Light comes first
Everything else on this page assumes the plant gets enough light. A desert cactus wants the brightest window in the house, ideally south-facing, with several hours of direct sun. A plant kept in dim light stays alive for a surprisingly long time, but it stops being a cactus in shape: new growth comes out pale and narrow, and the thin new section never thickens again.
If a plant has spent the winter indoors, move it into strong sun gradually over two or three weeks. Cactus skin sunburns, and the scar is permanent — a bleached patch that later turns corky and brown.
Watering by season
The rule that matters is not how often, but how completely. When you water, water until it runs out of the drainage hole, then do not water again until the soil is dry all the way through. A splash every few days keeps the top wet and the roots thirsty, which is the worst of both worlds.
| Season | Roughly how often | What the plant is doing |
|---|---|---|
| Spring | Every 10–14 days | Waking up; growth restarts, roots take water again |
| Summer | Every 7–10 days | Active growth; a pot in full sun can dry in under a week |
| Autumn | Every 3–4 weeks, tapering | Slowing down; cut back as light drops |
| Winter | Once a month or not at all | Dormant — see below |
Treat the table as a starting point, not a schedule. Pot size, soil mix, window, and how hot your rooms run all shift it. The reliable test is weight: lift the pot after watering and again when you think it is dry. You will learn the difference in a month, and it works better than any moisture meter.
Rot is the one mistake you cannot undo. Overwatering shows up first at the base as a soft, darkened, slightly translucent patch. If you catch it early, unpot the plant, cut above the damage with a clean blade until the flesh is uniformly pale and firm, let the cut dry for a week or two, and re-root the top. Once rot reaches the growing point, the plant is finished.
Soil and pots
Bagged cactus compost is usually still too rich and holds too much water. Cut it heavily with grit: roughly half mineral to half organic is a good default, and for slow desert species — Ariocarpus, Copiapoa, most of the white-spined ones — go further, up to 70 per cent mineral.
- Mineral part: pumice, lava grit, coarse quartz sand or calcined clay, 2–5 mm. Not builder's sand — it packs into concrete.
- Organic part: any peat-free potting compost, sieved to remove the fibrous lumps.
- Top dressing: a centimetre of grit keeps the neck of the plant dry and discourages fungus gnats.
Pots must have drainage holes; no exceptions, whatever the shop says about a layer of stones in the bottom. Unglazed terracotta dries faster and is more forgiving if you tend to overwater; plastic is fine if you are disciplined. Size up by one pot at a time — a small plant in a large pot sits in wet soil for weeks.
Repotting
Every two to three years, in early spring, before growth starts. Let the soil dry out first so the root ball crumbles away instead of tearing. Bare the roots, trim anything black or mushy, and check the neck for mealybugs while you have a clear view of it.
Then the part everyone gets wrong: do not water it in. Repot into dry mix, leave the plant dry for a week or two, and only then water. Cut roots need time to seal; watering straight away is the most common way to rot a plant that was perfectly healthy an hour earlier.
Winter rest
This is what separates a plant that flowers from one that merely survives. From roughly November to March, most desert cacti want to be cold, dry and bright: somewhere around 8–12 °C, with little or no water, and as much light as they can get.
An unheated spare room, an enclosed porch, or a windowsill behind a drawn curtain all work. Kept warm and watered through the winter instead, a cactus keeps growing in weak light, and the result is the pale drawn-out tip you cannot fix later — and, most years, no flowers at all. Nearly every species sets buds in response to that cold dry spell, not in response to feeding.
Feeding
Very little, and only during growth. A low-nitrogen fertiliser — tomato feed at half the recommended strength is the standard cheap answer — once a month from April to August is plenty. Nitrogen-heavy houseplant food produces soft, swollen, pale growth that splits and scars, and makes the plant far more attractive to pests.
The usual problems
Thin, pale new growth
Etiolation: not enough light, often combined with winter watering. The existing narrow section will never thicken. Move the plant to stronger light and accept the scar, or cut back to healthy tissue and regrow.
Brown, hard patches near the base
Usually corking, which is normal ageing — woody tissue forming as the plant matures. It is firm and dry. Rot, by contrast, is soft and wet. Press gently: firm means leave it alone.
White fluff in the ribs or on the roots
Mealybugs. Dab what you can see with a cotton bud dipped in surgical spirit, then check the roots at the next repot — root mealybugs look like specks of flour on the root ball and are far more damaging than the ones you can see.
Fine webbing and rusty, stippled skin
Spider mites, which thrive in hot dry air. They leave permanent scarring on the older skin, so treat quickly, raise humidity around the plant slightly, and keep new arrivals away from the rest of the collection for a few weeks.
Propagation
Offsets are the easy route: twist off a pup in spring, leave it on a dry shelf until the wound has calloused over — a few days for a small one, two weeks for a large one — then set it on dry mix and wait for roots before watering.
Seed is slower and much more interesting. Sow in spring on a sterile mineral mix, keep the pot covered, humid and warm at around 25 °C, and be patient: germination takes days for some genera and months for others. Seedlings stay tiny for their first year and want more water and less sun than adult plants — the usual advice about drought does not apply to them yet.
A short FAQ
Why has it not flowered in years?
Almost always no winter rest, sometimes not enough light, occasionally just age — some species will not flower until they are ten or more.
It has gone soft and wrinkled. Too little water?
Check the roots before you reach for the watering can. A shrivelled plant with wet soil has lost its roots to rot, and more water finishes it off. Wrinkling with bone-dry soil and healthy roots is genuine thirst, and the plant plumps up within a few days of a proper soak.
Can it live in a room without a window?
No. Grow lights work, but a dark corner with occasional daylight does not — that is the single most common reason these plants slowly fail indoors.