Mimosa (Albizia julibrissin)

The filigree leaves and showy flowers of the Persian silk tree, also called the mimosa tree, make it hard to deny its direct relationship to the mimosa. Persian silk trees are generally not very hardy, hence their limited distribution in Central European landscape gardening. Keep reading to learn how to grow Persian silk trees in your garden.

I. Appearance and Characteristics 

Albizia julibrissin is known by a wide variety of common names, such as Persian silk tree and pink siris. It is also called Lankaran acacia or bastard tamarind, though it is not too closely related to either genus. The species is called Chinese silk tree, silk tree or mimosa in the United States, which is misleading—the former 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.

It is a species of tree in the family Fabaceae, native to southwestern Asia and eastern Asia. The genus is named after the Italian nobleman Filippo degli Albizzi, who introduced it to Europe in the mid-18th century.

Albizia julibrissin is a small deciduous tree with a broad crown of level or arching branches, growing to 5–16 m (16–52 ft) tall. Its bark is dark greenish gray, becoming vertically striped with age. Its leaves are large and frond-like: They are bipinnate, divided into 6–12 pairs of pinnae, each with 20–30 pairs of leaflets. Individual leaflets are oblong, 1–1.5 cm (0.4–0.6 in) long and 2–4 cm (0.8–1.6 in) broad. The true leaves are 20–45 cm (8–18 in) long and 12–25 cm (5–10 in) broad.

The flowers bloom throughout the summer in dense inflorescences, which resemble starbursts of pink silky threads. The true flowers have small calyx and corolla (except the central ones), with a tight cluster of prominent stamens, 2–3 cm long and white or pink with a white base. They have been observed to attract bees, butterflies and hummingbirds. Its fruit is a flat brown pod 10–20 cm (4–8 in) long and 2–2.5 cm (0.8–1.0 in) 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

Original habitats of the tree include regions from Iran (Persia) and the Republic of Azerbaijan to China and Korea.

1. julibrissin is widely planted as an ornamental plant in parks and gardens, grown for its fine leaf texture, flowers and attractive horizontal canopy. Other positive attributes are a fast growth rate, low water requirements and the ability to thrive planted in full sun in hot summer climates. It is frequently planted in semi-arid areas like California’s Central Valley, central Texas and Oklahoma. Although capable of surviving drought, growth will be stunted and the tree tends to look sickly. As such it should be given infrequent, deep waterings during the summer, which will benefit growth and flowering.

II. How to Grow and Care

Sunlight

The tree tolerates partial shade but is generally considered intolerant of shade. A. julibrissin will thrive in full sunlight, promoting fuller foliage and abundant flowering. 

Temperature and Humidity

Though it thrives in the higher temperatures of the southern USDA zones, the silk tree is tolerant of low temperatures. Young plants, however, are frost-tender and will not survive hard winters.

Watering

The tree’s habit of naturalizing in wet to dry sites shows that A. julibrissin is adaptable regarding moisture conditions. Giving a young tree that is freshly planted a thorough weekly soaking is essential until the roots have been established. After the first season, no supplemental watering should be needed.

Soil

The silk tree tolerates a wide range of soil conditions and is even adapted to poor varieties. The tree’s nitrogen-fixing properties allow it to grow well in soils where other plants would suffer.  

It can also handle acidic to moderately alkaline soil pH with ease and is a good choice for landscapes that need solutions for high-alkaline areas.

Fertilizing

Fertilise your silk tree in the spring with a high-quality slow-release fertiliser. Since these trees are not very salt-tolerant, opt for a primarily organic fertiliser rather than a mineral one.

Planting Instructions

Aesthetically speaking, it is best to plant just one solitary Persian silk tree. The best time to plant it is after the last frosts around mid-May. To do this, first dig a generous planting hole. Mix the excavated soil with sand or gravel to optimize permeability in clay-rich, heavy soils. Then simply place the plant in the hole, press down and water everything in.

Planting a Persian silk tree in a container: Albizia julibrissin varieties with a slightly lower growth height are particularly suitable as container plants. Especially in regions with harsher climates, this option is ideal for safe overwintering. Start by creating a drainage layer of expanded clay at the bottom of the container, and then follow this with a well-drained, humus-rich soil. Our Plantura Organic All Purpose Compost is well-suited for this purpose, as it contains added lime and magnesium as well as organic fertiliser to optimally support your Persian silk tree throughout the growing period. Due to its high humus content, it is rich in nutrients and also retains water well, making it ideal for container and bed plants.

Pruning

Prune the tree in fall or winter during its dormant period. If you prune early in the tree’s life, you can narrow it to a central trunk, though some prefer to allow it to have several smaller trunks. Regardless of that choice, it’s best to prune the top of the tree to a flat shape, then prune back the branches to five or six buds on each. Remove dead, diseased, or weak limbs at any time during the year.

