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Repot as the plant wakes in spring, once you can see fresh growth or fresh root tips but before it is growing hard. Roots damaged at that point heal quickly because the plant is about to run. Repotting in autumn or winter leaves cut roots sitting in cold compost with no ability to seal, and that is a common route into rot.
Most cacti want moving every two to four years. Growth in a pot slows long before the plant is genuinely potbound, so use the calendar and the state of the compost rather than waiting for roots at the drainage hole.
| Signal | What it means | Action |
|---|---|---|
| Roots circling out of the drainage hole | Genuinely full | Move up one pot size in spring |
| Compost stays wet for weeks | The mix has broken down | Repot into fresh gritty mix, same pot size |
| White crust over the surface | Salts from feeding and hard water | Replace the top inch, or repot fully |
| Plant wider than the pot and unstable | Top heavy | Move to a wider, heavier pot, not a deeper one |
| No new growth for two full seasons | Exhausted compost or dead roots | Unpot and inspect the roots |
Handling a spiny plant without injury
This is the part that stops people repotting at all, and it is entirely solvable. Gloves are the worst option: fine spines and glochids go straight through fabric and then stay in the glove for the next job.
- Make a newspaper strap Fold several sheets of newspaper into a strip about two inches (5 cm) wide and long enough to wrap right around the plant. Cross the ends behind, hold them like a handle, and lift. This handles anything up to a large globular plant.
- Use foam or tongs for the biggest plants For a heavy columnar cactus, hold it between two thick pieces of upholstery foam, or use kitchen tongs padded with cloth. Take the weight from underneath rather than gripping the body.
- Deal with glochids properly The fine hair-like bristles on some cacti break off in skin invisibly. Do not rub the area. Press adhesive tape over the skin and peel it away, or use tweezers under good light.
- Work on a tray Repot over a shallow tray or a sheet of newspaper, so the spines and grit that come off are collected instead of ending up in the carpet where they are found later with bare feet.
Keep a fine pair of tweezers and a roll of tape with the potting kit rather than looking for them after a spine goes in. Spines left in the skin can turn a small puncture into a swollen and lasting sore.
The mix
The purpose of the mix is speed of drainage and air around the roots, not fertility. Standard peat or coir compost on its own holds far too much water and shrinks away from the pot wall when it eventually dries, so the next watering runs down the gap and never wets the root ball.
A working mix is roughly half mineral grit and half open organic material. Anything from fifty to seventy per cent grit works, with the higher figure for plants that stay in a cool damp room or under glass with poor ventilation.
| Component | Share by volume | Purpose |
|---|---|---|
| Coarse grit or pumice, 2 to 5 mm | About half | Drainage and air. Stops the mix collapsing |
| Loam-based potting mix | About a third | Holds a little water and nutrient, keeps some weight |
| Coir, bark fines or peat-free compost | The remainder | Keeps enough moisture between waterings |
| Grit top dressing, 5 to 10 mm | A layer half an inch (1 cm) deep | Keeps the neck dry, which is where rot starts |
Do not put broken pot pieces in the bottom for drainage. In a small pot they only raise the level at which water sits. A single layer of coarse grit, or nothing at all, is better.
Doing the repot
Work with the plant dry. Compost that has not been watered for a fortnight falls away from the roots cleanly, the root ball holds together, and dry roots break far less than turgid ones.
- Choose the pot Go up one size only, about an inch (2 to 3 cm) more across. A large pot holds a mass of wet compost the small root system cannot use, and that is where rot begins. Clay is more forgiving than plastic in a damp room.
- Ease the plant out Squeeze a plastic pot or tap the rim of a clay one on the edge of a bench. Never pull on the body of the plant. If it is stuck, run a knife around the inside wall first.
- Clean up the roots Crumble away the old compost by hand. Cut back any root that is brown, hollow or smells sour, back to white firm tissue, and look closely for the white waxy specks of root mealybug while you are there.
- Let cut roots dry If you cut anything more than a stray fine root, stand the bare plant somewhere dry and shaded for two to seven days so the cuts seal before it goes into compost.
- Pot at the same depth Hold it so the old soil line sits at the same height, trickle mix around the roots and tap the pot to settle it. Do not press the compost down hard. Finish with the grit top dressing right up to the neck.
- Wait before watering Keep it dry and out of direct sun for seven to ten days. Watering into fresh cuts is the one thing that turns a routine repot into a dead plant.
Afterwards
Expect no visible growth for a few weeks. The plant is building roots before it does anything on top, and a repotted cactus that sits still through most of a season is behaving normally.
Hold off feeding for at least a month. Fresh compost carries enough for that period, and a dilute high-potash feed once a month through the rest of the growing season is all it needs after that.
Write the repotting date and the pot size on a label pushed into the compost. Two years later that label is the only reliable way to know whether a plant is due, because a cactus gives almost no outward sign.