How often should I water an indoor cactus?

Water thoroughly, then let the soil dry out completely. That is usually every 2 to 4 weeks in spring and summer and only enough to stop shrivelling in winter, which can mean once a month or less.

Where should I put a cactus indoors?

In the sunniest window you have, usually south or west facing. Cacti need abundant light; in a dim spot they grow thin, pale and lopsided.

Why is my cactus turning soft or mushy?

A soft, mushy or blackening base almost always means rot from too much water or poor drainage. Cut away healthy parts above the rot and re-root them in dry gritty mix.

How do I get my cactus to flower?

Give it a cool, bright and dry winter rest, with nights around 7 to 13°C and very little water, then resume watering in spring. Mammillaria, Rebutia, Gymnocalycium and Lobivia flower readily indoors.

Are cacti safe for cats and dogs?

Most true cacti aren't considered poisonous, but spines can injure pets. Some plants sold as cacti, such as euphorbias, have toxic sap, so check the exact plant.

Cactus Care Indoors: Light, Watering and Winter Rest

Cactus care indoors is mostly about doing less. Cacti store water in thick stems and come from places with strong sun and long dry spells, so they need abundant light, gritty soil and deep but rare watering. The University of Minnesota Extension sums it up: they need only modest water and fertiliser, but lots of light. Most indoor cacti die from overwatering or a dark spot, not neglect. This guide covers desert cacti such as Mammillaria, Opuntia and barrel cacti; jungle cacti like the Christmas cactus need different care.

Cactus care at a glance

Need	What works	Avoid
Light	Sunniest window; south or west	North windows, shelves away from glass
Water	Soak, then dry completely	Little sips; wet soil
Soil	Half potting mix, half coarse sand or grit	Peat-heavy compost
Pot	Terracotta with drainage	Pots or dishes without holes
Summer temperature	18–30°C	—
Winter temperature	Cool, about 7–13°C for flowering types	Frost; warm, dark winters with water
Humidity	Dry air (10–30%) is fine	Steamy rooms
Feeding	Once or twice in late spring/summer	Regular rich feeding

Light

Cacti want as much light as you can give them. Put them right against a south- or west-facing window. If you don't have a sunny window, a grow light works: UMN Extension suggests lights 15–30 cm (6 to 12 inches) above the plants for 14 to 16 hours a day. The grow light cost calculator estimates what that costs to run, and grow lights for houseplants explains the options.

Sign	Meaning
Even, compact growth with strong spines	Light is right
Thin, pale, pointed growth at the top	Etiolation: not enough light
Leaning hard towards the window	Not enough light; rotate
Yellow, white or brown patches on the sunny side	Sunburn after a sudden change

Turn cacti a quarter turn every couple of weeks. When moving a plant into stronger sun or outdoors, do it gradually.

Watering

Use the soak and dry method. Water thoroughly until it runs out of the drainage holes, empty the saucer after a few minutes, then let the soil dry out completely. UMN Extension warns that repeated shallow sprinklings often cause distorted growth.

Season	Typical frequency
Spring	Increase gradually as light increases; every 2–3 weeks
Summer	Every 2–4 weeks; outdoors, check more often
Autumn	Every 4 weeks, tapering off
Winter	Only enough to prevent shrivelling; often once a month or less

To check if soil is dry, push a wooden skewer to the bottom of the pot; it should come out clean. Lifting the pot works too: dry pots are much lighter. If in doubt, wait another week; a cactus recovers from thirst far more easily than from rot.

Symptom	Likely cause
Soft, mushy, black or brown base	Rot from overwatering
Shrivelled, wrinkled, dull stem	Too dry for too long, or root damage
Yellowing from the base upwards	Overwatering
Split stem	Large watering after a long drought
Corky brown patches at the base of old plants	Often normal ageing

See overwatered plant signs and root rot treatment.

Soil and pots

UMN Extension suggests a mix of one part potting soil and one part coarse sand. Many growers use pumice, perlite or grit instead of sand. The test: moisten the mix and squeeze it; when you open your hand, it should fall apart. Use the soil mix calculator to work out quantities.

