Background

Cacti belong to an extensive group of tough, yet beautiful plants known as succulents, which are unique for their ability to store water in thick, fleshy tissues during periods of rainfall, then later draw upon those reserves in times of drought. Some succulents use plump, bulbous leaves as reservoirs, while others store water in their stems. Cacti are the largest single family in the latter (or stem-storing) group, and they differ from most other succulents in two important respects: First, cacti always lack true leaves, a feature which reduces their overall need for water. Second, they possess areoles, (small, cushion-like indentations on the stems of cacti from which flowers, fruits, hairs and spines protrude).

Broadly speaking, there are two kinds of cacti, desert and forest. Desert cacti account for the vast bulk of known cacti species. Since growing and caring for these two types is quite different, they will be presented separately.

Desert Cactus

Desert cacti range in size from tiny, ground-hugging types no bigger than a few inches high, to towering giants that rise to heights of forty feet and up. Desert cacti also vary widely in form: Some are columnar, others globular, others branching. Nearly all of them, however, have wax or hair-covered stems that are green to blue-green in color. The coating of wax or hair helps to conserve moisture by minimizing the effects of evaporation.

In a natural setting, desert cacti typically receive their full year's allotment of water over the course of a short-lived rainy season, during which they also grow and flower. Blooms appear on mature plants only, and the flowering period is often very brief.

Outdoor Desert Cactus Care

Planting. Desert cacti can be grown as garden plants throughout the Southwestern United States, as well as in other regions where climactic conditions are reasonably conducive to their needs. They prefer a coarse, grainy mix of sand, rock and organic matter, which occurs naturally in the desert Southwest, and can be replicated in other parts of the country by combining rich, organic matter such compost or pre-packaged garden soils  and planting mixes with either washed grit or builder's sand. A raised bed can work well for cacti so long as it is properly constructed and the soil beneath it drains with reasonable efficiency.

Light, Temperature and Humidity. The full range of desert cacti -- and other succulents, for that matter -- can be reliably grown outdoors nearly sunny spot where the climate is arid and winter lows don't dip below 40-50 degrees Fahrenheit. As annual rainfall levels increase and average winter lows decrease, however, significant limitations come into play. Northern gardeners, in particular, will find themselves restricted to only the most cold-hardy types for outdoor planting, such as the prickly pear cactus, the claret cup cactus and Darwin's cactus. Even then, there are no wintertime guarantees if you live north of zone 5 or 6.

Watering. Cacti rarely if ever need supplemental watering when planted outdoors. Watering should be especially in fall and winter months, when plants go dormant, and excessive watering can lead to root and crown rot.

Feeding. Cacti are light feeders and should not be fed a diet rich in nitrogen, as it will make them grow leggy and weak. Feed in the spring and summer months with low-nitrogen plant food especially formulated for cacti, or use all purpose plant foods at ¼ strength. Discontinue feeding in fall and winter.

Indoor Desert Cactus Care

The conditions inside most homes--relatively high heat and very low humidity-are ideal for most  desert cacti. That makes them among the easiet of houseplants to care for, and can be successfully grown indoors by even the most cautious indoor gardeners.

Potting and Repotting. When buying new cacti, look for plants that are healthy, insect-free and disease-free, with plump, well-colored stems  and visible evidence of new growth. Avoid plants that exhibit misshapen growth, have poor coloring (specifically, a dull, dry cast) or seem to be limp or shriveled.

• To remove the cactus from its old pot, take several layers of newspaper and gently wrap them around the cactus. Gently separate the cactus from the pot and shake the loose soil from the roots.
• Select a new pot that is the same size or slightly larger. Cacti prefer to be slightly rootbound, so don't increase the pot size unless necessary. Choose a potting mix specially formulated for the drainage needs of cacti.
• Fill the new pot about ¾ full with potting mix. Carefully position the cactus in the soil and add more potting mix around the roots, firming gently as you go to eliminate air pockets.
• Wait several days before watering. This allows roots time to heal and reduces risk of root disease.

Light, Temperature and Humidity. A west or south-facing window sill is nearly always a preferred choice for desert cacti, owing to their need for bright, direct sunlight. Rotate plants ¼ turn every week or two to avoid plants that lean toward the light. Most cacti will benefit from being allowed to go dormant overwinter. Since this requires temperatures of 45-55 degrees F, you may have to move pots to a cooler spot or even place them in an unheated room or cold frame until the resting season is over. At other times of the year, a room-temperature environment should keep your cacti happy. Avoid drafty conditions, especially cold, wintry drafts.

Watering. In fall and winter, water only enough to prevent the plants from drying out completely, then gradually increase the frequency of watering beginning in early spring. From late spring through end of summer, water thoroughly whenever the potting mix feels dry an inch or so below the surface.

