Cactus/Temp

Taxobox
name = Cacti

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image_width = 250px
image_caption = Echinocereus triglochidiatus ssp. "arizonicus"
regnum = Plantae
divisio = Magnoliophyta
classis = Rosopsida
subclassis = Caryophyllidae
ordo = Caryophyllales
familia = Cactaceae
familia_authority = Juss.
subdivision_ranks = Subfamilies
subdivision =
*Pereskioideae (leaf cactuses)
*Maihuenioideae
*Opuntioideae (Prickly pears and chollas)
*Cactoideae (True cactuses):"This is a translation of the article (retrieved 7 Dec 2006) as requested for the use of the page "

The Cactaceae or cactuses are a family within the order of the Caryophyllales.

Description

History and rise

Adaptation to dry conditions

A plant adapted to dryness can pursue several underlying strategies for survival:
*Reduction and alteration of the outer surface in order to reduce evaporation of water;
*Rapid water absorption should moisture become available;
*Capability of water storage;
*Metabolic adaptation

Surface reduction and alteration

Surface reduction and alteration entail a thickening of the leaves, then a reduction in the size of the leaf and finally no leaves at all are formed, and the leaves are transformed into thorns (cf the difference between thorns and prickles). A cactus thorn is also capable of absorbing moisture.

Water absorption

Water storage

The epidermis thickens, ribs running the length of the plant body make possible alterations in size, similar to an accordion, and to an extent the plant provides its own shade by a thick covering of thorns. In some succulents can be seen the formation of a large napiform root. This takes over the water storage and can be many times bigger than the visible portion of the plant. An example is Turbinicarpus, which above ground has a height of two centimetres and a diameter of three centimetres, but possesses a taproot more than half a metre in length.

pecialisation

Metabolism

Classification

The Cactaceae are divided into four sub-families:Pereskioideae (leaf cactuses), Maihuenioideae, Opuntioideae (prickly pears and chollas) and Cactoideae (true cactuses). The Cactaceae family contains about 125 to 130 genera with 1800 – 3000 species.

Summary of genera

The following genera have been recognised up to now and at some point described either validly or invalidly. The question of the validity of the descriptions according to the rules of the IOS is dealt with in the separate articles on individual genera. Many genera have been validly described, but re-combined or incorporated in other genera in the light of further scientific discoveries.

Cultivation

Because cactuses occur on the American continent from Canada to Patagonia in numerous different environments, such as deserts or semi-deserts, steppes, high mountains or tropical rain forest, it is not possible to formulate one single set of instructions for care that would be valid for all cactuses. However, most cactuses from Mexico and the Andes have similar requirements: they need a lot of light, must not be watered excessively and need to undergo a dormant period in order to be able to develop flowers. For that reason during the winter they are kept cool and dry.

The frequently cultivated cactuses of the genera "Echinopsis", "Parodia", "Gymnocalycium", "Echinocereus", "Echinocactus", "Opuntia" and "Mammillaria" can be successfully cultivated in this way. Cactuses of the genera "Selenicereus", "Hylocereus", "Epiphyllum", "Schlumbergera", "Rhipsalis" and "Disocactus" need more water and even in winter should be kept relatively warm and well watered. Many of these cactuses cannot tolerate full sunlight, but should be kept in semi-darkness.

Light requirements

In their native habitat spherical cactuses often grow in semi-darkness behind rocks or beneath clumps of grass and small shrubs. In central Europe however the sunlight is overall less, and for that reason here these cactuses need to receive as much light as possible, e.g. on a south-facing window sill, in a sunny propagation bed or forcing house or in the open air if protected from the rain. Sufficient light is necessary for the cactuses to be able to develop their typical forms of growth, coloration and thorn cover and to be stimulated into flowering. Cactuses that are kept too dark "grow rank", that is, they grow very straggly and take on a light green coloration. Such cactuses are vulnerable to diseases and pests.

Cactuses which have adapted themselves to high levels of sunlight by means of thick thorn or hair cover or of a reflective waxy layer require especially high light levels; cactuses with less strongly developed succuluence and with fewer thorns generally tolerate less light. To these belong the leaf cactuses ("Epiphyllum"), Christmas cactuses ("Schlumbergera") and cactuses of the genera "Rhipsalis".

Water requirements

Succulent cactuses are adapted to dry conditions and for that reason can survive for long periods without any water provision. In order to grow they require moisture, but compared to many other house plants they are very sensitive to over-watering and standing water. Because cactuses at low temperatures only grow very slowly and lose very little water in evaporation, they should be watered only during their growing period and at temperatures of more than 15 °C. In order to prevent the occurrence of any rot fatal to cactuses, the soil should be allowed to dry out thoroughly between waterings. If cactuses despite damp earth appear desiccated, that is a symptom of root damage, which would be made worse by additional watering. For that reason in such a case the plant should be kept absolutely dry and the roots checked for pests and rot.