Always use pots with drainage holes. Terracotta dries faster and is heavier, which helps tall cacti stay upright. Avoid the decorative dishes without holes that many cacti are sold in.

Repotting a cactus safely

Repot in spring, every two to four years or when the cactus outgrows its pot.
Protect your hands with thick gloves, or wrap the cactus in a folded strip of newspaper or a towel.
Tip it out and loosen old soil from the roots. Remove any dead or soft roots.
Pot into dry mix one size up, at the same depth as before.
Wait about a week before watering so damaged roots can heal.

The repotting pot size calculator helps pick the next size.

Winter rest and flowering

Many desert cacti flower only after a cool, bright, dry winter. UMN Extension describes it as a combination of good light, dry soil and cool nights. A windowsill in an unheated room or a cool conservatory is ideal.

From late autumn, stop feeding and cut watering right back.
Keep the plant bright and cool, ideally 7–13°C at night.
Water only enough to stop it shrivelling.
In spring, as light increases, start watering again gradually.
Buds often appear from the areoles near the top.

Mammillaria, Gymnocalycium, Lobivia and Rebutia flower readily indoors this way.

About glued-on flowers

The brightly coloured straw flowers glued onto small shop cacti aren't real. Pull them off carefully; the glue can damage the stem.

Feeding

Cacti have low nutrient needs. Feed once or twice in late spring or summer, when they're growing, with a fertiliser higher in phosphorus than nitrogen at half strength, as UMN Extension recommends. The fertilizer dilution calculator helps with dosing.

Summer outdoors

Cacti benefit from a summer outside once nights are warm. Start them in a semi-shaded, sheltered spot and move them gradually into more sun. Avoid the hottest midday sun at first, and check them more often because pots dry out faster outdoors. Bring them in before nights get cold, and check for pests hiding between the spines before they go back on the windowsill.

Propagation

Method	Works for	How
Offsets (pups)	Mammillaria, Echinopsis	Cut or twist off, dry for a few days, pot in gritty mix
Stem cuttings	Opuntia pads, columnar cacti	Cut, let the wound dry until callused, set into slightly moist sand
Seed	Most species	Slow; needs warmth and humidity at first

Water sparingly until roots form. See how to propagate houseplants.

Pests and problems

Problem	Cause	What to do
White cottony tufts	Mealybugs	Dab with alcohol on a cotton swab; see mealybugs on plants
Flat brown bumps	Scale	Remove with alcohol-dipped swab
Wilting with damp soil	Root mealybugs or rot	Unpot and inspect roots
Fine webbing, dull patches	Spider mites	See spider mites on plants
Rot spots	Too much water, too little light	Improve conditions; cut out rot

UMN Extension notes pests are a rare concern for cacti when light and watering are right. See houseplant pests.

Desert vs jungle cacti

	Desert cacti	Jungle cacti (Christmas, Easter cactus)
Native habitat	Arid regions	Tropical forests, growing on trees
Light	Full sun	Bright indirect
Water	Dry out completely	Lightly moist in growth
Spines	Usually many	Few or none

For holiday cacti, see Christmas cactus care. For other fleshy plants, see succulent care indoors and echeveria care.

Pet safety

The spines are the main risk to pets: cats and dogs that bat or bite a cactus can end up with spines in their mouth, paws or eyes. Some popular "cacti" aren't cacti at all, such as the African milk tree and other euphorbias, which have irritating, toxic sap. Check your plant's exact name with the plant toxicity checker, and keep spiny plants out of reach.

Bottom line

Put your cactus in the sunniest window, plant it in gritty soil in a pot with holes, water deeply and then let it dry out completely, and give it a cool, dry winter. Feed rarely, repot every few years in spring, and expect flowers once it gets that winter rest.

Sources: University of Minnesota Extension, growing cacti and succulents; ASPCA Toxic and Non-Toxic Plant List.