot — and gives the pot a clean, finished appearance.

Watering in This Mix

Water thoroughly but infrequently. In spring and summer (the active growing season), water when the mix is completely dry — typically every ten to fourteen days in a terracotta pot, longer in plastic. In autumn and winter, reduce to once a month or less. Some species — particularly desert cacti — benefit from a completely dry rest period from October through February.

When you water, water generously. A brief splash does not penetrate this well-draining mix adequately and leads to only the top centimetre becoming moist while the roots lower in the pot stay dry. Water slowly and thoroughly until it flows from the drainage holes, then leave it alone.

Fertilising

Because the compost fraction is modest, cacti in this mix benefit from a dilute cactus fertiliser once a month during the growing season. Use half the recommended dose — these are plants that evolved in nutrient-poor environments, and heavy fertilising produces weak, floppy growth that is more susceptible to pests. A low-nitrogen formulation is preferable; high nitrogen pushes leafy growth that looks lush but compromises the structural integrity of the plant.

What Goes Wrong

Rot at the base — Almost always a watering issue. Check that the drainage hole is unobstructed. If rot is present, remove the plant, cut back to clean tissue, let the wound dry for several days, and repot into fresh dry mix.

Slow or no growth — Normal during winter dormancy. Check for root mealybugs if growth is poor during the growing season: look for white powdery fluff at the root zone when you unpot.

Wrinkled, shrivelled appearance — Underwatering. Give a thorough soak and most plants will plump up within twenty-four hours.

Pale or etiolated growth — Not enough light. No mix adjustment will compensate for insufficient sun. Cacti need a minimum of four to six hours of bright direct light daily.