Feeding. Desert cacti should be fed once or twice a month throughout the spring/summer growing season with a fertilizer formulated especially for the low-nitrogen needs of cacti. Never fertilize a plant during its resting period or when its soil is dry.

Forest Cacti

Forest cacti in most instances do not closely resemble their desert-dwelling relatives. Most have either strap-like or segmented, leaf-like stems that branch out from the center of the plant and arch downward in a trailing growth habit. In addition, the forest types used as houseplants tend to flower more abundantly and for longer periods than most desert cacti, producing brightly colored, showy blooms that are always eye-catching and sometimes quite fragrant. Christmas, Easter and Thanksgiving cacti are popular examples of forest cacti.

Forest Cacti Care

Repotting. Re-potting should occur annually just after a plant's bloom cycle has ended, unless the cactus is from the epiphyllum group. Epihyllums flower more freely when slightly potbound, so repot them only when necessary and increase the pot size as minimally as possible when you do.

Light, Temperature and Humidity. Forest cacti enjoy a well-lit situation, but they'd rather not face the sun directly, so a site near an east-facing window sill is generally the best choice. In general, 55 to 70 degrees F. is the optimal temperature range for active forest cacti. When a plant is resting in fall and winter, cooler 50-55 degrees F. temperatures are generally preferred.. They prefer higher levels of humidity than exist in most centrally heated and air conditioned homes, so. Regular misting throughout the fall and winter helps. Plants enjoy spending summers outdoors in warm, humid climates.

Water. Different species of forest cacti live and grow according to different schedules of rest (or dormancy), activity and flowering. Make sure you know the seasonal schedule of the plant you've chosen.During the resting phase, water only enough to keep the potting mix from drying out completely, then slowly increase the frequency of watering once the rest period ends and buds begin to form. When flowers appear, water the cactus whenever the surface of the potting mix feels dry. Continue watering in this manner while the plant remains active.

Feeding. Forest cacti should be fed once or twice a month throughout the spring/summer growing season with a fertilizer formulated especially for the low-nitrogen needs of cacti. Never fertilize a plant during its resting period or when its soil is dry.

Christmas Cactus

The Christmas cactus gets its name from the holiday time frame during which it flowers -- specifically, mid-November to late January. Healthy plants produce elegant, fluted blooms that cascade down from serrated, branching stems in vibrant shades of white, pink, purple or red, depending on the variety. Care for Christmas cacti is as described in the forest cacti section, with the following exceptions to encourage the plant to rebloom.

Seasonal Care of the Christmas Cactus

Light. Beginning in September, plants begin to form flower buds in response to longer periods of darkness and cooler night temperatures. These characteristics occur naturally in many climates if the plant is outdoors (be sure to shield plant from any street lights or other artificial lights. Indoors, plants can be kept in a dark room, closet or under a box for 14 hours each night.

Temperature. In order for bud formation to occur, plants need to experience nighttime temperatures between 55 and 65 degrees F. and daytime temperatures no higher than 75 degrees F. Again, this often occurs naturally outdoors. Indoors, the plant might have to be moved from a site in the living quarters during the day to a cooler basement or unheated room at nights.

Watering and Feeding. During this period reduce watering and cease feeding completely.

Buds should begin to form in 4-6 weeks, after which you can eliminate these control measures. While the plant is in bloom, move it carefully, as sudden movements and jarring may cause the plant to drop buds and flowers. Also, keep the plant out of bright sunlight, extremely warm or drafty conditions.

When blooms cease in the latter part of January, your Christmas cactus will need a rest. Keep the cactus at a coolish 55 degrees F and water infrequently -- only enough to keep the plant from completely drying out. Beginnin in April, resume normal care of the plant.

Insect and Disease Control

Relatively few insect pests afflict cacti. The singlemost disease concern is root or crown rot, which can occur in poorly drained conditions or if excessive watering occurs. If soft, dark, mushy tissue develops, it should be cut out or removed with a sterilized knife. The wound should be treated with sulfur powder or other fungicide indicated for that purpose. Correct drainage problems, and water sparingly.

Propagation

Cuttings from most cacti root easily, but the process is slightly different than that of other types of houseplants.

• Take stem, offset or leaf cuttings in spring or summer. Allow cutting to air dry for several days to several weeks, depending upon size. Cutting should start to wilt, and a hard callus will form at the wound.
• Dip the callused end a rooting hormone according to instructions.
• Place the cutting into a pot filled with slightly dampened cactus potting mix. Do not feed or water for several weeks after planting.

Once new growth begins to develop, begin a regular watering and feeding program as described in plant care sections above.