Laburnum × watereri (A.C.Rosenthal & Bermann) Dippel

The existence of crosses further blurs the already subtle points of distinction between the two wild species of laburnum. Hybrids were first scientifically decribed (from the Italian South Tyrol and from southern Switzerland) by the German botanist Leopold Dippel in 1893 (Dippel 1893); several cultivated Laburnum clones which had previously been placed under L. anagyroides or L. alpinum (or under both by different sources) were retrospectively assigned to this new entity. The description of the hybrid’s wild distribution, at the head of this entry, is Dippel’s; very little research seems to have been conducted since, and Royal Botanic Gardens Kew 2024 in fact limits the natural range, for whatever reasons, to Switzerland alone. The staggered flowering time of the parent species, with L. alpinum opening its blooms just as L. anagyroides finishes, is probably a strategy that has evolved to limit cases of hybridisation. It is possible that the cold, late springs of the 19^th century in northern Europe, by compressing the flowering times of the two laburnum species, made hybridisation more likely in the nurseries of that period than would seem to be the case today.

It should be emphasised, firstly, that the various cultivated clones are likely to differ in the ways in which they recombined the features of their parents. Laburnum × watereri is widely described as bearing the longest racemes in the genus, but this was originally described as a feature particular to ‘Vossii’. The clone ‘Watereri’ combined glabrous young twigs with sparsely hairy leaves (Bean 1981), but Dippel’s adoption of this plant’s name for the new hybrid species was a nomenclatural necessity and does not imply that ‘Watereri’ was ‘typical’ in these respects. Secondly, the difficulties in identifying laburnums mean that some garden clones have never been assigned to either of the parents or to the hybrid with much degree of confidence. ‘Columnaris’, ‘Sunspire’, ‘Vossii Goldleaf’ and ‘Vossii Pendulum’ are sometimes treated under L. × watereri but are described or discussed in this account under L. anagyroides.

Laburnum × watereri (at least in the clone ‘Vossii’) has long been recommended as the ‘best’ laburnum to grow. Hybrid vigour is certainly likely to make the flowering display more spectacular than either parent’s, and these hybrids tend to show reduced fertility, with few seed-pods to draw vigour from the plant (or to tempt hungry children). It is therefore only to be expected that L. × watereri ‘Vossii’ is by far the most frequently offered laburnum among nurseries prestigious enough to feature in the RHS Plant Finder (Royal Horticultural Society 2024), though it might be naive to assume that all of this stock could be traced directly back to the laburnum sold by the Dutch nurseryman de Vos in the 1870s. Since ‘Vossii’ has become the gold standard for laburnums, and since it is much easier to raise seedlings (from either parent) than it is to make grafts of genuine material, it would seem quite likely that many of the plants to have been sold over the centuries as ‘Vossii’ are nothing of the sort. Even allowing for the difficulties inherent in identifying laburnums, a large majority of the trees that can be observed during a suburban walk in southern England could reasonably be classified as pure L. anagyroides, with pure L. alpinum appearing to be more conspicuous further north. (Another possible explanation for this anomaly is that hybrids such as ‘Vossii’ quickly exhaust themselves through flowering so abundantly, while the parent species can be sufficiently long-lived to have survived into an era when laburnums are no longer planted very much.)