The Albizia Bonsai: The Silk Tree That Sleeps

The Persian silk tree (Albizia julibrissin), often also called "Silk Tree" or "Mimosa Tree", is a bonsai that brings a touch of the exotic and of delicacy. Belonging to the Fabaceae family (legumes), it stands out for two unique features: its summer pink flowers shaped like a soft, scented powder puff, and its compound leaves which, thanks to a phenomenon called nyctinasty, fold up on themselves at sunset or when touched, as though the plant were going to sleep. It is a fast-growing species, ideal for those who like to see quick results.

Origin & species

Native to a wide belt running from Iran to Japan, it was introduced into Europe in the 18th century by a Florentine naturalist who brought it back from Constantinople (hence the Italian common name). It is a deciduous tree that in nature takes on an umbrella shape. The variety "Summer Chocolate", with bronze/purple leaves, is much appreciated in bonsai for its colour contrast, but it is slightly more delicate than the typical green species.

Exposure and microclimate

The Albizia is a child of the sun. It loves heat and strong light.
Place it in full sun for most of the year. Light is essential for flowering and for keeping the leaves (which tend to be large) to an acceptable size.
In summer, if you live in a scorching area, a little shade during the hottest hours prevents the tender leaves from wilting excessively through transpiration, but do not deprive it of light, otherwise the branches will stretch out far too much.

Watering

It needs regular water but will not tolerate constantly "wet feet".
Water generously when the soil mix begins to dry out at the surface. In summer it drinks a great deal because of its large leaf surface. However, reduce water in late summer to help the wood harden off before winter. If the leaves close during the day (and it is not night), the plant is telling you it is thirsty!

Water quality

It is not particularly fussy. Tap water is fine. Being a legume, it adapts well to different pH levels, although it prefers neutral or slightly acid soils.

Substrate

Drainage is essential, but nourishment is also needed to sustain the rapid growth.

• Recommended mix: 50% Akadama (for structure), 30% pumice and 20% humus or general-purpose soil mix.
• Adding an organic component is important to retain the moisture needed during the hot months, since the Albizia suffers if the root ball dries out completely.

Repotting

This is done in late spring (May), when temperatures are steadily mild and the buds are swelling.
The root system is vigorous and fleshy. Repot young specimens every 2 years. During repotting you can shorten the thickest roots, but try not to disturb the central root ball too much. Use slightly wider pots to accommodate roots that spread horizontally.

Fertilizing

Watch out for nitrogen!
Like all legumes, the Albizia fixes atmospheric nitrogen. If you use fertilizers that are too high in nitrogen, you will get huge leaves, long internodes and no flowers.
Use a balanced organic fertilizer (balanced NPK) in spring, and switch to one high in potassium (K) and phosphorus (P) before summer to encourage flowering.

Flowering and fruiting

It flowers in midsummer (June-July) with pink/white inflorescences made up of long, silky stamens.
After the flowers it produces long, flat pods (legumes) containing the seeds. On a bonsai these pods drain energy and look unattractive if left in quantity: remove them as soon as they form so as not to weaken the tree.

Structural pruning

Carried out in late winter, before the tree wakes up.
Albizia wood is fairly brittle and tends to snap if bent badly, but cuts heal reasonably well. Remove branches growing vertically or crossing the trunk. Since it tends to lose its lower branches if they do not get light, prune the apex decisively to give strength to the lower part.

Maintenance pruning

This is a plant that grows before your eyes. During the growing season, shorten branches that extend beyond the outline.
However, if you want flowers, remember that these are borne on the current year's growth (at the tips of the new branches). If you keep trimming everything back until July, you will cut away the buds!

Pinching

Carried out in spring on strong shoots that are not needed for flowering. Trim back to 2-3 leaves to keep the canopy compact. If the aim is structure rather than flowers, pinch regularly throughout the summer.

Wiring & bending

The bark of the Albizia is thin and delicate and marks very easily.
Apply the wire in late spring on young branches that are still flexible. Protect the branch with crepe paper or raffia before putting on the wire. Check every 15 days: the branches thicken quickly and the wire bites in no time.

Diseases and pests

• Acacia psyllid: a small insect that attacks new shoots, deforming them.
• Cottony mealybug: often nests in the forks of the branches.
• Fusarium: a vascular fungus that can strike if the soil does not drain well (it causes sudden wilting of branches).

Winter & protection

It is deciduous and loses its leaves. It withstands moderate cold but fears prolonged hard frosts, especially on the roots.
In northern Italy, protect the pot with mulch or move it into a cold greenhouse. Beware of late spring frosts, which can burn the first delicate shoots.

Yearly calendar

• January-February: dormancy, structural pruning.
• March-April: waking up, repotting (late spring).
• May-June: fertilizing (low nitrogen), selective pinching.
• July-August: flowering (a real show!), generous watering.
• September-October: removal of pods, PK fertilizer.
• November-December: leaf fall, protection.

Useful products for this bonsai

• Akadama Hard Quality (for healthy roots)
• Fertilizer for flowering (low nitrogen for the Albizia)
• Medium scissors (for green pruning)
• Copper-coloured aluminium wire (gentle on the bark)
• Cut paste (to protect fragile cuts)
• Worm humus (to enrich the substrate in summer)

Frequently asked questions about the Albizia

Why do the leaves close during the day?
If the leaves close while there is still light, the plant is under water stress (it is thirsty) or is suffering excessive heat. Water it at once and move it into the shade for a few hours.

Is it the same as the Mimosa?
It is often called "Mimosa", but the true Mimosa (Acacia dealbata) has little yellow pompom flowers that open in February/March. The Albizia has pink powder-puff flowers that open in summer. They are cousins (same family), but different plants.

Why doesn't it flower?
The Albizia flowers at the tips of the new branches produced during the year. If you keep pruning the branches to keep it in shape until summer, you cut the flowers away. To see it in bloom you have to let it grow freely (and a little untidy) from May onwards.