As cacti vary so much in size from perhaps one inch to 30 feet or more there are many species available to the grower to suit almost any situation or condition. Although the best place to grow a collection of cacti is in a greenhouse, there are many kinds which can be grown quite well in a sunny window.
Although all cacti can go for long periods without water, it is essential that they are provided with an adequate supply during the growing period or they cannot flourish.
To grow cacti well, and flower them, it is imperative to provide them with a porousas the roots soon rot if they are wet for days on end. Many types of potting soils could be used and recommended, even different ones for each genus; it is possible, however, to grow practically all types of cacti in one kind of potting compost. The art of growing cacti is in the watering and the amount given can vary according to the type of compost. Plants can only obtain their nourishment in a liquid form. Only small quantities of water should be given, this will ensure that they receive correct nourishment.
Potting composts for Cacti
A very good potting compost for cacti may be made up from a compost with few added nutrients, to which is added a sixth part of coarse sand to make it more porous. Some additions of broken brick or granulated charcoal may be incorporated in the added sand. If it is desired to mix a compost for general use, the following will be found quite reliable. Take 2 parts of loam, 1 part of peat and 1 part of sharp, coarse sand. Mix well and to each bushel add 1 oz of ground chalk or limestone 1/2 oz sulphate of potash, 1.1 oz of super phosphate and 11 oz of hoof and horn grist. All the globular and columnar types of cacti may be grown in this compost, while for the epiphytes some average branded potting compost may be used, as these plants will benefit from the richer soil. The very spiny-types of cacti do not require heavy feeding with fertilisers and as long as they are repotted at least every two years they will grow quite well. If these plants are fed too liberally they will become lush, open in texture, and be very liable to rot off in the winter. Also it will be found that the spines formed when the plant has been fed with fertilisers may not be as stout and well coloured as if the plant had been grown harder. When making up the cactus compost it is very important to find a good loam as a basis for the mixture. An ideal type is the top spit from an old-standing meadow. Unfortunately these meadows are becoming few and far between and the loam is often only the under spit after the top turf has been removed. The peat is not so important but the sand must be very sharp and coarse. Silver sand is useless for cactus compost and the type known as washed grit, or river grit is the best.
The potting compost should not be used immediately after it has been mixed and a lapse of a fortnight at least is desirable before potting. The time to repot varies considerably, being determined by many factors. Some cacti are very slow growers and so may be left in their pots for two or three years while others may need a move twice a year. Many cacti never flower because they have been in the same stale, worn-out soil for many years. With fairly frequent watering during the growing period the roots of the plant use up the nourishment in the soil, and clearly there can be little food value left in it after about a year.
The best time for repotting is during the growing period, which with most cacti will be between March and September. Once new growth is seen on a plant it can be repotted. When dealing with a fairly large collection it will be found better to make a start with the larger pots. These can then be cleaned for use with other plants which may need a bigger pot. It is also a good plan to make a clear place in the greenhouse and place all repotted plants there so that none may be missed. The pots should be clean and well crocked. It is unnecessary to place a large number of crocks in the pot as they will only take up valuable space which would be better occupied by good soil. The best way to crock a pot for a cactus is to cut as large a piece of broken flower pot as will lie in the bottom of the pot. This large crock will then form a kind of platform when the plant is removed the next time. If a stick is pushed up through thehole the crock will force the whole ball of soil up in the pot, whereas if a number of small pieces of crock are used it is possible to damage the roots when trying to remove the plant another time.
Place some of the coarsest particles of compost over the crock and then a little soil. Remove the plant from the old pot and hold it by the root system. Gently work all the old soil away from the roots. If any appear dead they should be cut away. Now rest the plant in the pot and gradually work in some fresh compost. Because most of the plants are spiny it may not be possible to work the soil in with the hands as is possible with ordinary plants. A tablespoon can be used to insert the soil and it can be gently firmed in with an old table-knife handle. A wooden stick must not be used as it would catch in the spines and break them. Once a spine is broken it will never grow again.
See that the plant is in the same relative position in the soil as it was before. See also that at least 1/2 inch of space is left at the top of the pot for watering. The plant should look right in the new pot; do not use one too large so that the plant looks lost or one so small that there is no room for soil as well as the base and roots of the plant. For the globular kinds of cacti a pot which is 12 mm 1/2 inch) bigger all round than the plant will do for pots up to 88 mm (3.5 inches) in diameter, but for a larger plant a pot at least 25 mm (1 inch) larger all round must be provided. This will not be sufficient for many of the taller growing types as the pot must be large enough to form a firm base to stop the plant and pot from falling over.
Plastic pots may be used, especially for small plants; they do not appear to dry out as quickly as clay pots. Once the plant is potted it is important to insert the label, and a good plan is to put the date of repotting on the back. This is a useful guide in a large collection. As it is essential that the soil should be able to discharge all surplus water as soon as possible, the pots should not be stood on a flat surface. Some coarse gravel makes an ideal base on which to stand the pots. If slats are provided in the greenhouse it is better to cover them with corrugated asbestos sheeting on which the gravel may be placed. Any plants stood on shelves must have a saucer containing gravel under them to allow the free removal of surplus water.