There are 43 accepted species in the genus Aeonium. They are distributed from Macaronesia through North Africa to the Arabian Peninsula. Most species are found only on the Canary Islands, where they may be restricted to one or two islands, and are usually found growing on cliffs or rocky areas, in well-drained soil in full sun or partial shade. The minimum temperature to cultivate most Aeonium species is -3 degrees Celsius, if they are kept on the dry side. Consequently, most species are unsuitable for growing outside in the United Kingdom, except in mild areas where there is perfect drainage. As a result, tree houseleeks make good pot plants for a cool conservatory, where they require little watering given their succulent nature. However, taller varieties will need additional support, as they can become top heavy. Tree houseleeks are grown in the Arid House at Oxford Botanic Garden, alongside other succulents and cacti.
Tree houseleeks are characterised by a rosette of fleshy leaves. There is considerable variation in habit among species. A mature plant of Aeonium arboreum may reach one metre in height, bearing rosettes atop woody, branching stems, whilst Aeonium tabuliforme has ground-hugging rosettes, which look like plates up to 30 centimetres in diameter. The inflorescences of tree houseleeks, which arise from the centre of the leaf rosettes in rounded or pyramidal panicles, comprise many star-shaped flowers. Once the rosette has flowered it will die, and in cultivation is best removed. Additional branching produces more rosettes, except in the single rosette types where the whole plant dies after setting abundant seed.
There are many striking tree houseleeks, some of which have varietal names. Aeonium arboreum ‘Schwarzkop’ has deep purple, nearly black, leaves, against which the yellow flowers look very striking. Aeonium arboreum ‘Atropurpureum’ is similar, but with red-flushed leaves. Other species have been selected for their leaf variegation, such as Aeonium haworthii ‘Variegatum’, which has tri-coloured rosettes that start out lemon-yellow and become green with a red edge as they age. It is important to remember that good light is needed to maintain these colours. During winter, colours are often less striking but, as the days lengthen, colour intensity is restored.
Tree houseleeks are easy to propagate by tip cuttings. Other methods of propagation include leaf cuttings and seed. Pests are not usually too much of an issue, although mealy bug aphids and vine weevils can sometimes be a problem.
Huxley, A (1999) The New Royal Horticultural Society dictionary of gardening. Groves Dictionaries Inc.
Phillips, R and Rix, M 1997. Conservatory and indoor plants. Pan Books.