Cacti, with limited exceptions, are succulents with the typical high interior to exterior ratio that stems from the need to endure periods of drought in their native habitats. Succulents are a broad category of plants that have adapted fleshy leaves or stems for water storage. The relatively large interior to exterior area ratios of these plants reduces loss of water into the atmosphere. A cactus also has unique characteristics that separate it from other succulents, primarily areoles from which spines, branches and flowers sprout. They are also, with few exceptions, native only to the Americas and the surrounding islands.
Areoles are the well-defined, small, fuzzy bumps on a cactus. The tufts and spines emerging from the areole are more evident in some species than in others. As a cactus grows, new areoles form. Each areole of a cactus can produce only one flower in its lifetime.
There are two general categories of cactus: jungle and desert. You have probably seen both. Schlumbergera x buckleyi, or Christmas cactus, is an example of a jungle cactus. The more familiar desert cacti originate in the arid regions of the Americas and take a variety of shapes and sizes, from low, branching structures to tall, tree-like shapes.
Cacti need strong, extended light exposure and in the northeastern United States that usually means giving your cactus the sunniest possible place in your home. Plants that receive inadequate light will grow thin and stretched in appearance. Full sunlight is also needed to stimulate bud formation and flowering. Your plant will benefit from being turned daily so that all sides receive light. Without regular rotation, the cactus is likely to become deformed. You can supplement natural sunlight with full-spectrum, fluorescent, artificial light.
The near absence of leaves in desert cacti helps them to conserve water. While cacti have adapted to survive long periods of dryness in their native environments, they do need regular watering in your home. Water thoroughly once the top inch of potting mixture has gotten dry. That could be as often as once a week in hot, dry home or only every few weeks in a more humid, cool house.
Watering from the bottom is preferred as splashing water on the cactus may cause unsightly markings. Place your cactus pot in a basin of water and allow the water to wick upwards through the drainage hole into the soil of the plant for ten minutes or so and then check that water has adequately moistened the potting mixture before removing. Allow the water to drain for a few minutes once your cactus is lifted from the basin; do not let the plant stand in a wet dish. It is essential that you avoid over-watering so do not give water if the soil is still moist. Keep humidity below 30% to the extent possible.
In the fall and winter, desert cacti experience a non-active growth period and it is important to adjust your care to preserve the health of your cactus. Water less frequently and always before noon so that all the water can be absorbed or evaporate during daylight. Water only enough to keep the plant from shriveling and err on the side of too little water.
The normal active growth period for desert cacti is March through September. During this period, a typical, warm household temperature is fine. During the non-active period, however, your cactus needs to rest with continued strong sunlight in a room of 40 to 45°F. An unheated room is fine, provided the temperature does not drop much below 40°F.
If your plant must stay in its summer position, pay close attention to adjusting to a winter watering schedule and lower the temperature of the house slightly if possible.
Cacti prefer a nutrient-rich, rocky soil with excellent drainage. Many commercial cacti and potting soil mixes rely too heavily on peat which holds moisture in the soil longer than is desirable. A proper cactus soil should drain in less than a minute after watering and it is essential that the cactus is kept in a pot with a drain hole. Like other succulents, cacti need soil to dry out between waterings or their delicate roots will rot.
You can find many recipes for potting mixtures that will provide a good combination of organic material and quick draining media but in general, mixing three parts grit or sand (for drainage) with one part potting soil (for organic material) and one part coir, peat or shredded bark (for structure) creates a suitable soil for most cacti.
Cacti have a wide, shallow root system to maximize water collection in their natural habitat. In the constricted space of a pot, compacted roots and poor drainage will quickly lead to root rot. In the early spring, remove your plant from its pot to see if the roots are beginning to fill the space. If so, move the cactus to a pot just one size larger. Do not use too large a pot or the soil will hold more moisture than your plant can absorb and tempt root rot. You can use a folded length of newspaper to protect you and your cactus from hand to spine contact.
Desert cacti need to be fed approximately every two weeks during active growth. A tomato fertilizer, high in potassium, will encourage flowering. Do not use a high nitrogen fertilizer or your cactus may get soft, damaged patches. If your plant is not in a cactus-mix soil but in a standard potting mix, greater nutrition will already be present and you can reduce the feeding frequency. No feeding is necessary during the fall/ winter rest period.
It may be surprising to know that desert cacti are generous flowerers. Under the right conditions, a mature cactus will enjoy a prolonged flowering period annually. Cactus plants that originate in areas with dramatically plentiful sunlight are not likely to get enough light to flower on a windowsill, but most cacti will flower indoors if you are careful with your care.
If possible, gradually move your cactus to a sunny position outdoors during the hot summer months. The intense and prolonged sunshine is great for your plant. Keep an eye on its watering needs.
It is very important to differentiate the non-active growth period of your cacti and to give them a rest. From October through February, most desert cacti need reduced water, food and temperature, though direct sunlight should continue.
The best time for propagation is spring or summer. Offsets appear at the base of many cacti and can be removed with a knife or, in some cases, a gentle tug. Allow any cut surface to harden before pressing the cut end just slightly into the soil of a seedling tray. Expose to gentle light and water lightly until roots form, after which you should treat the young cacti as mature plants. Repot the following spring.
Using the same technique, it is possible to root a stem section from a columnar cactus or root a branch offset of a columnar cactus with branches.