Aeonium is a genus of succulent flowering plants in the stonecrop family Crassulaceae, commonly known as tree houseleeks. The genus was formally established by Webb and Berthelot in 1836 in their Phytographia Canariensis, and today comprises roughly 35 accepted species placed in the subfamily Sempervivoideae and tribe Aeonieae.
Plants in this genus are small to medium evergreen succulents that form distinctive, colourful rosettes of thick, waxy leaves at the tips of their stems. Rosette diameter ranges from just a few centimetres in compact species to around 30 cm in larger ones. Growth habit is variable: some species stay low and clumping, while others develop woody, branching stems and grow into shrubs up to about 1.5 metres tall, giving rise to the common name "tree houseleeks." Stems are woody or fleshy, leaves are persistent, and the small star-shaped flowers are borne in terminal clusters, with free petals typically divided into 6 or 12 sections.
A defining feature of the genus is that most species are monocarpic at the rosette level: once a rosette produces its tall, branched central inflorescence, that rosette dies back after flowering. In practice the plant usually persists because it branches freely or produces basal offsets, so a mature specimen continues as a colony of rosettes at different ages. Aeonium simsii is a notable exception that does not follow this monocarpic pattern.
The genus is centred on the Macaronesian archipelagos, with the great majority of species endemic to the Canary Islands and a smaller number occurring on Madeira and the Cape Verde Islands. A handful of further species are found in Morocco and in East Africa — including Ethiopia, Somalia, Kenya, Uganda and Tanzania — and on the Arabian Peninsula in Yemen. Several familiar species, including A. arboreum, A. haworthii, A. canariense, A. decorum, A. tabuliforme and A. undulatum, are grown worldwide as ornamentals, and a number of cultivars such as 'Zwartkop', 'Sunburst', 'Blushing Beauty' and 'Cyclops' are widely traded in the succulent trade.
Etymology
The genus name Aeonium derives from the ancient Greek αἰώνιος (aiōnios), meaning "ageless" or "everlasting" — a reference to the persistent, evergreen rosettes that the plants maintain through the dry season.
Distribution
Aeonium is centred on the Macaronesian region of the eastern Atlantic. The Canary Islands are by far the richest centre of diversity, with most accepted species endemic to the archipelago. Smaller numbers of species occur on Madeira and on the Cape Verde Islands. Beyond Macaronesia, the genus extends to Morocco in north-west Africa, to several East African countries (Ethiopia, Somalia, Kenya, Uganda and Tanzania), and to Yemen on the Arabian Peninsula. Outside its native range, the genus is widely cultivated and some species have become naturalised in mediterranean-climate areas, including California.
Ecology
Aeoniums are leaf succulents adapted to the seasonally dry, mediterranean-type climates of their native islands and mountain habitats. Most species are monocarpic at the rosette level: an individual rosette grows vegetatively for several years, then produces a single large terminal inflorescence and dies after flowering and setting seed. Because most species branch freely or produce basal offsets, the parent plant typically persists as a colony of rosettes of different ages and only some rosettes flower in any given year. Aeonium simsii is a noted exception that does not follow the monocarpic pattern.
Cultivation
Aeoniums are among the most widely grown ornamental succulents. They prefer full sun to partial shade, where they develop their best leaf colour, and tolerate a wide range of well-drained soils of loam or sand with acidic to neutral pH; in containers a cactus mix is recommended to ensure drainage. They are highly drought tolerant and need only minimal water, and during their winter dormancy watering should be reduced to just enough to keep the leaves from shrivelling. The genus is generally rated for USDA hardiness zones 9–11; in cooler climates plants should be moved indoors before night temperatures drop to around 40°F (4°C). Common pests include aphids, mealybugs, spider mites, scale and ants, while overwatering causes root rot and excessive sun can scorch leaves. Many species and cultivars have been awarded by the Royal Horticultural Society; popular selections include the dark-leaved 'Zwartkop', the variegated 'Sunburst', 'Blushing Beauty' and 'Cyclops'.
Propagation
Aeoniums are easily propagated in spring or summer by seed, division of offsets, or stem and rosette cuttings — the same techniques used across most rosette-forming Crassulaceae.
Taxonomy notes
Aeonium was established by Webb and Berthelot in 1836 in their Phytographia Canariensis (volume 1, page 184, table 28) and is the type genus of the tribe Aeonieae within subfamily Sempervivoideae of the family Crassulaceae. The genus currently comprises roughly 35 accepted species; the GBIF backbone records 166 descendant taxa in total, reflecting numerous synonyms, infraspecific names and hybrid epithets in addition to the accepted species.
History
The genus was formally described by Philip Barker Webb and Sabin Berthelot in 1836 in Phytographia Canariensis, the botanical component of their landmark Histoire Naturelle des Îles Canaries, in which they documented the flora of the Canary Islands.
Conservation
The genus Aeonium is not currently recorded in the IUCN/ISSG Global Invasive Species Database, although several species are cultivated worldwide and have escaped into mediterranean-climate regions outside their native range.
