The popularity of cacti means many books are devoted to their cultivation. Cacti naturally occur in a wide range of habitats and are then grown in many countries with different climates, so precisely replicating the conditions in which a species normally grows is usually not practical. A broad distinction can be made between semidesert cacti and epiphytic cacti, which need different conditions and are best grown separately. This section is primarily concerned with the cultivation of semidesert cacti in containers and under protection, such as in a greenhouse or in the home, rather than cultivation outside in the ground in those climates that permit it. For the cultivation of epiphytic cacti, see Cultivation of Schlumbergera (Christmas or Thanksgiving cacti), and Cultivation of epiphyllum hybrids.
Growing medium
The purpose of the growing medium is to provide support and to store water, oxygen and dissolved minerals to feed the plant. In the case of cacti, there is general agreement that an open medium with a high air content is important. When cacti are grown in containers, recommendations as to how this should be achieved vary greatly; Miles Anderson says that if asked to describe a perfect growing medium, "ten growers would give 20 different answers". Roger Brown suggests a mixture of two parts commercial soilless growing medium, one part hydroponic clay and one part coarse pumice or perlite, with the addition of soil from earthworm castings. The general recommendation of 25–75% organic-based material, the rest being inorganic such as pumice, perlite or grit, is supported by other sources. However, the use of organic material is rejected altogether by others; Hecht says that cacti (other than epiphytes) "want soil that is low in or free of humus", and recommends coarse sand as the basis of a growing medium.
Watering
Semi-desert cacti need careful watering. General advice is hard to give, since the frequency of watering required depends on where the cacti are being grown, the nature of the growing medium, and the original habitat of the cacti. Brown says that more cacti are lost through the "untimely application of water than for any other reason" and that even during the dormant winter season, cacti need some water. Other sources say that water can be withheld during winter (November to March in the Northern Hemisphere). Another issue is the hardness of the water; where it is necessary to use hard water, regular re-potting is recommended to avoid the build up of salts. The general advice given is that during the growing season, cacti should be allowed to dry out between thorough waterings. A water meter can help in determining when the soil is dry.
Light and temperature
Although semi-desert cacti may be exposed to high light levels in the wild, they may still need some shading when subjected to the higher light levels and temperatures of a greenhouse in summer. Allowing the temperature to rise above 32 °C (90 °F) is not recommended. The minimum winter temperature required depends very much on the species of cactus involved. For a mixed collection, a minimum temperature of between 5 °C (41 °F) and 10 °C (50 °F) is often suggested, except for cold-sensitive genera such as Melocactus and Discocactus. Some cacti, particularly those from the high Andes, are fully frost-hardy when kept dry (e.g. Rebutia minuscula survives temperatures down to −9 °C (16 °F) in cultivation) and may flower better when exposed to a period of cold.
Propagation
Cacti can be propagated by seed, cuttings or grafting. Seed sown early in the year produces seedlings that benefit from a longer growing period. Seed is sown in a moist growing medium and then kept in a covered environment, until 7–10 days after germination, to avoid drying out. A very wet growing medium can cause both seeds and seedlings to rot. A temperature range of 18–30 °C (64–86 °F) is suggested for germination; soil temperatures of around 22 °C (72 °F) promote the best root growth. Low light levels are sufficient during germination, but afterwards semi-desert cacti need higher light levels to produce strong growth, although acclimatization is needed to conditions in a greenhouse, such as higher temperatures and strong sunlight.
Reproduction by cuttings makes use of parts of a plant that can grow roots. Some cacti produce "pads" or "joints" that can be detached or cleanly cut off. Other cacti produce offsets that can be removed. Otherwise, stem cuttings can be made, ideally from relatively new growth. It is recommended that any cut surfaces be allowed to dry for a period of several days to several weeks until a callus forms over the cut surface. Rooting can then take place in an appropriate growing medium at a temperature of around 22 °C (72 °F).
Grafting is used for species difficult to grow well in cultivation or that cannot grow independently, such as some chlorophyll-free forms with white, yellow or red bodies, or some forms that show abnormal growth (e.g., cristate or monstrose forms). For the host plant (the stock), growers choose one that grows strongly in cultivation and is compatible with the plant to be propagated: the scion. The grower makes cuts on both stock and scion and joins the two, binding them together while they unite. Various kinds of graft are used—flat grafts, where both scion and stock are of similar diameters, and cleft grafts, where a smaller scion is inserted into a cleft made in the stock.
Commercially, huge numbers of cacti are produced annually. For example, in 2002 in Korea alone, 49 million plants were propagated, with a value of almost US$9 million. Most of them (31 million plants) were propagated by grafting.
