Aeonium
Aeonium is a genus of subtropical succulents in the family Crassulaceae, best known for their flat, symmetrical rosettes of thin leaves held aloft on branching, woody stems. Most species hail from the Canary Islands, with a handful reaching Madeira, Morocco and East Africa. Unlike many succulents, aeoniums are winter growers: they are lush and active through the cool, damp months and slip into dormancy over hot, dry summers — a rhythm that catches out many newcomers.
Description
Aeoniums are grown above all for their rosettes, which range from tight buttons a few centimetres wide to broad, plate-like crowns 30 cm or more across. The leaves are typically thin, smooth and somewhat waxy — more like a fleshy leaf than a plump water-storing organ — and are often fringed with fine hairs or "eyelashes" along the margins. Colour varies enormously, from apple-green through chocolate-purple to near-black, and many forms flush deeper red in strong light.
Growth habit spans from low, stemless mats to shrubby plants a metre or more tall, with rosettes carried at the tips of bare, branching stems that scar attractively where old leaves have dropped. Most species are monocarpic at the rosette level: a rosette that flowers will die after setting a large, conical spray of small starry blooms, usually yellow, white or pink. On a well-branched plant only the flowering head is lost, and the rest carries on.
Distribution and habitat
The genus is centred on the Canary Islands, where different species have radiated to fill nearly every niche — coastal cliffs, laurel forest, sunny rock faces and high volcanic slopes. Further outposts occur on Madeira, in the mountains of Morocco, and across to Ethiopia and the Arabian Peninsula. In their native range the plants take advantage of cool, wet Mediterranean-type winters and rest through the parched summer, often shrivelling their rosettes into tight buds to conserve moisture.
Notable species
- Aeonium arboreum — the classic shrubby "tree aeonium"; parent of many dark cultivars.
- Aeonium 'Zwartkop' — an intensely dark-purple to black-leaved selection, one of the most popular of all succulents.
- Aeonium tabuliforme — the "dinner-plate aeonium", a flat, stemless, tightly overlapping rosette.
- Aeonium haworthii — a freely branching, small-rosetted shrublet known as the "pinwheel".
- Aeonium canariense — large, soft, velvety green rosettes hugging the ground.
- Aeonium undulatum — tall, single-stemmed, with a big glossy rosette on a bare trunk.
Cultivation
The single most important thing to understand about aeoniums is their winter-growing cycle. Water and feed them through autumn, winter and spring, when they are actively growing, and ease right off in the heat of summer, when they naturally rest. A summer-dormant rosette that curls its leaves inward and drops the outer ones is behaving normally, not dying — resist the urge to soak it.
Grow them in bright light for the best colour and the most compact rosettes; too little light stretches the stems and fades dark forms toward green. A free-draining but not purely mineral mix suits them, as their thin leaves appreciate a little more moisture-holding than most desert cacti. They are frost-tender and are best kept above freezing; in cold climates grow them in pots that can come indoors, or as summer container plants. See Watering and Repotting for general technique. Aeoniums have shallow roots and top-heavy rosettes, so a wide, stable pot helps.
Propagation
Aeoniums are among the easiest succulents to propagate from stem cuttings. Take a rosette on a length of stem, let the cut end callus for a few days, and root it in a gritty mix during the growing season — autumn is ideal. Many branching species also offer side-rosettes that can be removed and rooted the same way (see Propagation — cuttings and Propagation — offsets). Species can be raised from seed as well, which is the main route for producing new variation, though named clones are kept going vegetatively so they come true.
Hobby and cultivar notes
Aeoniums have been selected and hybridised extensively for leaf colour, rosette size and variegation. Dark-leaved lines such as 'Zwartkop' (also sold as 'Schwarzkopf') and softer variegated forms like 'Sunburst' and 'Mardi Gras' are staples of the succulent trade. Variegated forms often want a touch more light to hold their pattern, and being lower in chlorophyll they can grow more slowly and sunburn more easily than plain green plants. Cristate (crested) forms, whose growing point has fasciated into a fan or ruffle, also turn up in the trade. Because the genus crosses readily, a great many garden hybrids exist; when in doubt, care follows the same winter-growing pattern as the species above.
Common problems
- Summer "collapse" — often just normal dormancy; a rosette pulling its leaves into a bud and shedding outer leaves in heat is resting, not rotting.
- Etiolation — thin, stretched stems and pale, loose rosettes from too little light.
- Rot — from overwatering, especially if watered heavily during summer dormancy or left cold and wet.
- Pests — aphids on the soft flower stalks, and mealybugs tucked into the rosette centres (see Pests and diseases).
See also
- Crassulaceae — the stonecrop family
- Aeonium arboreum · Aeonium tabuliforme · Aeonium 'Zwartkop'
- Propagation — cuttings · Soil and potting mix · Watering · Repotting