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The four important factors regarding the correct cultivation and health of cacti are: water, light, air, soil. They must have ample light and air. Light is most important to cacti, since the stem has taken over the function of the leaf, and often the plant body is densely covered with spines which cut down the amount of light entering the chlorophyll-bearing tissues.
Direct sunlight is a different matter. There are many plants in their native habitat which only occur under the protection of shrubs, grass or overhanging rocks, such as, many of the smaller cacti including Mammillarias, Rebutias, Frailea and Noctocactus. Thus a plant grown in the centre of Australia, at Alice Springs, would require more protection from the direct heat than a similar plant grown near the coast at Mount Gambier in the south-east of South Australia.
A general rule is that a densely-spined or thick-skinned plant is more able to stand the full sun than a thinly-spined or spineless one which would do better in a more protected position. To enable the plants to breathe more readily in collections that grow in the cities, it is advisable to spray them every few days to clean them of the accumulated dust from the air. The larger plants will grow better out in the open rockery, where they can benefit from the maximum fresh air, sun, wind and also the occasional shower.
Some parts of South Africa, Australia and New Zealand experience heavy frosts and snow, but there is no need to worry,
as some cacti enjoy the frost in winter. There are some which need the frost in winter to enable them to flower, eg. Echinocereus and some Mammillarias. Too much frost will of course severely damage or even kill them.