Light
Cacti require four to six hours of bright sunlight daily. However, some species of both desert and forest cactus can burn in direct sunlight. Locate your cactus near a sunny window, and opt for a spot that receives filtered bright light in the summer and direct light (like a south- or west-facing window) in the winter. You can move your cactus outdoors during the summer to provide ample light requirements, but do so only when night temperatures warm to 50 degrees Fahrenheit or above.
Soil
Desert cactus grows best in a fast-draining soil mix designed specifically formulated for cacti. They also grow well in regular potting soil amended with sand, pebbles, or perlite to increase drainage and aeration. Forest cactus likes well-draining soil, as well, but can often grow just fine in a regular potting mix.
Water
During the spring and summer, when your cactus is actively growing and blooming, give it a hearty drink every ten days, allowing the water to drain thoroughly. During the winter rest period, decrease waterings to once every four weeks (and every six weeks for some desert species). The soil should be dry to the touch in between summer waterings and mostly dry in the winter.
Temperature and Humidity
Cactus prefers hot temperatures, ranging from 70 to 80 degrees Fahrenheit. In the winter, the plant prefers a cool-down period, with temperatures hovering near 55 degrees Fahrenheit. In its natural habitat, desert cactus is accustomed to very chilly nights and some species can even withstand nights that dip to 35 degrees Fahrenheit. Still, any indoor plant that’s not hardened off needs to be protected from winter drafts. Cactus prefers humidity levels of 40 to 60 percent, which is fairly easy to achieve in most homes. Forest cacti like the air slightly more humid than desert varieties. So if you see your succulent plant wilting, mist it occasionally.
Fertilizer
Cactus can survive some of the harshest conditions on earth. Any effort to fertilize is welcomed, but is not necessary. Some gardeners yield poor results with standard houseplant fertilizers (probably due to the improper ratio of nutrients), so seek out a specialized organic cacti fertilizer that contains more phosphorus than nitrogen. Fertilize your cactus two to three times a year, only during the growing season, and refer to the manufacturer’s recommended amounts. Reduce or eliminate fertilizer during the winter.
Types of Indoor Cactus
Several varieties of cacti—from those that feature traditional spines to those that look like other succulent plants—are suitable for growing indoors. A few favorite types for indoor cultivation include:
Bunny Ear (Opuntia microdasys) cactus is native to Northern Mexico and features stem-like pads that resemble rabbit ears. This variety needs to be handled with care because the pads contain yellow glochids (barbed bristles) that look cottony, but contain numerous spines. Bunny Ears cactus bears white flowers and can grow up to two to three feet tall. One of the most popular varieties of houseplants, Old Lady Cactus (Mammillaria Hahniana) resembles a pincushion, complete with hair and plenty of spines. This cultivar is round in shape, with attractive purple flowers, and can grow up to four inches tall and eight inches wide. Easter Cactus (Rhipsalideae gaertneri) is a forest cactus that features spine-free, segmented stems and bright star-like flowers in white, red, and pink. It’s one of the easiest cacti to grow indoors, and best suited for beginner growers. Star Cactus (Astrophytum asterias) also called sand dollar cactus or sea urchin cactus is a star-shaped mound and produces yellow flowers. This small variety grows one to two inches tall with a diameter of two to six inches and is often found in indoor terrarium gardens.
Pruning
Generally speaking, a cactus doesn’t really need major pruning unless you are trying to control its growth. Removing dead or damaged parts, however, can be done with clean, sharp garden shears. Gardeners typically trim their cactus only to remove new offshoots (or pups) to propagate new plants. When doing so, always wear protective gardening gloves so that you don’t injure yourself in the process.
Propagating Indoor Cactus
If your cactus produces offshoots (or pups), you can use them to propagate other plants. Most pups grow at the base of the plant, sharing nutrients and water from the mother plant, while others form along the stem or on the pads. Harvesting and propagating pups contributes to the health of the mother plant while allowing you to replicate the plant. Here’s how to propagate cactus from offshoots: Your new cactus should develop strong roots in four to six weeks.
How to Grow Indoor Cactus From Seed
Both desert and forest cacti can be grown from seed, but it takes patience. Additionally, you will need to acquire cactus seeds, which can only be collected from the plant if it flowers. Some cacti might never flower indoors, so buying packaged seeds from a nursery might be your only option. Most cactus seeds need to be stratified (fooled into thinking they’ve experienced winter) before planting. This can be done by placing seeds in moistened peat and then storing them in the refrigerator until they crack open (in approximately four to six weeks). After the stratification period, prepare a pot with cactus potting mix and plant the seeds as deep as they are wide. Water them lightly, and then cover the pot with plastic and place it in a bright location out of direct sun. Most cacti will germinate in about three weeks, and then you can remove the plastic covering during the day. In about six months, seedlings should be ready for their own pots.
Potting and Repotting Cactus
Cacti are slow-growing plants that will rarely need repotting. Actually, many species of cacti will bloom better when they are slightly root-bound. Cacti should be repotted at the beginning of the growing season only when they need fresh soil or are suffering from rot. To do so, first, put on a pair of protective gloves. Next, remove your plant from its current pot using a clean trowel to loosen the roots. Fill the bottom of a terra-cotta or clay pot with a fast-draining cactus potting mix. Add your plant, backfilling around the sides and taking care to cover the roots, and lightly water.
Overwintering
An indoor cactus needs special care in the winter, however, that typically means less attention, not more. For starters, make sure to locate your cactus in your sunniest window. Because the winter sun sits low in the sky, this will allow your cactus to thrive without burning. Next, make sure to stop fertilization during this dormancy period, and reduce waterings back to once a month, at most.
Common Pests and Diseases
All types of cactus can suffer from infestations of mealybugs, scales, fungus gnats, and spider mites. Symptoms include shriveled leaves, a mold-like coating, and the appearance of bugs on the stems or in the soil. In most cases, it’s possible to carefully wash pests off using a spray from the sink hose or cotton swabs. Most plant pests have grown resistant to insecticides, and using chemical insecticides indoors should be your last resort. Overwatered cacti can suffer from fungal rot which looks like dark, sunken spots on the stem that eventually turn mushy. Bacterial rot can also cause your cactus to ooze a black liquid. If either of these conditions presents itself, remove the affected areas of the plant and treat it with a diluted hydrogen peroxide solution.
How to Get Indoor Cactus to Bloom
A blooming cactus or succulent is a treat, as most do not bloom indoors. In order to aid in the process, you’ll need to recreate the cactus’ natural habitat, which requires warm daytime temperatures and cool nighttime temperatures of 50 to 55 degrees Fahrenheit. If your ambient house temperature is warmer than that, it might require relocating your plant to a garage or basement at night (something most home gardeners wouldn’t bother doing). In order to flower, cacti also need to receive at least four to six hours of sunlight and the proper amount of water (but not too much!). Perhaps the most important factor for blooms is to let your cactus experience a period of dormancy, when sunlight and water are reduced. It is during this time that some forest cacti (like the Christmas cactus) bloom.
Common Problems with Indoor Cactus
The most common mistake cactus gardeners make is overwatering in the winter. This can lead to rot either at the base of the plant or at the tips where new growth appears. If the rot is advanced, it might be necessary to start a new plant from cuttings or discard the whole mother plant entirely. The use of non-organic fertilizer can also damage your cactus because most chemical fertilizers contain heavy metals that eventually poison the plant. And because cacti have no protective bark or leaves, physical injury is common, causing a bump to lead to an infection. To prevent this, make sure to keep your plant healthy so that it will callus over before it becomes infected.
