Cacti are a group of plants with peculiar shape and size and mostly adopted for desert life. The adverse environment has made these plants unusual in appearance and provided morphological, anatomical, and physiological safeguards to thrive and flourish under such condition.
The Cactaceae are perennial herbs or shrubs, annual species being non-existent, with more or less fleshy stems, usually jointed. The leaves are rarely perfect and permanent, but are generally absent or rudimentary or fugacious or reduced to mere minute scales. In Pereskia the leaves look like normal flattened leaves as in other dicots.
In the genera Echinopsis, Rebutia, Lobivia, Echinocactus, Astrophytum and Lophophora, the stem is globular or avoid or cylindrical, very fleshy, usually with many ribs, acute or rounded, straight, wavy or spiral, more or less notched, with areoles and spines and the flowers are borne on the areoles at the top or at the sides.
The stem of Opuntia is always a typical joint whether it is globular, cylindrical or flattened. The joint is, therefore, a stem which when properly formed ceases to grow at the apex, but continues to grow in size and thickness until it reaches its full development and then the rudimentary fleshy cylindrical leaves drop off and the spines grow to their full size.
Spines are of two types. The real spines, vary in length according to species and in most cases very stiff, acute,needlelike, awlshaped, tortuous, arched or hooked and develop on the outer or lower areole or bud. On the upper or inner areole, just above the spines hairs develop which are flexible or more or less stiff bristles. The spines and bristles of cacti are often a terror particularly in the operations of planting and repotting.
The largest and finest flowers of the Cactaceae are mostly nocturnal. They open at sunset and display all their glory in the dark hours of the night and droop and fade away at dawn.
Cacti can tolerate wide range of temperature. Very high temperature or very low temperature adversely affects the growth of many species of cacti. At a temperature above 350C under the blazing sun, plants may show sign of scorching or turn yellow. In such condition the plants should be shaded off. Most epiphytic species require half-shade and a moist atmosphere.
Soil and Manures & fertilisers
The soil for cacti should be porous and permeable. Almost all species thrive well in a porous calcareous soil. Lime mortar from old buildings, hard limestones, marble or even crushed valves of oysters or other shell fish, roughly powdered and passed through a sieve to remove the fine dust, provide an excellent material to add to the soil. Some old leafmould, thoroughly rotted should form a part of the compost. Very well-rotted cow manure is also an useful ingredient in the compost for the cacti. Undecomposed organic matter is sure to give trouble and causes rotting of roots. Chemical fertilizers force growth and due to excessive succulence the plant becomes susceptible to drought and infection. Small quantities of bone meal, superphosphate or basic slag may be added to the compost. The compost should be watered and allowed to remain for few months before using it. A small quantity of charcoal powder also helps to keep the soil porous and sweet and prevents infection of roots. On the other hand, a soil, which keeps moist or forms a green coat or crust on the surface, is either badly drained or unsuitable and should be changed without delay.
Planting and Potting of cacti
Many hardy cacti are also grown in ground particularly in the rockeries. The land should be sloping and the soil porous and calcareous. Garden loam 2 parts, sand, old mortar or limestone gravel and leafmould 1 part of each and some crushed charcoal are suitable for Cleistocactus, Expostoa, Oreocereus, Borzicactus etc. For Cereus, Cephalocereus, Pachycereus, Lemaireocereus, Harrisia and other Cereus, use garden loam 2 parts, old mortar or limestone gravel, leafmould or manure 1 part of each: some crushed charcoal may be added.
The best time for planting and potting is just at the commencement of the growing season in spring. The soil should not be too wet and it is always safe to place some dry soil beneath and around the roots. If roots are damaged at the time of planting, watering should be stopped for few days to allow the wounded roots to dry and heal. In order to promote new roots little watering is done to keep the compost just moist. Water may be given more freely later on, when the temperature rises and new growth develops.
Well-baked earthen pots are suitable for cacti, glazed or painted pots hamper the aeration of the roots. In potting or repotting cacti, it is safe to select the pots just large enough to contain the plant. Pots which are too large have two disadvantages. When watered they retain too much moisture, retain it too long and appear as constant danger to the plant. Moreover, the roots take too long to reach the sides of the pot and spread, thus causing a delay in growth and flowering.
The pots should be filed for about one-third with crocks or other drainage material, the large curved pieces to be placed on the holes and bottom and the hole topped over by a thin layer of thick gravel, shingle or crocks, broken into small pieces.
