Depending upon the classifier, 300 plus to about 600 species of annuals, biennials, perennials and subshrubs from the northern hemisphere and tropical mountains just south of the Equator. Perhaps also in Madagascar and temperate South America, much depending upon the botanical authority. They are varied in habit but generally have succulent, often very thick and fleshy leaves. The small starry to bell-shaped flowers can have three to eleven free petals (most species have five or six), arranged in dense to loose cymes, panicles or racemes. With the exception of the species in section Rhodiola (6) which are dioecious, sedum flowers are hermaphrodite and seed is usually freely set. Some botanists have removed the more distinctive species and placed them in genera of their own. However, opinions among taxonomists are not unanimous and for the purposes of this Encyclopaedia the following 'splits' are retained in Sedum: Gormania, Clementsia, Hylotelephium, Meterostachys, Orostachys, Rhodiola, Sinocrassula and Telephium.
All the more decorative, smaller, hardy species can be grown on the rock garden, raised bed, scree or dry wall and several have long been favourite for this purpose. Well drained soil and a sunny site are basic requirements for most species. Any necessary further comments on cultivation will be found under the species descriptions. Propagation by seed in spring, division where possible in late summer or spring, leaf and stem cuttings in summer.
For convenience, the many diverse species have been classified into eight groups based on habit, foliage and floral characters. In some cases the groupings also indicate cultural preference.
7. Mexican Group
12. Telephium (Hylotelephium)
DESCRIPTIONS OF SUBGENERA AND GROUPS
1. Afrosedum (less than ten species)
Small erect or spreading subshrubs with small fleshy, blunt, terete or somewhat flattened leaves. Flowers yellow in short stalked inflorescences. Best grown in the alpine house, dry over winter as this group originates from the Iberian Atlantic Islands, the Atlas Mountains and areas adjacent to the Great East African Rift Valley.
2. Aizoon (about ten species)
Perennial plants rising from a woody rootstock and (with one exception) dying back in winter. Flowers yellow, in cymes. The thin, flat leaves are opposite (in one instance ternate) and usually dentate. The stems are erect or decumbent and the fruit can be colourful. Siberia, northern China and islands adjacent to north-eastern Asia.
3. Cyprosedum (several species)
Mostly biennial rosetted plants with terminal, many flowered panicles of small, yellow to reddish flowers with spreading petals. From the mountains of Crete and Cyprus. Need specialist culture.
4. Epeteium (several species)
Small annuals or biennials with mostly flat leaves. Cymes of white or yellow flowers (in one instance sky blue). From lower, wetter temperate areas. Will self-seed if the soil is left undisturbed. Although this group is often dispersed by modern taxonomists, all the species have much in common as far as the alpine gardener is concerned.
5. Genuina (hundreds of species)
Mostly low, perennial, evergreen mat-formers without a rootstock (roots fibrous), composed of creeping stemlets and ascending shoots. The flowering branches arise between the leafy ones, the latter persisting while the flowering stems die annually. Leaves generally fleshy, terete or flattened, often imbricate and nearly always alternate though sometimes (especially in the American species) crowded into rosettes. Flowers mainly white, pink or yellow. Most of the common, typical species are in this group.
6. Meterostachys (one species)
Tiny monocarpic rosettes of terete, callus-tipped leaves reducing to a compact winter bud. Japan. Alpine house cultivation in winter.
7. Mexican group (a few contrasting species from very diverse subgenera brought together for convenience).
High altitude plants, some of which have been highly favoured by succulent plant and alpine specialists. Half-hardy.
8. Orostachys (about twelve species)
Biennial or perennial, dense monocarpic rosettes from northern Asia (Urals to Japan). The terminal panicles, racemes or spikes of whitish flowers are outstanding. Choice specimen plants for the alpine house, though some will grow outdoors with shelter.
9. Prometheum (two species)
Biennial, monocarpic rosettes very similar to Sempervivum. Inflorescence in a wide panicle, crowded at the stem tips. Pedicels densely-leaved beneath the flowers. Petals pink or red, erect with recurved tips. Asia Minor to Caucasus.
10. Rhodiola (about sixty species)
Herbaceous perennials with annual, unbranched stems rising from a fleshy caudex. The thinly succulent, usually dentate leaves are normally alternate. The four or five-partite dioecious or hermaphrodite flowers are produced in cymes. Male flowers have rudimentary carpels and the female flowers have no stamens and often have small or absent petals. From subarctic tundra (right around the northern polar zone) to high alpine regions further south and rocky coasts. Most frequent in the central Asian mountains. Choice specimen plants, sometimes difficult to maintain and propagate, with the exception of a few species like S. rosea.
11. Sinocrassula (about six species)
Biennial or perennial rosetted plants with dark, succulent, finely spotted or lined leaves. The flowers, like Crassula, have only a single whorl of five stamens. China.
12. Hylotelephium (about twenty-five species)
Mostly tall herbaceous perennials with large, flat, thinly succulent leaves which can be alternate, opposite or ternate, carried on rising or decumbent stems from a carrot-like rootstock. Cymes, corymbs or umbels of bright pink or white (rarely green and never true yellow) flowers are produced in late autumn. Widespread across Eurasia to Japan, plus one American species.