Laburnocytisus, also known as Adam’s laburnum or broom laburnum, is a fascinating horticultural specimen. It is a small tree resulting from a graft-chimaera between two species: Laburnum anagyroides (laburnum) and Chamaecytisus purpureus (syn. Cytisus purpureus) (broom). This unique tree bears some shoots typical of one species, some of the other, and some that are a peculiar mixture of both “parents.” The plus sign (+) denotes that the generic name is created for a graft-chimaera. The plant can also be denoted by the formula Laburnum anagyroides + Chamaecytisus purpureus. .

The cultivar ‘Adamii’ of Laburnocytisus is a legume, belonging to the pea family Faboideae (or Papilionaceae, formerly Leguminosae). Only one cultivar, ‘Adamii,’ is known to have arisen from this graft. Although sometimes described as if it were a single species, (+)Laburnocytisus adamii, it is not a single species, and this notation does not adhere to the International Code of Nomenclature for Cultivated Plants.

Most of its branches resemble the laburnum, featuring foliage with three leaflets (3-palmate) that are 3–6 cm long. However, these branches also bear dense clusters of broom-like shoots, which also have three leaflets but are only 1 cm long and have a darker green coloration. During late spring or early summer, the tree produces flowers. Some branches bear long (20–30 cm) racemes of yellow laburnum flowers, while others yield dense clusters of purple broom flowers. Remarkably, most branches also produce coppery-pink flowers on short (8–15 cm) racemes, which represent an intermediate between the two “parents.” The leaves on these shoots also display an intermediate appearance. As the tree matures, the proportion of broom and mixed tissues tends to decrease, with the laburnum characteristics becoming more dominant. Laburnocytisus typically reaches a height of 7 m (rarely 8 m) and is hardy to USDA plant hardiness zone 5 in northern Europe. It thrives in moderately fertile, moist but well-drained soil and should be grown in a sunny position to encourage optimal flowering.

The plant originated in the nursery of M. Adam near Paris in 1825, probably as an accident;^[1]Chamaecytisus purpureusis normally a low-growing plant, and grafting it onto a straight trunk of a related species would be expected to create an attractive, semi-weeping standard.

In theory, other + Laburnocytisuscould be developed in the same way but using different “parents”.

A graft-chimaera is not a true hybrid but a mixture of cells, each with the genotype of one of its “parents”; it is a chimaera, created by grafting, in which the tissue of one plant grows within an outer envelope of the second plant. In the case of + Laburnocytisus‘Adamii’, laburnum forms the core, surrounded by the broom.^[1]Such plants are often called “graft hybrids”, but as they are not true hybrids the use of this term is now discouraged.

+ Crataegomespilusis a graft-chimera between hawthorn (Crataegus) and medlar (Mespilus) which arose in a similar manner. There are two distinct cultivars that have arisen from this graft: + Crataegomespilus‘Dardarii’ and + Crataegomespilus‘Jules d’Asnieres’.^[2]

The Bizzaria of Florence (Citrus medica + C. aurantium), which is probably the first graft chimera obtained, is a graft between the Florentine citron and sour orange.