During the dormant period succulent cactuses should not be watered at all. During this time they often wither somewhat from loss of water, but that will very rapidly put itself right with the first waterings after the end of the dormant period.

Less succulent cactuses require more water and tolerate less dryness; they should therefore be watered even in winter according to their need.

Nutrient salts

Because many cactuses grow relatively slowly and in ther dry native habitat very little humus formation takes place, they require fewer nutrient salts than other plants: in particular, their nitrate requirement is less. Special cactus fertilizers are commercially available that have a reduced nitrogen content compared to normal fertilizers. If these fertilizers are used according to the accompanying instructions over-feeding can be avoided. Over-fed cactuses are less likely to flower and are vulnerable to diseases.

Soil

Soil for dryness-loving cactuses must be porous and coarse-grained. and must contain little decomposable organic matter that could offer a feeding ground to saprotrophic fungi, potentially fatal to cactuses. Normal commercial soil for flowering plants is too strongly pre-fertilised and for that reason unsuitable. Many cactus lovers mix their soil themselves; a standard mixture consists of the following components: a quarter measure of high-grade peat-free planting soil, a quarter measure of quartz sand and two quarters of a fine-grained water-retentive material such as lava, pumice, Seramis (a clay substrate brand from Masterfoods) or LECA 2-4.

"Epiphyllum", "Gymnocalycium" and "Cereus" tolerate a rather greater proportion of humus, "Ariocarpus", "Turbinicarpus" and "Astrophytum" rather more mineral components.

Plant containers

For cactuses pots of synthetic materials or of glazed earthenware are very suitable. Unglazed earthenware pots are porous and moisture evaporates through their outer surface. This causes the root ball to cool and soluble salts to proliferate on the pot wall. The roots follow the nutrient salts and grow along the walls of the pot, partly adhering to them. The relatively lower temperature causes the roots to grow more slowly; and both prevent the formation of a dense root ball.

Cactuses with pronounced taproots, such as those of the genus "Ariocarpus", need tall pots, as in these plants the root is sometimes a good deal larger than the body of the plant above ground. The best time for re-potting is the beginning of the growth period in spring. After re-potting cactuses should not be watered, but instead allowed to stand dry for a week, in order to allow damage to the roots - an entrance point for fungi - to heal.

Pests and diseases

Insect pests

Three kinds of pest are constantly encountered on cacti:
mealybugs (Pseudococcidae), root mealy bugs (Phylloxera) and spider mites (Tetranychidae).

Mealybugs are insects between 1 and 3 millimetres long, which are particularly noticeable from their woolly webs in the crown and root cap areas of the plant. They are also known as "woolly lice".Root mealy bugs are less conspicuous and are usually noticed during re-potting. They are white, about a millimetre long and build little white webs all over the roots.

Spider mites are especially dangerous, as they are usually only noticed by the damage they cause, when it is generally too late to save the affected plants. Spider mites are very small, not more than half a millimetre long, and spread mostly in warm dry conditions, very rapidly. With a lens (x6 or more) a fine web can be discerned during attacks. The epidermis of the plant takes on a brownish discoloration at the points of attack, at first in the crown area, and later over the whole plant.

All three pests cause damage by sucking out the sap.

With the correct means it is not especially difficult to counter these pests:
*Lots of fresh air and the proper provision of plant food with phosphate and potassium
*Meticulous observation: if a plant becomes sickly or exhibits unusual discoloration, a pest may be the cause.
*Preventive measures against pest infestation and immediate remedial action as soon as an attack is discovered.

Systemic remedies are much to be preferred, as these work not only on contact but also by spreading throughout the whole plant.

The remedy is sprayed over the plants in the prescribed dose and also administered by watering. Treatment is generally undertaken in spring (mid May) and autumn (beginning of September). Care must be taken to repeat the treatment after 10-14 days, on account of the short reproductive cycle of the pests, although a different pesticide should be used.

Other pests

Scale insects, nematodes and woodlice are of minor significance, but snails can sometimes cause immense damage, although they can be dealt with by the use of snail and slug pellets. The larvae of dark-winged fungus gnats (Sciaridae) are dangerous to seedlings, as they live in damp earth and damage the roots and bodies of young plants. They may be dealt with by yellow glue [or lime?] tablets to eradicate the flies as well as by watering with, for example, Bacillus thuringiensis var. israelensis.

Fungi

Preventive measures are also available against fungi:
*Lots of fresh air and light the proper provision of plant food with phosphate and potassium
*Keeping the plants dry: the earth must be dry throughout before the next watering take place. In cool weather it is better not to water at all.
*Meticulous observation: if a plant becomes sickly or exhibits unusual discoloration, a fungal attack may be the cause.
*Avoiding injuries, particularly to the roots, for example during re-potting.
*Sterile working conditions and practices during sowing and propagation.

