This popular and widely grown tree is a cross between Scotch laburnum (L. alpinum) and common laburnum (L. anagyroides) which has larger and more numerous flowers than either of its parents. Hybrid vigour is certainly at work here.
Laburnums thrive in even fairly poor soil which is well drained. They flower best in full sun and far less well in partial shade. At Caerhays we once grew this as a specimen plant on the main lawn beside the summer house until the tortoises got poisoned from eating the seed pods which fell into their enclosure below the tree. The tree had to go as the seeds (and leaves) are highly toxic if eaten and do appear attractive and pea-pod-like to young children. Before the tree got too large, and before the tortoise drama, it was a family job to remove all the seed heads after flowering with dire warnings all round and then a quick trip to the bonfire.
L. x watereri ‘Vossii’ has a maximum height of 20-25ft in maturity so it is not a huge tree. The dense racemes of yellow flowers appear in May (sometimes perfect for the Chelsea Show) and are around 18-24ft long.
L. ‘Vossii’, like other species of laburnum, can readily be trained up and over an archway or pergola so that the fragrant flowers hang down below the structure. One of the most renowned features of Bodnant Gardens in North Wales is their long laburnum avenue at the top of the garden. Similar features exist and have been widely copied elsewhere. The one at Castle Howard in Eire is very fine too. Serious annual pruning is needed to contain the stems of individual trees along a pergola to stop them trying to become upright trees and to tie them in on the old wood to the ironwork (or wood) of the pergola or avenue structure. Laburnum flowers appear on each season’s new growth. L. ‘Vossii’ is totally hardy and can be propagated from seed set in a cold frame in the autumn. If you have other Laburnum species or Laburnocytisus trees growing nearby the seedlings may well not come true to the original tree when they reach flowering size.
