The genus Crassula consists of over 300 species. Its centre of distribution is southern Africa with 150 species which are widespread but concentrated mainly in the semi-arid, winter rainfall areas. Crassula also occurs in Europe, America, Australia, New Zealand and the southern islands.
Most members of this family are characterised by having thick, fleshy leaves typically arranged in symmetrical, opposite pairs of neat Aloe-like rosettes. It is this characteristic of neatly and symmetrically arranged leaves that make it easy to recognise members of this family. The flowers usually have four to five petals and the fruits are made up of four or five separate parts (carpels). An interesting feature is the prominent nectar glands within the flowers.
Crassulas have an extraordinary metabolism that allows them to photosynthesize normally without losing much water through their leaves, known as Crassulacean Acid Metabolism or CAM. This gives them the ability to survive and thrive in dry regions or areas that experience recurring droughts.
All plants need carbon dioxide (CO2) for photosynthesis. Most plants take in CO2 during daylight hours through their stomata (pores in the leaves) and can’t avoid losing water at the same time through these open pores. In Crassula the stomata are closed during the day but open at night so that when they take in the CO2 they lose the least amount of water. The CO2 taken in at night is stored in the form of organic crassulacean acids. During the day, these acids are broken down and the CO2 is released and re-used in the photosynthetic process.
In this way they lose much less water yet can photosynthesize normally during the daylight hours. Furthermore, during extremely dry periods they won’t even open their stomata at night, and will recycle the CO2 within the cells. They will not grow but the cells will be kept healthy – this is known as CAM-idling.
During wet conditions the leaves become swollen and distended, during dry spells they become flattened. Flowers are pollinated by butterflies and the seeds are dispersed by the wind.
It grows easily and is best planted on rockeries in full sunlight. Propagation is easily effected by division, leaf cuttings or seed. Seed germinates within 3 weeks and plants should flower in the fourth year. Leaf cuttings can be made during spring or summer and rooted in clean sand. They must be kept moist. Sulphur should be applied as a fungicide to wounds. In the Western Cape direct transplanting is best done in autumn to coincide with the rainy season, or the young plants must be watered regularly until established.
References: Guide to Garden Succulents – Gideon F Smith & Ben-Erik van Wyk