The funguses encountered on cacti are "Phytophthora", "Fusarium", "Pythium", "Botrytis" and "Helminthosporium". Fungi thrive best in humid conditions. Healthy, austerely raised plants are scarcely susceptible to fungal infections. Favourite points of entry for fungi are injuries to the roots and to the root cap, remains of flowers and fruit, and stab wounds from the thorns of other plants.

There are several visible symptoms of a fungal attack. The plant may:
*lose all its roots;
*dry out from inside;
*dissolve into mush from the inside;
*rot away from the root cap upwards;
*rot away from the crown downwards;
*be covered, at least partly, in mould.

If a fungal infestation is discovered on a plant, then the whole plant, with the container and the soil, must be destroyed. With valuable plants, if the attack is from below, it is possible to attempt to save the upper part by cutting it away with a knife disinfected in alcohol after every cut, until no discoloration of the vascular bundles can be seen. The part that has been cut off can then be rooted or grafted (see Propagation).

The use of fungicides is unnecessary if plants are kept in austere dry conditions.

Other diseases

Occasionally cacti are attacked by viruses or bacteria: the only remedy is the destruction of the entire plant with its container and soil. Signs of such attacks may be black spots, reddish sunken spots, brown-grey raised spots or scurfy spots. On no account should the affected plants be used for compost.

Some cacti, for example those of the genera "Cereus" or "Notocactus", twist from below as they get older. While this may not be desirable aesthetically, it is entirely normal and not detrimental to the plant.

In conditions of strong sunlight together with a low temperature many cacti take on a reddish tinge. This is not a sign of sickness, but a natural protective mechanism. If the plant is moved to somewhere less exposed to the sun, the redness will fade after a short time.

Propagation and sowing

Rooting

Plant shoots or cuttings from the heads of cacti can be rooted. The shoots or cuttings are cut off with a sharp knife disinfected with alcohol, shoots at the narrowest part. The cuts are painted with charcoal or aluminium powder (to which may be added some rooting powder), and the shoots or cuttings are then laid in a shady dry place for seven to ten days until the cuts are dried out: the bigger the wound, the longer the drying-out time. The pieces are then placed in a container of quartz sand, which is moistened from time to time. After a time the cuttings form new roots. As soon as these are a centimetre long the cutting can be potted up like a normal cactus. Leaf cacti ("Epiphyllum") can also be rooted in a glass of water.

Grafting

Some cacti grow in cultivation only with great difficulty on their own roots; other cacti as cuttings form roots extremely slowly, or their cut-off shoots and cuttings are too small to be able to form roots before they dry out. These cacti are very suitable for grafting. In grafting, a cactus (the graft) is induced to grow together with another cactus (the stock). The stock is mostly a robust cactus such as "Cereus", "Eriocereus" or "Echinopsis", which takes over the provision of nutrients to the graft. The graft and the stock are both cut perfectly level with a sharp knife or razor blade, and then the cut surface of the graft is pressed onto the cut surface of the stock. It is important to take care that there is no air between the two cut surfaces and that the vascular bundle rings of both plants cover each other, or at least overlap. The graft is then strengthened with rubber bands (injuries to the crown are avoided by the use of protective foam rubber), and the container with the new graft is placed in a shady spot. After a week at most both plants will have grown together and the artificial supports can be removed.

Growing from seed

Growing from seed is the cheapest and most interesting method of propagation, even if it sometimes requires a certain amount of patience.

Cactus seeds are cleaned of the adhering flesh of the fruit, dried and dusted with fungicide powder. Then the seeds and scattered in a seeding tray on fine-grained soil (coarsely-sieved cactus soil) and not covered, because cacti germinate with light. Then water is added carefully from the bottom, a transparent cover is added and the tray put somewhere light and warm (25 - 30 °C). (It is important not to forget to label it). Until the seeds have germinated the soil must not be allowed to dry out. Germination takes place after one to six weeks. From now on the seed tray must be regularly aired but the soil must still be kept damp. When the seedlings are 5 mm tall they can be transplanted into larger pots in which they have a greater distance from each other than in the seed tray. The potted-out plants are no longer covered and after a transition period of three months during which they are watered rather more than is usual, are treated like normal cactuses. the best time for sowing seed is in February, as the plants have a long growth period ahead of them and by autumn will have grown so much that they can be over-wintered like normal cactuses.

Quellen

External links

* http://www.kaktus-und-kuebel.de/
* http://www.kakteenweb.de/
* http://www.kakteenfotos.de/
* http://www.thater.net/cactaceae/
* http://www.cactus-mall.com/cacmalde.html