Coral Tree

What is a coral tree?

When the coral tree flowers, it hangs out clusters up to 60 centimetres long, each with up to 80 fleshy, scarlet flowers. That is what gave the genus its names: Erythrina from the Greek for red, coral tree from the colour of the blossom. Erythrina crista-galli, the cockscomb coral tree, has the southernmost natural range of its genus and is adapted to cooler winters than its tropical relatives. In Argentina and Uruguay, where it is called ceibo, it is the national flower.

For a planting on poor ground, the flowers are the lesser part. The coral tree is a legume, forms root nodules with nitrogen-fixing bacteria, and tolerates hard cutting. Among the nitrogen-fixing plants it is one of the few trees of real size that come from a climate with cool winters.

Origin and growth

The ceibo is native to Uruguay, southwestern Brazil, northern and central Argentina, southern Paraguay and southeastern Bolivia. Its centre lies in the river system of the Paraná and Uruguay. There it grows along rivers and streams, in wetlands and marshy grasslands, and in the Paraná delta it forms pure stands on flood-prone islands. In its homeland it is described as a pioneer that recolonises disturbed ground.

It usually grows 5 to 10 metres tall, up to 15 or 20 in humid places, often with a stout trunk and corky, deeply furrowed bark. The wood is white, light and soft. Young twigs and leaf stalks carry curved prickles. The leaves have three leaflets and fall in winter. The pods are 10 to 20 centimetres long with one to six hard seeds. Its flowers are visited by bees and by hummingbirds.

Requirements

The coral tree likes sun and warmth and is sensitive to frost. It copes with cold by shedding its leaves and the current year's shoots, which die back and resprout from older wood. Growers in California report little dieback down to about minus 7 °C and root survival to about minus 10 °C. Its origin shows that it values water: in Brazil it is rarely found on dry sites. In cultivation it has proved adaptable to urban conditions and different soils.

Role in the soil: nitrogen, biomass and mulch

In its homeland the ability to fix nitrogen is seen as the reason the ceibo prevails on poor soils and prepares them for other plants. The partners of E. crista-galli itself are little studied; in related coral trees researchers have mostly found Bradyrhizobium in the nodules, alongside Rhizobium in some species. More on these partners is in the article on rhizobia.

For biomass the genus has a long record in the tropics. In Central America the related Erythrina poeppigiana is pollarded as a shade tree over coffee, and its prunings stay on the ground. The ceibo tolerates the same treatment: in botanic gardens it is cut back hard and resprouts. Its thin, papery leaves and soft young shoots make a light mulch; the wood itself is light and poorly lignified.

In the food forest

In a food forest the coral tree occupies the lower canopy: a small tree that can be kept at the height its neighbours need. In syntropic agriculture such trees are cut back regularly to let light in and to feed the soil with their prunings; the coral tree can do this work for many years. Its open, leafless winter crown lets light through in the cool season. It can serve as a nurse tree for sun-sensitive young trees, and its spring flowers feed bees.

In the Mediterranean climate

The coral tree has a long history in southern Europe. It was cultivated in Lisbon by 1695, and one of the best-known specimens grows in the botanic garden of Coimbra, planted around 1800. Notable trees stand in Seville and Barcelona, in Nice, which lists the species among the trees recommended for the city's future climate, and in Italy, for example in the Vatican Gardens, in Viareggio, in San Remo and at La Mortella on Ischia. A grower in southern France reports that frost damages the late shoots but that the tree returns every year.

Most of these trees grow in parks and gardens, often with water nearby. On a dry slope with five months without rain, the riverside origin is the open question.

The coral tree is not on the EU list of invasive species. In eastern Australia it has escaped along watercourses and is regulated as a weed; in New Zealand it is naturalised. Both are moist, frost-free river landscapes. Where water is scarce, the risk is much lower; seedlings near streams should still be removed.

Propagation and care

The coral tree grows easily from seed and from cuttings: softwood cuttings in spring and early summer, semi-ripe ones in late summer, hardwood cuttings at the end of winter. Pruning is best done in late winter, when frost damage can be seen and the cut wood goes straight to the ground as mulch. Gloves help against the prickles.

How Terra Humana understands it

At Sa Medada the coral tree is meant to be a support tree of the middle years: longer-lived than the shrubs of the first seasons, but in service of the trees that will one day form the canopy. Whether a tree from the river meadows of the Paraná can do this work on a dry Sardinian slope is something only the coming summers will show.