Hybrids between L. alpinum and L. anagyroides have arisen on several occasions in gardens and have also been found in the wild. The group-name for such hybrids, wild and cultivated, is L. × watereri, the type being the laburnum raised at the Knap Hill nurseries at some unknown date before 1864, and distributed as L. vulgare watereri. The distinguishing characters of the Knap Hill laburnum are: Young stems glabrous as in L. alpinum. Leaflets thinly appressed-hairy beneath and hence intermediate in indumentum between the parents. Inflorescences dense as in L. alpinum but with flowers almost as large as in the other parent. Pods intermediate, being less hairy than in L. anagyroides and neither winged nor much thickened at the suture.
cv. ‘Parkesii’. – The Knap Hill laburnum, although the typical form of the cross, was not, however, the first of the group to be put into commerce. In 1842, J. D. Parkes, a nurseryman of Dartford, advertised in the Gardeners’ Chronicle (p. 705) a new laburnum which became known in gardens as L. parkesii or L. vulgare parkesii. These names have been given as synonyms of L. × watereri with the result that the Parkes and the Knap Hill forms became confused. They seem in fact to have been very similar, but according to Nicholson (The Garden, Vol. 27 (1884), p. 519) the Knap Hill form was the finer of the two, and, according to Parkes, his form had ‘pendent’ branches.
cv. ‘Vossii’. – This beautiful hybrid, raised in Holland late in the 19th century, is now the commonest laburnum in gardens and has superseded the older forms of L. × watereri. The racemes are up to 2 ft in length and the young stems are appressed-hairy, not glabrous as in the typical form of the cross.