Propagation

Cuttings

Silk tree is easily propagated through seed or cuttings. To propagate through cuttings, take the following steps:

• Choose a 6-inch healthy stem in late spring. Choose a branch that has not yet bloomed.
• Remove all but the top two or three leaves, dip the stem into water, and then into rooting hormone.
• Immediately place the cutting into a 4-inch pot that has been filled with high-quality potting soil.
• Place the pot in a plastic bag, then set it in an area of sunny yet indirect light, with a temperature of about 70 degrees Fahrenheit. Keep the soil moist.
• Expect roots to grow in three to four weeks.

Seed

Silk trees love to spread seeds, so harvesting them is not difficult. Collect seeds in the fall and store them in a cool, dry place until spring. To grow this tree from seeds, take the following steps:

• For the best results, scrape each seed gently with a file or rasp until the hard outer coating is slightly breached.
• Place the seeds in an insulated container and pour hot, almost boiling water, over the seeds.
• Seal the container and allow them to sit in the hot water for 24 hours.
• Dry the seeds and gently press them into the middle of a 3-inch pot filled with compost and perlite in equal amounts. Place the seed one inch deep.
• Place the pots outside, preferably against a shady, south-facing wall.
• Keep the compost mixture moist. Seeds can take anywhere from seven days to several months to germinate.
• When starting as seedlings, it’s fine to keep the silk tree in its container until it begins to show roots through the drainage holes. At that point, it’s time to transplant the tree to its permanent home in your landscape. When doing this, carefully place them in soil that has been amended with the same type of compost they began growing in.

Potting and Repotting 

When repotting a silk tree seedling, choose a container that is at least 4 inches larger than the last pot. As the tree grows, you will need to repot it into a larger pot. If you are potting a larger tree, make sure you choose a container that is twice the size of the root ball. Any container material will do for a Persian silk tree but make sure it has drainage holes. Water weekly because plants in containers typically need more moisture.

Overwintering

This tree can tolerate temperatures down to about 25 degrees Fahrenheit but does much better in areas where the winters aren’t as harsh. No matter the zone, make sure your silk tree is planted in full sun and in an area where it is protected from strong winter winds.

Pests and Diseases

Common Pests

A variety of insects will feel quite at home in the silk tree, including soft-bodied creatures like mites and mimosa webworms. Cottony cushion scales are also attracted to this tree.3 Look into insecticidal treatments to help eliminate them.

Common Problems 

The Persian silk tree is sometimes plagued by mimosa wilt, a fungus that attacks the vascular system of the tree. The foliage will begin to appear yellow and wilting, then turn brown and fall off the branches. The branches will also begin to fall from the tree. Unfortunately, most trees with this problem die within a year, and it can spread to other plants in the vicinity. If your silk tree is infected, immediately remove it from the landscape.

III. Persian silk tree varieties

One thing in which the many varieties of Albizia julibrissin differ is their degree of frost hardiness. Take this into consideration if you plan on keeping this tree somewhere where it is not warm.

• Albizia julibrissin ˈOmbrellaˈ: this commercial variety is quite similar to regular silk trees, but it is very vigorous and tolerates temperatures of -15 °C for a short time. Albizia julibrissin ˈOmbrellaˈ grows umbrella-shaped and reaches a height of about 8 – 10m.
• Albizia julibrissin ˈErnest Wilsonˈ: this variety is much more frost-hardy than the regular species. It can withstand temperatures down to about -23 °C. Due to the somewhat stocky growth with a height of 4 – 5m, it is more round in shape.
• Albizia julibrissin ˈSummer Chocolateˈ: with its unusual foliage, this commercial variety makes quite an impact. The leaves, which are fresh green when they first appear, later turn a chocolate brown and finally turn golden-yellow in the autumn. The ˈSummer Chocolateˈ variety of silk tree reaches a height of about 2 – 4m and is hardy to about -17 °C.
• Albizia julibrissin ˈPendulaˈ: the peculiarity of this variety lies in the arching, overhanging shoots with pink flowers. It can reach a height of up to 7m and can withstand temperatures down to about -17 °C.
• Albizia julibrissin ˈEvey’s Prideˈ: the deep brown-red foliage of this variety adds exciting pops of colour to any garden. It grows up to 7m high and bears bright pink flowers. This variety is winter hardy down to about -17 °C.

IV. Uses and Benefits 

Silk tree wood may be used to make furniture. It is also reportedly psychoactive as well, with the flowers being used to make teas. In Chinese medicine it is considered an antidepressant.