Cactus care indoors: desert cacti, forest cacti and how they differ

Light

Desert cacti need the strongest light in the house. Clemson Extension recommends windows facing south or west, where they receive direct sunlight for several hours each day, and adds that artificial light may be necessary to supplement it. In University of Illinois Extension terms that means the direct level of about 1,500 foot-candles, right in front of a south window.

Insufficient light gives weak, misshapen plants, the RHS warns. The giveaway is a narrow, pale, pointed tip on a round cactus, or a column that tapers and leans toward the glass. That section will never fatten again. A summer outdoors helps, but Clemson advises partial shade for the first few weeks so the skin can adjust; a cactus moved straight from a windowsill into full summer sun can scorch.

Water

Clemson calls proper watering the most important factor in growing cacti, and its growing-season advice surprises people. From March to September, when cacti grow rapidly, it says to water them as any other houseplant that prefers to dry slightly between waterings. The RHS agrees: water thoroughly once the surface of the compost feels dry in spring and summer, and do not let it dry out excessively while the plant is growing. A finger pushed into the mix, as the University of Illinois Extension recommends for all houseplants, is the best guide.

The other half of the year is the opposite. In winter, while the plants are dormant, Clemson says to let them get very dry between waterings, just enough to keep them from shriveling, which usually means a light watering every few weeks. The RHS sets the rest from November to the beginning of March, with minimal or no watering. Warmth plus water in winter produces weak growth and rot.

Whatever the season, Clemson's rule is absolute: the plant must never stand in water. Empty saucers and cachepots after every watering. Forest cacti are the exception to all of this. Clemson says rain forest cacti need substantially more water and should never dry out completely in spring and summer, and the RHS notes winter-flowering types such as Christmas cactus need regular watering in winter with a rest in summer.

Soil and pH

Cacti actually prefer a rich soil, Clemson says, as long as it drains freely. Packaged cactus mixes work, or combine equal volumes of peat-based potting soil and very coarse sand or grit. The RHS uses two parts loam-based compost to one part horticultural grit or sharp sand and tops the pot with a layer of grit, which keeps the base of the stem dry. Forest cacti want more organic matter; Clemson suggests two parts peat moss, one part garden soil and one part sand for them.

Humidity

Desert cacti are well adapted to the dry conditions found in homes, according to Clemson, and the RHS says they cope well with the dry air of central heating. Too much humidity can actually harm them, contributing to corky scab. Forest cacti differ: Clemson notes they prefer at least 50 percent humidity but tolerate less.

Temperature

Clemson gives 65 to 85°F (18 to 29°C) as the preferred range during the growing season and 45 to 55°F (7 to 13°C) for dormant cacti in winter. The RHS asks for winter nights of 46 to 50°F (8 to 10°C). Most cacti tolerate normal house temperatures year round, but Clemson notes they are more likely to flower and prosper if kept cool in winter, ideally in a sunny, cool room or close to a window. Keep them off radiators, and do not let wet plants freeze.

Fertilizer

Clemson recommends a low-nitrogen fertilizer such as 5-10-5 once a month from June through September, or a single application of time-release fertilizer in spring. The RHS feeds monthly from April to September with a high-potassium liquid feed, such as a tomato fertilizer, or a specialist cactus food. Do not feed in winter, and do not feed freshly repotted plants, Clemson adds.

Work out the dose for your watering can with the fertilizer dilution calculator →

Repotting

Cacti like to be slightly pot bound and should be repotted only as needed. Clemson names April as the best month, just as summer growth starts, and recommends a pot only slightly larger than the old one, since an oversized container stays moist and leads to rot. Wear heavy leather gloves, or wrap the cactus in several layers of newspaper; the RHS suggests folded newspaper as tongs or an oven glove. Wait several days before watering so damaged roots can heal.

• Roots fill the pot or grow out of the drainage hole
• The cactus is top-heavy and tips its pot over
• Offsets have crowded the surface to the rim

Propagation

Clemson lists seeds, cuttings, grafting and offsets as the ways to multiply cacti. Clump-formers such as many Mammillaria produce pups that can be detached once rooted. Columnar types and branching Opuntia pads root from cuttings: the RHS advises letting the cut end dry and callus, which can take from a day to a week, then setting it in a gritty mix kept only slightly moist.

Seed is slow but rewarding. The RHS sows in spring, leaves small seeds uncovered, keeps the tray at about 70°F (21°C) in bright light out of direct sun, and warns that seedlings can take up to 12 months to reach a handling size.

Methods: Offsets (pups), Stem cuttings, callused before planting, Seed, Grafting · Best season: spring and summer.

Common problems

Clemson identifies too much water as the main problem, with root and stem rots coming from a mix that drains slowly or from watering too often. Aphids, spider mites and mealybugs are the major pests, and scale is harder to control; a heavy infestation may mean discarding the plant. The RHS adds oedema blisters from overwatering, shriveling from underwatering, discolored patches from cold and corky scab from excessive humidity or brightness.

Not sure which one it is? Try the plant problem diagnosis tool →

Is the cactus toxic to cats and dogs?

The true cacti on the ASPCA list, including old man cactus, tree cactus (Opuntia species), thimble cactus, Easter lily cactus and Christmas cactus, are all non-toxic to cats and dogs. The danger is physical: spines, and the fine barbed bristles of prickly pears that Clemson warns can cause more pain than large spines. Watch out for pencil cactus, which is not a cactus at all but Euphorbia tirucalli. The ASPCA lists it as toxic to dogs, cats and horses, with an irritant latex sap that can cause vomiting, though it calls its toxicity generally over-rated.

Check any other plant against the ASPCA lists →