+Laburnocytisus 'Adamii' (Poiteau) Trel.

Graft hybrids occur when the tissues of different taxa fuse at the point of grafting, making a more or less stable plant with features intermediate between the parents, and which needs to be propagated vegetatively. They tend to be curiosities rather than things of beauty, but Adam’s Laburnum stands out because its parents, although closely enough related to cooperate in this fashion, happen to look so different. In 1825, Jean Louis Adam, a nurseryman from Vitry near Paris, was grafting dwarf purple brooms (Cytisus purpureus) onto a stock of Laburnum anagyroides. This was a common practice at the time, Laburnum being very easy to raise from seed and Cytisus purpureus not being able to reach a significant height on its own. Unusually in its genus, Cytisus purpureus carries purplish-pink flowers; these cluster along the stems, while its trifoliate leaves are proportionately smaller than those of the tree-sized Laburnum anagyroides. Adam noticed a branch with intermediate foliage and floral features growing from the point of one of his grafts, and quickly propagated this and put it into commerce (Bean 1981).

With its short racemes of muddy purplish-yellow coloured flowers, the graft hybrid hardly improves on either parent, but its saving grace lies in the reliable if random fashion in with which patches of both parents’ tissues break out all over the tree, contrasting pure yellow with rich purplish-pink flowers. The patches of the slow-growing, tiny Cytisus leaves look a bit like witches’ brooms, while the strong-growing patches of pure Laburnum contribute to the hybrid’s rather unexpected if ungainly vigour. A tree growing in fertile Cotswold limestone at Great Corsham parish churchyard in Wiltshire, UK, had a trunk 59 cm thick in 2005 – respectable dimensions for any laburnum – and finally blew down in 2014 (Tree Register 2024). Adam’s Laburnum is also hardy enough to grow at the Arnold Arboretum in Massachusetts (Dirr 2009) and at the Oslo Botanical Garden (University of Oslo Botanic Garden 2024).

Unlike the true laburnums, Adam’s Laburnum seems to have retained its minor popularity as a garden tree, probably because of its eccentricity. To its advantage, it seldom produces many of the laburnum parent’s toxic seeds. As of 2024, one curious and lamentable online phenomenon was the widespread use of garishly photoshopped fake images to sell this scarcely spectacular, but interesting, plant.