Laburnum Fabr.

The two species of Laburnum, L. alpinum (Mill.) Bercht. & J.Presl and L. anagyroides Medik., represent one of very few genera of trees which are endemic to Europe and absent from the more diverse temperate biomes of eastern North America and east Asia. Their hanging chains of bright yellow flowers place them among the easiest kinds of trees to recognise, but the two species only differ subtly and also hybridise, both in the wild as as garden plants. A third species, L. caramanicum (Boiss. & Heldr.) Benth. & Hook.f., from the eastern Mediterranean, is recognised by a few authors, including Bean 1981, but is generally now placed within its own genus, Podocytisus Boiss. & Heldr., as it is in this account. Another genus, Petteria C. Presl, contains one related species, P. ramentacea (Sieber) C.Presl, a shrub whose vernacular name is Dalmatian Laburnum. In horticulture, Laburnum anagyroides was also a parent of the curious chimaera, + Laburnocytisus adamii (Poit.) Trel.

Among smaller flowering trees, laburnums seem unusually long lived, although the planting date is not always secure. A now very reduced L. alpinum reputedly planted in 1601 survives at the Hortus Botanicus Leiden, The Netherlands, flowering each year from younger stems surrounding the now dead main trunk (Monumental Trees 2024), but the authenticity of this date is questionable (R. van Vugt, pers.comm. 2024). Another Laburnum alpinum of reputedly from the mid-1700s survives at Attadale in the Scottish Highlands (T. Christian pers. comm. 2024). A bushy plant which survived until the 1980s at Abercynrig in Wales was supposed to be contemporaneous with the garden itself, started in 1749 (Tree Register 2024). This plant was identified as Laburnum anagyroides. In Wales, Scotland and Scandinavia at least, L. alpinum is perhaps more likely to grow large and live long, The largest laburnum to have been recorded within the natural range is a tree with a single trunk just over one metre thick in northern Liguria, Italy (Monumental Trees 2024); this is also attributed to L. anagyroides, bt these two species are so similar in ecology and appearance that it often makes sense to treat both taxa together.

Laburnum, the classical Latin name for these trees, has come to serve in English as their vernacular name too. The German name is Goldregen (‘golden rain’); in contemporary English ‘Golden Rain Tree’ has been rather unaccountably hijacked by the Chinese Koelreuteria paniculata, whose softer yellow flowers open later in summer and, unlike falling rain, stand stiffly above the foliage.

Laburnums develop a striking purplish brown hard heartwood called ‘false ebony’ which, although it never comes in good straight lengths, has been used in furniture making and also for woodwind instruments, including the traditional Highland bagpipes (Bean 1981; Wikipedia 2024).

Laburnums are very easy trees to grow, tolerating urban pollution and maritime exposure and able to thrive in poor soils because, like most Fabaceae, their roots bear nodules containing bacteria which can fix nitrogen in the soil; they dislike waterlogging, and are also susceptible to honey fungus. Both species are hardy, and are in fact more likely to reach their limits towards the south of the climate zones covered by Trees and Shrubs Online; high summer night-time temperatures in the south-eastern United States seem to trouble them in particular (Dirr 2009).

Both Laburnum species have long been cultivated in European gardens north of their natural range, reaching Britain by the 16th century at the latest (Edwards & Marshall 2019). They grow readily from seed, but W.J. Bean recommended removing the seed-pods – which are not in the least ornamental – in order to promote year-on-year flowering and to increase the tree’s lifespan (Bean 1981), though this does seem like a counsel of perfection. Prior to recent developments in Magnolia breeding, laburnums were the only fully hardy trees to bear spectacular yellow blossom in late spring. Their popularity as garden plants peaked during the 19th and 20th centuries; since then, climate change has started to entice northerly gardeners to experiment with Fabaceae species from the southern hemisphere which are evergreen and whose hallmark yellow flowers open rather earlier in spring, such as Acacia baileyana or Sophora tetraptera. In May and June, however, it still becomes suddenly clear than laburnums remain among the commonest flowering trees in the gardens of north-western Europe.

Another factor in the fall from favour of laburnums in recent decades has been their reputation as poisonous trees. (Their seeds have evolved to scatter from the tree as the pods open, rather than to be transported by birds or animals that might eat them.) More precisely, laburnum’s distinctive conspicuousness means that, like a patch of yellow flowering ragwort (Senecio jacobaea), they tend to stand in the popular imagination as representatives or as scapegoats for toxic plants, which of course really exist all around us in great numbers. In the 1970s, when there would have been many newly-planted laburnums whose ripening seed-pods will still have been within an inquisitive child’s reach, R.M. Forrester (Forrester 1979) reported that in an average year in England and Wales, over 3000 children would be admitted to hospital with suspected laburnum poisoning (John Grimshaw clearly remembers being warned against them as a child in the 1970s). However, W.J. Bean’s ominous warning that ‘children have been known to die from eating [the seeds] when green’ (Bean 1981) was possibly repeated from Loudon (1838) who said much the same thing sensationalist: the poisonous compound, an alkaloid called cytisine, is present in all parts of this thoroughly unpalatable tree but would need to be taken in quite large doses for symptoms to become life-threatening, and the great majority of the hospital admissions seem to have been merely precautionary. Laburnum poisoning is also only likely to be common in cultures where small children play unsupervised, and where they know that peas come in pods, rather than in freezer packs from the supermarket. Nevertheless, medical attention should be sought if any part of a Laburnum is ingested. Laburnum poisoning is however a serious issue in the case of livestock, especially horses (Forsyth 1954).

Despite being toxic to some livestock, laburnum found an unexpected use in the 19th century as a hedging plant in a few of the more upland parts of Britain including south-west Shropshire, Carmarthenshire and Cumbria; some of these plantings still survive as individual veteran trees of L. alpinum (Mabey 1996; Ancient Tree Inventory 2024). Seedlings can also grow up, well north of the species’ natural ranges, particularly when alkaline rocks are exposed, for example among railway ballast.

The genus has produced a limited number of ornamental sports, but none of these improve obviously on the beauty of the wild species or their hybrid; caught up as they have been in the genus’ fall from garden popularity, many of these cultivars now seem to be either extinct or headed that way. Due to the similarity of the two species – and the existence of hybrids – it is often unclear to which taxon a particular cultivar really belonged.