Cactus Pests and diseases

 

A range of pests attack cacti in cultivation. Those that feed on sap include mealybugs, living on both stems and roots; scale insects, generally only found on stems; whiteflies, which are said to be an "infrequent" pest of cacti; red spider mites, which are very small but can occur in large numbers, constructing a fine web around themselves and badly marking the cactus via their sap sucking, even if they do not kill it; and thrips, which particularly attack flowers. Some of these pests are resistant to many insecticides, although there are biological controls available. Roots of cacti can be eaten by the larvae of sciarid flies and fungus gnats. Slugs and snails also eat cacti.

Fungi, bacteria and viruses attack cacti, the first two particularly when plants are over-watered. Fusarium rot can gain entry through a wound and cause rotting accompanied by red-violet mold. "Helminosporium rot" is caused by Bipolaris cactivora (syn. Helminosporium cactivorum); Phytophthora species also cause similar rotting in cacti. Fungicides may be of limited value in combating these diseases. Several viruses have been found in cacti, including cactus virus X. These appear to cause only limited visible symptoms, such as chlorotic (pale green) spots and mosaic effects (streaks and patches of paler color). However, in an Agave species, cactus virus X has been shown to reduce growth, particularly when the roots are dry. There are no treatments for virus diseases.

Cactus Cultivation

 

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.

Cactus Conservation

 

All cacti are included in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which "lists species that are not necessarily now threatened with extinction but that may become so unless trade is closely controlled." Control is exercised by making international trade in most specimens of cacti illegal unless permits have been issued, at least for exports. Some exceptions are allowed, e.g., for "naturalized or artificially propagated plants". Some cacti, such as all Ariocarpus and Discocactus species, are included in the more restrictive Appendix I, used for the "most endangered" species. These may only be moved between countries for non-commercial purposes, and only then when accompanied by both export and import permits.

The three main threats to cacti in the wild are development, grazing and over-collection. Development takes many forms. The construction of a dam near Zimapan, Mexico, caused the destruction of a large part of the natural habitat of Echinocactus grusonii. Urban development and highways have destroyed cactus habitats in parts of Mexico, New Mexico and Arizona, including the Sonoran Desert. The conversion of land to agriculture has affected populations of Ariocarpus kotschoubeyanus in Mexico, where dry plains were plowed for maize cultivation, and of Copiapoa and Eulychnia in Chile, where valley slopes were planted with vines. Grazing, in many areas by introduced animals, such as goats, has caused serious damage to populations of cacti (as well as other plants); two examples cited by Anderson are the Galápagos Islands generally and the effect on Browningia candelaris in Peru. Over-collection of cacti for sale has greatly affected some species. For example, the type locality of Pelecyphora strobiliformis near Miquihuana, Mexico, was virtually denuded of plants, which were dug up for sale in Europe. Illegal collecting of cacti from the wild continues to pose a threat.

Conservation of cacti can be in situ or ex situ. In situ conservation involves preserving habits through enforcement of legal protection and the creation of specially protected areas such as national parks and reserves. Examples of such protected areas in the United States include Big Bend National Park, Texas; Joshua Tree National Park, California; and Saguaro National Park, Arizona. Latin American examples include Parque Nacional del Pinacate, Sonora, Mexico and Pan de Azúcar National Park, Chile. Ex situ conservation aims to preserve plants and seeds outside their natural habitats, often with the intention of later reintroduction. Botanical gardens play an important role in ex situ conservation; for example, seeds of cacti and other succulents are kept in long-term storage at the Desert Botanical Garden, Arizona.

Other uses of cactus

 

Cacti have many other uses. They are used for human food and as fodder for animals, usually after burning off their spines. In addition to their use as psychoactive agents, some cacti are employed in herbal medicine. The practice of using various species of Opuntia in this way has spread from the Americas, where they naturally occur, to other regions where they grow, such as India.

Cochineal is a red dye produced by a scale insect that lives on species of Opuntia. Long used by the peoples of Central and North America, demand fell rapidly when European manufacturers began to produce synthetic dyes in the middle of the 19th century. Commercial production has now increased following a rise in demand for natural dyes.

Cacti are used as construction materials. Living cactus fences are employed as barricades around buildings to prevent people breaking in. They also used to corral animals. The woody parts of cacti, such as Cereus repandus and Echinopsis atacamensis, are used in buildings and in furniture. The frames of wattle and daub houses built by the Seri people of Mexico may use parts of the saguaro (Carnegiea gigantea). The very fine spines and hairs (trichomes) of some cacti were used as a source of fiber for filling pillows and in weaving.

Ornamental plants

 

Cacti were cultivated as ornamental plants from the time they were first brought from the New World. By the early 1800s, enthusiasts in Europe had large collections (often including other succulents alongside cacti). Rare plants were sold for very high prices. Suppliers of cacti and other succulents employed collectors to obtain plants from the wild, in addition to growing their own. In the late 1800s, collectors turned to orchids, and cacti became less popular, although never disappearing from cultivation.

Many cacti, as well as other succulents, can make great houseplants, and once established, they don't require a lot of water or extra care. While all cacti are different, keep in mind that many will have a dormant period in winter where you reduce watering and lower light levels.

Cacti are often grown in greenhouses, particularly in regions unsuited to the cultivation of cacti outdoors, such the northern parts of Europe and North America. Here, they may be kept in pots or grown in the ground. Cacti are also grown as houseplants, many being tolerant of the often dry atmosphere. Cacti in pots may be placed outside in the summer to ornament gardens or patios, and then kept under cover during the winter. Less drought-resistant epiphytes, such as epiphyllum hybrids, Schlumbergera (the Thanksgiving or Christmas cactus) and Hatiora (the Easter cactus), are widely cultivated as houseplants.

Cacti may also be planted outdoors in regions with suitable climates. Concern for water conservation in arid regions has led to the promotion of gardens requiring less watering (xeriscaping). For example, in California, the East Bay Municipal Utility District sponsored the publication of a book on plants and landscapes for summer-dry climates. Cacti are one group of drought-resistant plants recommended for dry landscape gardening.