Albizia julibrissin is a species of legume in the genus Albizia, native to southern and eastern Asia, from Iran east to China and Korea.
The genus is named after the Italian nobleman Filippo del Albizzi, who introduced it to Europe in the mid-18th century, and it is sometimes incorrectly spelled Albizzia. The specific name julibrissin is a corruption of the Persian word gul-i abrisham which means 'silk flower'.
Albizia julibrissin is known by a wide variety of common names, such as Persian silk tree or pink siris. It is also called Lenkoran acacia or bastard tamarind, though it is not too closely related to acacias (Acacieae), let alone tamarinds (Caesalpinioideae). The species is usually called "silk tree" or "mimosa" in the United States, which is misleading - the former name can refer to any species of Albizia which is most common in any one locale. And, although once included in Mimosa, neither is it very close to the Mimoseae. To add to the confusion, several species of Acacia, notably Acacia baileyana and Acacia dealbata, are also known as "mimosa" (especially in floristry), and many Fabaceae trees with highly divided leaves are called thus in horticulture.
Its leaves slowly close during the night and during periods of rain, the leaflets bowing downward as if the tree were sleeping. In Japan its common names are nemunoki, nemurinoki and nenenoki which all mean "sleeping tree". Nemu tree is a partial translation of nemunoki.
A number of now-invalid scientific names (junior synonyms) are known for this plant:
Acacia julibrissin (Durazz.) Willd.
Acacia nemu Willd.
Albizia nemu (Willd.) Benth.
Albizzia julibrissin Durazz. (orth.var.)
Feuilleea julibrissin (Durazz.) Kuntze
Mimosa julibrissin (Durazz.) Scop.
Mimosa speciosa Thunb.
Mimosa speciosa Jacq. is Albizia lebbeck
Sericandra julibrissin (Durazz.) Raf.
Additionally, the Albizia julibrissin of John Gilbert Baker and authors referring to him is actually Albizia kalkora (Roxb.) Prain.
Albizia julibrissin is a small deciduous tree growing to 5–12 m tall, with a broad crown of level or arching branches. The bark is dark greenish grey in colour and striped vertically as it gets older. The leaves are bipinnate, 20–45 cm long and 12–25 cm broad, divided into 6–12 pairs of pinnae, each with 20–30 pairs of leaflets; the leaflets are oblong, 1–1.5 cm long and 2–4 mm broad. The flowers are produced throughout the summer in dense inflorescences, the individual flowers with no petals but a tight cluster of stamens 2–3 cm long, white or pink with a white base, looking like silky threads. They have been observed to be attractive to bees, butterflies and hummingbirds. The fruit is a flat brown pod 10–20 cm long and 2–2.5 cm broad, containing several seeds inside.
There are two varieties:
Albizia julibrissin var. julibrissin. The typical variety, described above.
Albizia julibrissin var. mollis. Differs in the shoots being densely hairy.
There is also a form, Albizia julibrissin f. rosea which has, in the past, been classed either as a variety or as a cultivar. This is a smaller tree, only growing to 5–7 m tall, with the flowers always pink. Native to the northeast of the species' range in Korea and northern China, it is more cold-tolerant than the typical form, surviving temperatures down to at least -25 °C.
Persian silk tree is widely planted as an ornamental plant, grown for the leaf texture and flowers. The broad crown of a mature tree makes it useful for providing dappled shade.
In the wild, the tree tends to grow in dry plains, sandy valleys, and uplands. It has become an invasive species in Japan and the United States, in the latter widely spread from southern New York, New Jersey, and Connecticut west to Missouri and Illinois and south to Florida and Texas; it is also cultivated in California and Oregon but is not invasive there. It also grows in Europe, for example, in Crimea. Breeding work is currently under way in the United States to produce ornamental plants which will not set seed and can therefore be planted without risk. However, in the eastern United States it is generally a short-lived tree as this species is highly susceptible to mimosa vascular wilt, a fungal disease caused by a species of Fusarium, though the disease does not seem to have seriously impacted its populations. Because of its invasive tendencies and disease susceptibility, it is rarely recommended as an ornamental plant in the US, though it is still widely planted in parts of Europe.
The flower colour of Albizia julibrissin varies from white, in A. julibrissin f. alba, to rich red-tipped flowers; cream- or pale yellow-flowered variants are also reported. Seedlings vary: those with darker pink flowers than the normal A. julibrissin are classed as A. julibrissin f. rosea, but some may be paler than others; the selected cultivar A. julibrissin 'Ernest Wilson' (also known as 'E.H.Wilson' or 'Rosea') is a cold-tolerant tree with deep pink flower colour. Other cultivars are becoming available: A. julibrissin 'Summer Chocolate' has red foliage ageing to dark bronze, with pale pink flowers; A. julibrissin 'Ishii Weeping' (or 'Pendula') has a drooping growth habit.
In Japan, Albizia julibrissin f. rosea is often used for non-traditional bonsai.
The bark or cortex is used to cure bruises and as a vermicide. The seeds are used as a food for livestock and wildlife, and the sweetly-scented flowers are a good nectar source for honeybees.