Laburnum


Laburnum, sometimes called golden chain or golden rain, is a genus of two species of small trees in the subfamily Faboideae of the pea family Fabaceae. The species are Laburnum anagyroides—common laburnum and Laburnum alpinum—alpine laburnum. They are native to the mountains of southern Europe from France to the Balkans.
Some botanists include a third species, Laburnum caramanicum, but this native of southeast Europe and Anatolia is usually treated in a distinct genus Podocytisus, more closely allied to the Genisteae.

Description

The Laburnum trees are deciduous. The leaves are trifoliate, somewhat like a clover; the leaflets are typically long in L. anagyroides and long in L. alpinum.
They have yellow pea-flowers in pendulous leafless racemes long in spring, which makes them very popular garden trees. In L. anagyroides, the racemes are long, with densely packed flowers; in L. alpinum the racemes are long, but with the flowers sparsely along the raceme. The fruit develops as a pod and is extremely poisonous.
The yellow flowers are responsible for the old poetic name 'golden chain tree'.
All parts of the plant are poisonous, although mortality is very rare. Symptoms of laburnum poisoning may include intense sleepiness, vomiting, convulsive movements, coma, slight frothing at the mouth and unequally dilated pupils. In some cases, diarrhea is very severe, and at times the convulsions are markedly tetanic. The main toxin in the plant is cytisine, a nicotinic receptor agonist.
It is used as a food plant by the larvae of some Lepidoptera species, including the Palearctic moth, the buff-tip.

Species

Accepted binomials

Laburnum comprises the following species:
The status of the following species is unresolved:
The following hybrids have been described:
There is also a graft hybrid, +Laburnocytisus adamii Lavallée.

Uses

Woodworking

Laburnum has historically been used for cabinetmaking and inlay, as well as for musical instruments. In addition to such wind instruments as recorders and flutes, it was a popular wood for Great Highland Bagpipes before taste turned to imported dense tropical hardwoods such as Brya ebenus, ebony, and Dalbergia melanoxylon. The heart-wood of a laburnum may be used as a substitute for ebony or rosewood. It is very hard and a dark chocolate brown, with a butter-yellow sapwood.

Cultivation

Laburnum species and hybrids are cultivated as ornamental trees for gardens and parks. They are also trained as espaliers on pergolas, for ceilings of pendant flowers in season. In its natural form, Laburnum is a shrubby, multi-branched tree, but it is often pruned to maintain a single trunk which displays the smooth green bark.
Gardeners are advised to remove the spent seedpods after flowering because they sap the strength of the tree and are the most poisonous part. Generally Laburnum does not perform well in hot climates, and has a reduced life-span if grown in climates with warm winters. Afternoon shade and the occasional deep watering are advisable in areas with hot, dry summers. They do best in climates with moderate winter and summer temperatures, ideally Oceanic climates like those of the Pacific Northwest and Northern Europe. Laburnum trees are ubiquitous in England, Wales, Scotland and Ireland, where they are commonly planted as lawn specimens or in shrub borders.
Most garden specimens are of the hybrid between the two species, Laburnum ×watereri 'Vossii', which combines the longer racemes of L. alpinum with the denser flowers of L. anagyroides; it also has the benefit of low seed production. It has gained the Royal Horticultural Society's Award of Garden Merit.