Jooni

An exemplary photo of a False Acacia bonsai

False Acacia

1 Species

Species

Robinia pseudoacacia

The false acacia, also called black locust (Robinia pseudoacacia), is a fast-growing deciduous legume with pinnate leaves, deeply furrowed bark and thorny shoots; mature trees produce fragrant, hanging clusters of white flowers in late spring. A false acacia bonsai is strictly an outdoor tree that needs full sun and a cold winter dormancy.

It is a good choice for beginners because it is tough, hardy and recovers well from pruning, though its long internodes, large compound leaves and brittle branches make fine ramification a longer-term project. Wear gloves when working on it, as the paired spines are sharp.

At a glance Robinia pseudoacacia

Difficulty
Beginner
Placement
Outdoors
Light
Full sun
Watering
Moderate
Min. temperature
-10°C (14°F)
Foliage
Deciduous

Fertilizing

Feed every 2–4 weeks from leaf-out in spring until late summer with a balanced or low-nitrogen bonsai fertilizer. As a legume, Robinia can fix nitrogen with the help of root bacteria, so nitrogen-rich feeds only produce long, coarse shoots.

  • A feed with a little more phosphorus and potassium in spring supports roots and flowering.
  • Do not feed for about four weeks after repotting.
  • Stop feeding in autumn and do not feed during winter dormancy.

Placement

Keep your false acacia bonsai outdoors all year in full sun, with at least six hours of direct sun a day. It tolerates light shade, but it then grows leggier and rarely flowers.

Shelter it from strong winds, which can snap its brittle branches. The species is very hardy; protect the roots in the pot only when temperatures fall below -10°C (14°F).

Pruning

Do structural and hard pruning in late winter or early spring, just before the buds break; cuts made then heal fastest. During the growing season, cut back long shoots that extend beyond the silhouette, always leaving a few leaves on each shoot.

  • Remove basal suckers promptly unless you want them for the design.
  • Never remove all growing tips at once, as this weakens the tree.
  • Wire young branches carefully with the correct gauge, as older wood is brittle; informal upright and multi-trunk styles suit it best.

Repotting

Repot young trees every 2–3 years and mature specimens every 3–5 years, in spring just before new growth appears. Remove thick, circling roots but keep as many fine feeder roots as possible.

  • An equal mix of akadama, pumice and lava rock works well; Robinia is undemanding about pH and grows from slightly acidic to slightly alkaline (pH 6–7.5).
  • After repotting, keep the tree in light shade and out of the wind for two to three weeks before returning it to full sun.

Watering

Water thoroughly when the surface of the soil has dried, then let it dry slightly again. False acacia copes with short dry spells far better than with wet feet, so never keep the soil constantly wet: overwatering is the main cause of root rot.

In hot summer weather you may need to water daily. Water less in autumn as growth slows, and in winter keep the soil only slightly moist.

Seasonal care

  • Spring: Prune and repot just before the buds break, then keep repotted trees in light shade for a few weeks. Start feeding once the leaves unfold.
  • Summer: Keep the tree in full sun, water as soon as the surface dries (often daily in hot weather) and cut back long shoots that break the silhouette. Shelter it from strong winds that can snap its brittle branches.
  • Autumn: The leaves turn yellow and drop. Stop feeding and reduce watering as growth stops.
  • Winter: Leave the tree outdoors to go fully dormant, protecting the roots only when temperatures fall below -10°C. Water sparingly so the soil never dries out completely, and plan the structural pruning for late winter.

Common problems

False acacia is a tough tree, but it has a few typical weak points.

  • Root rot: The most common cause of decline, brought on by overwatering or soil that stays wet. Use a fast-draining mix, let the surface dry slightly between waterings and never let the pot stand in water.
  • Aphids: Colonies of black aphids gather on soft shoot tips in spring and early summer, leaving sticky honeydew. Rinse them off with a strong jet of water or treat with insecticidal soap.
  • Rolled leaflet edges: Caused by the black locust gall midge, whose larvae develop inside the rolled-under leaf margins. Remove affected leaflets; healthy trees tolerate light infestations.
  • Broken branches and suckers: The wood is brittle and the roots readily throw up suckers. Wire carefully with the correct gauge, protect the tree from strong winds and cut suckers off at their base.
  • Toxicity: The bark, leaves and seeds are poisonous to people and animals, especially horses. Keep prunings away from pets and livestock and wash your hands after working on the tree.