Laburnum anagyroides Medik., Laburnum
Account Summary
Introduced, neophyte, a garden escape, very rare, but very probably under-recorded.
5 September 1998; Northridge, R.H.; roadside bank near Enniskillen Town.
Growth form and preferred habitats
This small to medium sized, deciduous, ornamental tree, 7-9 m high with a trunk rarely exceeding 20 cm in diameter has low, spreading arching branches when old. A native of the mountains of C & S Europe, it has for centuries been very popular in gardens everywhere and is known to quite frequently self-seed and naturalise in lowland parts of B & I. Laburnum usually occurs as a persistent, isolated garden escape, typically in acid soil on waste ground or in other marginal, occasionally or regularly disturbed habitats, eg along roadside verges or on railway embankments. It is, however, ecologically very undemanding and can occupy and survive in any type of soil (Hadfield 1957).
Flowering reproduction
Flowering occurs in May and June, the long, lax, dangling, bright yellow racemes of numerous pea flowers with a large standard petal, making the tree very conspicuous. Pollination is effected mainly by bumble-bees and the fruit is a narrow, pendulous, legume pod, 30-80 × 7-8 mm that hangs on the tree for some time (sometimes all winter), before splitting and twisting to explosively eject at least some of the dark brown or black seed (Cooper & Johnson 1998). Seeds are abundantly produced and germinate freely, and for this reason the plant is often used as a stock upon which to graft or bud other leguminous trees and shrubs. Laburnum trees have the reputation of being not long-lived (Hadfield 1957).
Toxicity
All parts of the tree, but especially the bark and the seeds contain a quinolizidine alkaloid toxin called cytosine (named after the old name of the plant – Cytisus laburnum) which resembles nicotine (Hadfield 1957; Cooper & Johnson 1998). The quantities of cytosine found in Laburnum and said to be dangerous or even fatal vary greatly (Cooper & Johnson 1998). The seeds are regarded as particularly poisonous and have killed children and stock, though it is said the leaves can be grazed without harm. Laburnums should not be planted near small garden ponds as the seeds and leaves may poison fish (Hadfield 1957). In general, Laburnum poisoning is rare, but a few fatal cases have been reported, including large animals such as horses from eating leaves and seed and dogs from chewing sticks broken from the tree.
With regard to human poisoning, it is very rare that anyone consumes a fatal dose, but treatment is considered necessary if more than five seeds are ingested by a child or 15 by an adult. The administration of activated charcoal, together with fluids, especially milk in the first six hours after ingestion, is the recommended treatment (Cooper & Johnson 1998).
Fermanagh occurrence
The solitary Fermanagh record listed above is of an individual tree which originated either from a nearby garden, or very much less likely, it might have been deliberately planted.
Species and hybrid variation
Sometime after its introduction to gardens in Britain towards the end of the 16th century, L. anagyroides formed a partially fertile hybrid (L. × watereri (Wettst.) Dippel), with the closely related L. alpinum (Mill.) J. Presl, the so-called 'Scottish Laburnum' although the latter has no real, valid origin connection with Scotland. The hybrid is a more vigorous and floriferous medium-sized tree than either of its parents and is the preferred garden Laburnum specimen today, although all three forms (the two species and their hybrid) are cultivated. The hybrid is often sold in the horticultural trade under the name L. × vossii hort. As one would expect, it is intermediate between its parents in degree of pubescence and in leaflet and fruit shape. The hybrid combines the more floriferous longer racemes (up to 50 cm long) of L. alpinum, with the larger (15-21 mm) flowers of L. anagyroides. The fruits of the hybrid are much fewer than the species, malformed, usually containing no, or just one or two seeds per pod and, remembering Laburnum toxicity, it the safer garden option to choose (Hadfield 1957; Stace et al. 2015).
The under-surfaces of the leaves and the young shoots of L. anagyroides are densely covered in appressed, silvery, silky hairs, while L. alpinum is almost glabrous and the hybrid is intermediate, ie it is only sparsely hairy. The upper suture of the pod is sharply ridged in both L. alpinum and the hybrid, but it is blunt and greatly thickened (ie much broader) in L. anagryoides (Hackney, C.R. 1989, Fig. 1.).
British and Irish occurrence
L. anagyroides is a quite frequent and widely scattered introduction across Britain from the far south and the Scilly Isles to Shetland. In Ireland, it is much less frequent, being rare and widely scattered in the RoI, although quite a lot more frequent in NI. Many of the records for L. anagyroides shown in the New Atlas hectad map could well be mis-identifications resulting from the hybrid being regularly overlooked (D.A. Pearman, in: Preston et al. 2002). The huge increase in records of L. anagyroides in Britain and in NI since the 1962 BSBI Atlas, probably merely reflects increased interest in recording alien trees in these areas.
While L. × watereri is capable of setting a very limited amount of seed, it is very much less likely to spread itself than L. anagyroides or L. alpinum. Seedlings of the latter are apparently very rare, even when many of the parent trees are planted as a hedge, as in the Slieve Gullion area of S Londonderry (Hackney, C.R. 1989). L. anagyroides has also been recorded more rarely in use as hedging on farm lanes in NE Ireland (Hackney et al. 1992). On the other hand, the presence of L. × watereri sometimes indicates deliberate hedge planting, as has been shown to have taken place in Cos Tyrone and Londonderry (H37, H40) (P. Hackney, Northern Ireland Flora Website 2005; McNeill 2010). The hybrid has also been used for hedging in three VCs in S Wales (VCs 44-6) (Chater 1996; Stace et al. 2015).
Uses
Laburnums are of no economic value although the wood is occasionally used as it is hard and durable and is similar to that of Robinia pseudoacacia (False-acacia). It takes a very fine polish and is sometimes used for inlays in furniture, for turning small objects and for veneers. The dark heartwood can be substituted for ebony. Older writers record that on the Continent it was regarded as supplying the longest lasting bows as weapons (Hadfield 1957).
Threats
None.