Cactus

Many species of cacti are highly prized by horticulturalists as botanical oddities and ornamental plants. These may be cultivated for their beautiful flowers, the aesthetics of their stems and spines, or merely because the plants have a strange-looking appearance. In addition, many people like to grow cacti because they are relatively easy to maintain—it does not matter much if you forget to water your cacti for a few days, or even a few weeks or more. In fact, over-watering is usually the greatest risk to most cacti that are kept as house plants, because too much moisture will pre-dispose these drought-adapted plants to developing fungal and bacterial diseases, such as soft-rot.

Virtually any of the native species of cacti of North America may be used in horticulture, as are many of the species of Central and South America. The genera Mammillaria and Opuntia are most commonly grown, but virtually any species may be found in cultivation around or in homes and greenhouses. One of the most common and familiar species is the Christmas cactus (Zygocactus elegans), a flat-stemmed, red-, pink-, or white-flowered species that is grown as a garden and house plant. This species blooms during the winter, and florists often induce this plant to bloom around Christmas-time, when it is commonly sold as a living ornament to brighten homes during that festive season. The candelabra cactus (Cereus peruvianus) is a tree-sized species native to South America that is commonly cultivated outdoors in hot climates, or in greenhouses in colder climates.

Many species of cacti can be rather easily transplanted from natural habitats into the vicinities of homes and businesses, where they may be used as central components of low-maintenance gardens in places where rainfall is sparse, and the development of grassy lawns would require an excessive use of scarce and expensive water. Wild cacti are also collected to grow in or around the home, and to develop private collections of these interesting plants.

Unfortunately, most species of cacti re-colonize disturbed sites very slowly and infrequently. Extensive losses of cactus habitat to industrial and residential developments, coupled with excessive collections of wild plants, have resulted in the populations of some species of cacti becoming endangered. In some areas, populations of wild cacti must be guarded against illegal, often nocturnal collecting of valuable plants for horticultural purposes. Unfortunately, it is difficult to protect many endangered cacti from poaching. This is because of the extensive areas that must be patrolled, in the face of multi-million-dollar profits that can potentially be made in the illicit cactus trade. Some species of cactus are now critically endangered in the wild because of excessive, illegal collecting, and this represents an important ecological problem in many areas.

The most commonly edible cactus fruit is that of Opuntia species, especially O. ficus-indica. The fruits of prickly-pears, sometimes known as apples or tunas, can be eaten directly or used to make a jelly. Prickly-pear fruits are considered to be a delicacy around Christmas time in some regions.

Peyote or mescal buttons (Lophophora williamsii) is a cactus containing several alkaloids in its tissues that are used as a hallucinogen and folk medicine. Peyote is important in the culture of some tribes of native Amerindians in the southwestern United States and Mexico, especially in the vicinity of the Rio Grande River. These aboriginal peoples use peyote to induce religious experiences and revelations. Peyote is also commonly used as a recreational drug by many people, and by several religious cults.

Some species of spiny cacti, such as Opuntias, are used as living fences, for example, to keep livestock out of gardens. The long, sharp spines of other cacti were used as needles in some of the earliest types of phonographs. The "wood" of the saguaro cactus has long been used by Amerindian peoples, and is still utilized to make crafts and novelty furniture.

A few species of cacti have become pests, or weeds, when they escaped from cultivation in places where they were not native, and were not controlled by diseases or herbivores. The best known example is that of a prickly pear cactus (Opuntia spp.) that was imported to Australia from North America for use as an ornamental plant and living fence, but became invasive and a serious weed of rangelands. This pest has now been almost completely controlled through the introduction of one of its natural herbivores, the moth Cactoblastis cactorum, whose larvae feed on the cactus.