Albizia julibrissin Seeds | Silk Tree

Categories：Tree

Silk Tree (Albizia julibrissin) — Silk Tree (*Albizia julibrissin*): Fabaceae, mimosoid clade — Albizia, native to Iran and Afghanistan east through the Himalaya to China, Korea and Japan.

LATIN NAME: Albizia julibrissin

COMMON NAME: Silk Tree, Persian Silk Tree

Plant Profile

The filaments open from rounded heads carried on stalks above the leaf axils, and they last through much of the warm season rather than in a single burst. Because the reward is nectar at the base of each filament, the tree is worked steadily by bees and butterflies, and in the southern United States by hummingbirds. Because the heads open in succession rather than in a single burst, a street lined with the tree stays pink for a whole season.

The twice-divided leaves carry many small leaflets, and each leaflet sits on a pulvinus, a swollen joint at its base. The pulvinus moves ions and water between cells on opposite sides, so the leaflets swing down together as the light fades and lift again in the morning. The movement is a sleep rhythm driven by an internal clock rather than a response to damage, and the whole crown appears to close at night and reopen with the light.

The reason a silk tree so often looks magnificent at fifteen years old and dead at twenty-five is a soil-borne fungus, Fusarium oxysporum f. sp. perniciosum, which blocks the water-conducting tissue of the roots and trunk. There is no cure. A wilting crown that yellows and dies back from the top is the usual sign, and the pathogen survives in the ground long afterwards, so a replacement planted in the same spot usually follows the first tree. Bark wounds at the base make infection more likely, which is one reason to keep a string trimmer away from the trunk.

Science & Biotechnology

An amino acid isolated from the seeds of this tree was named albizziin after the genus and became a laboratory tool in amino-acid research because it interferes with the handling of glutamine in cells. The bark and the flower heads carry triterpenoid saponins and flavonoids, and both are collected in Chinese practice for calming preparations, the bark being the part most often traded. The tree is therefore studied as a drug plant and as an ornamental at the same time.

Uses & Benefits

The genus carries the name of Filippo degli Albizzi, an Italian nobleman who sent seed from Constantinople to Florence in the 1740s and so introduced the tree to European gardens. The species epithet comes from a Persian phrase meaning silk flower, a direct description of the filaments. Both halves of the botanical name are therefore about people and trade routes rather than about the plant’s own structure, which is unusual enough to be worth remembering.

Growing from Seed

Seed from northern populations is hardier than seed from the south, and the selection named after the plant hunter Ernest Wilson is the one usually recommended for cold districts because it flowers deeply pink and survives harder winters. Even so, a severe frost can cut a young tree to the ground, and it will then regrow from the base as a multi-stemmed shrub. In parts of the southeastern United States the species seeds so freely that it is listed as invasive, so check local guidance before planting it near wild ground.

The fruit is a flat, papery pod that ripens from green to straw brown and holds eight to twelve seeds in a single row. The seed coat is hard and sheds water, a physical dormancy that keeps the seed asleep until the coat is breached. Nicking it with a file on the side away from the hilum, or soaking it in near-boiling water as it cools, is enough to let water in.

Sow treated seed in a gritty, open mix, covering it by about its own depth, and keep it warm and just damp. Seedlings grow fast and resent being held in a small pot, so move them into deep containers early. Plant out at five to eight metres apart, in full sun, into soil that drains. In the first summer, a deep soaking once the surface has dried suits the tree far better than frequent light watering, which encourages a shallow root system.

The tree flowers on new wood, so winter pruning costs nothing in bloom and is the moment to take out crossing branches and any dead spurs. Where the species is killed back by cold, cut to live wood once growth has started so the cuts can seal. The timber is light and the forks are narrow, so remove the weak junction growth while the branches are still thin enough to cut with a handsaw.

Our Take

I like this species most for a lesson rather than a border: put it beside the sensitive plant and you can show the difference between a nightly sleep rhythm and a rapid touch response in the same afternoon. For durable timber and the same pink pompom in a colder climate, the related Chinese albizia is worth the space, and lebbeck shows how large the genus becomes in the tropics.

Tree Seeds / Tree