The Aeonium are other-wordly plants that you might imagine growing in the deserts on the planet Mars. They form succulent rosettes with green, black or purple leaves and take a level of neglect that always endears them to busy homeowners.
The plants are most commonly grown indoors, often in conservatories where light levels are high. But they like to be stood outside in the summer and can add great interest to a patio display.
There are around 35 species of Aeonium, most of them native to the Canary Islands. The other species are found on Madeira and in north Africa. They are closely related to the genera Sempervivum (house leeks), Aichryson and Monanthes.
The species most commonly grown commercially is A. arboreum, which has thick sturdy bare stems with the potential to reach up to 1m tall, topped by rosettes of glossy green leaves. There are several excellent varieties, including the popular black-leaved 'Zwartkop' and the purple form 'Atropurpureum'. There is also one with enormous rosettes, appropriately named 'Magnificum'.
Another relatively tall species is A. balsamiferum, which also grows to 1m, has a branching form and is one of the hardiest forms. A. urbicum is of similar height and produces smaller rosettes in a wheel effect every few centimetres up the stem.
There are also low-growing species, some with no visible stem such as A. tabuliforme, which produces flat, green rosettes 25cm across. A. haworthii Award of Garden Merit (AGM) is just 30cm high and forms an attractive mound of bluish rosettes, making it a good container plant. Some, such as A. simsii, are small but form a branched system.
Aeonium do produce flowers - spikes made up of tiny yellow blooms. Having flowered, the rosette will die. This is not an issue for many species that produce offsets or side rosettes because the flowered stem can simply be cut off and new shoots will replace those removed.
Cutting older stems before they flower has the added benefit of preventing Aeonium becoming leggy. These cut stems can be used to create new plants. Cuttings take easily, though as with cacti, cut stem ends should be allowed to callous over before being put into a potting medium. The ideal compost is a gritty, loam-based mix.
Aeonium are not hardy so most people stand them out between May and September but bring them under cover for the rest of the year. It is a risk to overwinter them outdoors, even in sheltered areas, because they are likely to get too wet as well as too cold.
WHAT THE SPECIALISTS SAY
GILL STITT, owner, Shrubland Park Nurseries, Suffolk - "Succulents are always popular because of their geometrical shape, exotic appearance and easy cultivation. The purple 'Atropurpureum' and almost black-leaved 'Zwartkop' varieties of A. arboreum are much sought after.
"Many come from the Canary Islands, so they enjoy sun and heat and do well when grown in a conservatory. My favourite variety is probably A. arboreum 'Magnificum' - it has huge dinner plate-size rosettes.
"If grown in pots, A. arboreum and its varieties can get top-heavy. The stems are rather brittle so they need placing where they won't get knocked. They are not hardy, though may take a light frost if kept very dry, but we don't recommend trying to keep them outdoors over winter - the wet as much as the cold will be too much for them."
JULIE SMITH, co-owner, Hardy Exotics, Cornwall - "I love the different shapes, forms and colours of Aeonium. They look good all year round, so are great for containers and window boxes and are also ideal as house plants.
"My favourite is probably A. decorum 'Dinner Plate'. When mature, it has a thick branching stem about 1m tall and the rosettes are huge and it's a very sculptural plant.
"They must be grown in very well-drained conditions - we use about 25 per cent fine washed grit mixed in with a John Innes-type compost. Other tips are to water them sparingly and give them protection from frost."
NIKOLETT TOTH, nursery supervisor, Clifton Nurseries, London - "Aeonium should be treated like cacti - they should be watered sparingly at this time of year and need lots of light. In the summer they should be watered more often - about once a week or every 10 days.
"You need to be careful not to let them dry out too much, though they do cope with neglect quite well. They are semi-hardy. Some gardeners may be able to get them to survive outside all year round in London in a sheltered spot, though maybe not this winter.
"We stock a range of Aeonium in plastic pots ranging from 9-5cm in size. We also plant them up in containers. They look lovely in rusted containers with stones on top. We place them outside in the summer, displaying them with other succulents. The black-leaved 'Zwartkop' always sells well."
SPECIES AND VARIETIES
A. arboreum AGM (H1) is a shrubby succulent with glossy green leaves. Grow in a heavy pot for stability. Maximum height: 1m. Minimum temperature: 0-3 degsC.
A. arboreum 'Atropurpureum' AGM (H1) produces a mass of greenish rosettes, flushed purple, on branching stems. Height: 1m. Spread: 60cm.
A. arboreum 'Magnificum' features huge flattened rosettes of fresh green succulent leaves. Height: 45 cm. Temperature: 3-7 degsC.
A. balsamiferum forms tight compact rosettes that smell of honey. It is a branched species that forms a cone shape and one of the hardiest Aeonium. Height: 1m. Spread: 60cm.
A. 'Blushing Beauty' features large evergreen rosettes of red-purple flushed succulent leaves that curl up at the ends. Large, dense heads of yellow flowers are borne between March and June. Height: 70cm. Hardy to -4 degsC.
A. canariense has low-growing, very large rosettes to 30cm across. Side shoots develop after about three years and a large cone-shaped flower head appears with yellow flowers. Height: 15cm. Spread: 45cm.
A. 'Cristata Sunburst' has congested heads of half-hardy evergreen pink and cream rosettes. Height: 15cm.
A. decorum 'Variegatum' has striking coral-edged succulent cream rosettes that flower any time between April and November.
A. 'Dinner Plate' is a stunning form that grows huge dinner plate-sized rosettes on top of a single thick stem up to a metre high. Looks fantastic in a pot on the patio or in the conservatory.
A. haworthii AGM (H1) produces a mound of bluish rosettes 10cm across with a reddish edge. A good container specimen. Height and spread: 30cm.
A. haworthii 'Variegatum' is a well-branched spreading succulent with attractive rosettes. The new leaves are yellow with a pink margin, turning green with age but still retaining the pink margin. Panicles of pale yellow flowers appear in spring. Height: 60cm. Temperature: 0-3 degsC.
A. nobile has very large and heavy rosettes of brown-green leaves and heads of coppery red flowers between June and July.
A. simsii variegated is a low-growing branching succulent with soft, pale-green leaves with white margins. Height: 30cm.
A. tabuliforme AGM (H1) forms flat, fleshy green rosettes and produces yellow flowers between May and August. Height and spread: 25cm. Tender.
A. tabuliforme 'Cristatum' forms fleshy green rosettes 12cm across. Yellow flowers are borne between May and August. Tender.
A. urbicum forms a large open rosette with crinkly green leaves. Smaller rosettes are produced in a wheel effect every few centimetres up the stem. Height: 1m. Spread: 45cm.
A. 'Zwartkop' AGM (H1) is a popular form with very dark purple-black leaves. It responds well to pinching out, producing branches. Height: 60cm. Temperature: 3-7 degsC.