Cactii are similar to succulents in that they store reserves of water within them - the difference, however, is that succulents generally store water in their leaves while cactii store it in the stem, often having no leaves at all. There are many hundreds of varieties of cactus, the family of which is closely related to the Carnation (but you'd never know it as they are so different).
Most cactus come from the arid regions of the Americas, Africa and Madagascar but some can be easily grown in Australian gardens and many, even more easily grown, in pots. Many cacti produce very showy spring flowers and colourful fruits but most are known for their swollen, spongy stems that are often deeply ribbed and their prickly spines - developed to protect them from thirsty birds and animals. Suprisingly, cacti can be frost hardy (the desert can get very cold at night) but although they thrive in dry conditions they do demand a mineral rich soil and lots and lots of sunlight.
Growing in pots is the easiest but ensure that they get enough sunlight - a window ledge is best with a northerly aspect. Inadequate light will produce paler bodies and reduce the liklihood of flowering.
Cacti generally only grow in spring and summer so only need water at these times. Ensure that the soil is well drained and don't let residue water remain in the saucer but otherwise regular small amounts of water in summer are necessary. Cacti don't like humidity but if the weather does turn "moist" in summer then reduce the watering accordingly. In autumn and winter you should let them dry out completely. They may shrivel a little but don't worry, they'll recover their bulge again in spring.
Featured cacti, from left to right:- 1. Gymnocalycium Mihanovichii 2.Parodia Leninghausii 3. Mammillaria Matudae
Cacti don't have to be grown in sand but do make sure that it is well drained. A slightly acid soil that is low in organic content is best and a high sand content will always help the drainage. A gravel mulch to top off the pot will help keep their collars dry and look very good too.
A light sprinkling of a slow release, low nitrogen fertiliser in spring is all that is required. Avoid soluable fertilisers as they are often nitrogen rich which produces a rather lush and bloated growth.
Pest and Diseases:
Cacti are at their most susceptible to pests and diseases when they are under stress from high humidity or overwatering. The most common pest is the mealy bug which sucks the sap and weakens the plant. It shows as a whitish, sticky area at the top of the plant which can be dabbed with methylated spirit or sprayed with a systemic insecticide. The other main problem is botrytis fungus which affects the roots of plants that have been overwatered. Spray with a fungicide and propogate side shoots immediately - just in case it doesn't survive.
Take side shoots as cuttings in spring and pot in a terracotta pot (they dry out quicker than plastic). Don't overwater the cutting for a week or two as the roots develop best in dryer conditions with just a hint of water here and there.
This article provides an introduction to newcomers to cactus growing but if you'd like to know more in depth information on the subject then the Cactus and Succulent Society of Australia has plenty of resources to explore.
While the British Cactus and Succulent Society also has a comprehensive website, its best feature is a comprehensive photo gallery to help identify species.