If it is found that the roots have become pot-bound it is advisable to break off the pot with a sharp blow, instead of risking damage to the plant in trying to take it out and save the pot. The plant to be repotted should be kept dry so that it easily comes out without injuring the roots and crocks are removed carefully. The plant should be replanted just as deep down as in the previous pot. Unless the soil is in a perfectly dry condition, newly potted lightly sprayed once or twice a day and placed in shade for about one week, afterwards in half-shade for at least an equal period before removing them to full sun.
It is very important practice for cultivation of cacti. Under natural condition these plants are adapted to server drought. Their external structures and also the physiological condition clearly indicate very little requirement of water.
In the summer months, when the cacti grow, watering is done frequently. If these plants are kept in open in small pots daily watering may be necessary. Every time the soil is to be drenched completely so that excess water comes out through the drainage hole. At the resting period, during the winter months roots remain inactive and watering should be minimised gradually from the autumn. Water should not accumulate in the depression of the plant. In Echinopsis, Echinocactus, Rebutia the growing point on the top may rot. Fine spraying or syringing of water may be done in the morning.
Propagation of cacti
Propagation of cacti is done by both seeds and vegetative methods.
Cacti can be grown from seeds but the seedlings grow slowly and take long time to form a specimen. It is commonly used by commercial growers and amateurs who specialise and aim at a large collection of these plants.
The seed compost consists of equal quantity of leafmould and sand and some amount of loam, powdered brick and char coal. Seed pan or shallow pans are used for sowing and half the depth of the pot is crocked. The pans are filled with the compost leaving about 3 to 4 cms on the top. The soil is then gently pressed and levelled. Seeds are sown uniformly, pressed down in the soil and thin layer of sand spread on the top. The compost is watered by placing the pot in water and allowing it to enter through the bottom and side holes. The pot is covered with a paper or glass pane. Cacti can be sown successfully from March to September. Germination may take place from 7 days to one month. When the sedlings are 1.5 to 2 cm in diameter and in the case of tall-growing species unless about 4 to 5 cm high, they are taken out without injuring rootlets and planted in small pots containing the same compost is shade and watering is best done by spraying after a day or so. A number of seedlings may also be transplanted into a bigger pot at a distance of 3-4 cm.
Propagation by cutting of offset is an easy method of getting large plants and also the only way of perpetuating hybrids and valuable plants. Species with solitary globular stem cannot be propagated by this method. Tall-growing species with single stem sends out shoots if the top portion is cut which can be used as the cutting but the specimen plant is lost. The branches thus formed are also detached and multiplied.
The cuttings are cut clean with a sharp knife and kept in airy and shady place for drying. The period of drying may vary from seven days to one month. The cutting is then planted in sand and leafmould or in the usual compost and kept in semishade till rooting. During the time of root formation watering is done sparingly even in the summer months. Formation of roots in the cuttings is indicated by new growth on them.
Cutting of Cerei including Harissia, Selenicereus should be of covenient length. Pachycereus, Lemaireocereus are cut at a constriction. In branching species of Mammillaria and Rebutia, the clumps are divided into smaller clumps. Species of Trichocereus, Cleistocactus, Echinocereus, etc. are best propagated by separating the suckers which arise from the lower part of the stem. Cuttings of Epiphyllum are best taken across the broad part of the flat stem.
Propagation by grafting is a useful method of propagation. Generally a weak growing species is grafted on a strong growing species is grafted on a strong growing stock, the weak scion improves the vigour and stock is weakened. Species of Selenicereus, Trichocereus, Harrisia, Cereus, Opuntia, Cleistocactus, etc. are commonly used as stock.
The grafting may take the form of an inverted wedge. The top of the stock is cut as a wedge which is inserted in wedge shaped cavity at the lower end of the scion for proper union. It is very important that the sides of the wedge and cavity should come in close contact and the top of the stock fit in the bottom of the cleft.
For grafting Cephalocereus, Oreocereus and Espostoa, clean horizontal cut is made on both stock and scion. The scion is kept fixed on the stock by giving a moderate pressure by tying the scion with the potted stock. To avoid injury a piece of folded cloth is placed on the top of the scion and the string is tied on it. After a month the pressure is released. The cut end of the stock and scion instead of being horizontal may also be inclined at an angle of 45 degrees as in the case of tongue graft.
Cacti can also be grafted by making a clean cone or inverted pyramid at the base of the scion and inserted in a caving made into the upper end of the stock. The scion is then tied in the usual manner. Grafting is also done in the summer months when the humidity is very low. High humidity causes rotting.
Rebutia has many species of small globular plants producing offsets. The flowers appear on small plants and vary in colour R. deminuta, R. fiebrigii, R. marsoneri, R. senilis are popular.