The Sesbania Bonsai: The Hummingbird Tree

Sesbania punicea, poetically known as the Hummingbird Tree (or "Red Wisteria"), is a bonsai that carries with it the charm of South America. Its nickname is no accident: its clusters of deep red-orange flowers are so rich in nectar that they attract these birds in their native range. It belongs to the Fabaceae family and is prized for its spectacular flowering and for its seed pods which, once dry, rattle in the wind like maracas (hence the other name "Rattlebox"). It is an exotic plant, fast growing and of great visual impact.

Origin & species

Native to Argentina, Brazil and Uruguay, it is a deciduous shrub or small tree (it loses its leaves in winter). It has elegant compound leaves that partly close at night (nyctinasty). It is a pioneer plant that loves water and sun.

Position and microclimate

Sesbania loves full sun.
The more sun it gets, the more flowers it will produce. It does not mind scorching summer heat. On the contrary, shade inhibits flowering and weakens growth. In windy spots the flexible branches sway without problems, but watch out for small pots that could tip over.

Watering

In the wild it often grows near watercourses. In a pot, therefore, it drinks a great deal.
In summer, never let the soil mix dry out completely for too long: water generously and frequently. If it goes thirsty, the leaves yellow and drop quickly. In winter, with the plant bare, reduce watering and keep the soil barely moist.

Water quality

It is a tough, tolerant plant. It accepts tap water without particular problems, although water that is not too hard is always preferable to avoid a build-up of salts over time.

Substrate

It needs a soil that holds moisture but drains well to prevent rot.

• Recommended mix: 40% Akadama, 30% pumice and 30% universal soil mix or humus.
• The organic component is higher than with other bonsai precisely to meet its high water demand.

Repotting

Carried out in spring (April), when temperatures rise.
Sesbania has explosive root growth. Young specimens should be repotted every one or two years. It takes root pruning well, but always try to balance the canopy (if you cut a lot of roots, shorten the branches too).

Fertilizing

It is a hungry plant. Fertilize regularly from spring through summer.
To support flowering and vigorous growth, use a good quality solid organic fertilizer. Suspend feeding only during full bloom.

Flowering and fruiting

It flowers in late spring/summer. The red clusters are its strong point.
They are followed by pods (legumes) with four characteristic wings containing the seeds.
Note: if the bonsai is young, remove the pods as soon as the flowers fade so as not to tire the plant. If you want to hear the "rattle", leave just a couple of them.

Structural pruning

Done in late winter, shortening the long branches of the previous year.
Sesbania tends to grow fast and untidily: you need to be decisive with the shears to keep a compact shape and an interesting trunk.

Maintenance pruning

During the growing season, trim back the branches that get too long.
Remember that flowers form on new shoots: if you keep trimming until July, you will cut away the future flowers. Let the branches grow freely in late spring if you want flowering.

Pinching

It can be done on vigorous shoots in spring to thicken the canopy, but it should be stopped early if the aim is to see the red of the flowers.

Wiring & bending

Young branches are green and flexible, easy to wire.
However, they lignify quickly and become stiff. Apply the wire in spring and check it often, because the bark is tender and the fast growth leads to rapid wire marks.

Diseases and pests

• Aphids: they love buds and tender shoots.
• Whitefly: sometimes present under the leaves in summer if ventilation is poor.

Winter & protection

It is not an alpine plant.
In the south it lives outdoors. Further north it withstands brief light frosts (-2°/-4°C), but if the cold is intense it should be moved to a cold greenhouse or a sheltered spot (such as an unheated veranda). It suffers from prolonged frost on the roots.

Yearly calendar

• January-February: dormancy, pruning on bare wood.
• March-April: repotting, bud break.
• May-June: fertilizing, plenty of water.
• July-August: flowering, full sun, lots of water.
• September-October: pods ripening, autumn fertilizer.
• November-December: leaf drop, protection.

Useful products for this bonsai

• Quality universal soil mix (to retain water)
• Biogold fertilizer (complete, balanced feeding)
• Plastic drip tray (useful as a water reserve)
• Copper-coloured aluminium wire (to shape the branches)
• Angled branch cutter (for clean cuts)
• Multi-purpose insecticide (protecting the buds)

Frequently asked questions about Sesbania

Are the seeds poisonous?
Yes, highly toxic if swallowed. All parts of the plant contain saponins, but the seeds in the pods are especially rich in them. Keep the plant away from small children and pets that might chew on them.

Why do the leaves close in the evening?
It is a natural phenomenon called "nyctinasty" (night-time movement), typical of many legumes such as Albizia and tamarind. The plant "sleeps" to conserve warmth and moisture.

Can I keep it indoors all year?
No. It needs direct sun to flower and it needs to feel the change of seasons (and the winter cool) in order to go dormant. Indoors it would do badly, grow leggy and never flower.