The horticultural techniques of grafting have been understood and practiced for well over 1,000 years, enabling the perpetuation of selected forms of fruit and ornamental trees, shrubs, and vines. Through trial and error, the proper pairing of the rootstock and the desired scion has been determined for hundreds of plant species. Though a close genetic relationship between the rootstock and scion is generally necessary for success, in a few instances species from different genera within the same family have been successfully joined. Tomatoes (genus Lycopersicon), for instance, can be grafted onto the roots of potatoes (genus Solanum). Although both parts of a compatible graft union work in concert, each retains its individual genetic identity, which is delineated by the graft boundary. But of course, we know that the rules of horticulture and botany were made to be broken.
Chamaecytisus (formerly Cytisus) purpureus is a charming deciduous shrub native to southeastern Europe, forming a two-by-two-foot specimen smothered in spring with pretty pink-purple pea flowers. In 1824, the Parisian nurseryman J. L. Adam, with the intention of creating a standard, or tree, form, top-grafted this species onto the common golden chain tree, Laburnum anagyroides. What resulted is perhaps one of the most unlikely ornamental plants available today.
When Adam's newly grafted specimen flowered the following spring, he was astounded to see that the flowers were not those of Chamaecytisus purpureus but were bicolor—both yellow and purple—and intermediate in form between the rootstock and scion. His claim that he had created a hybrid between the two genera by the simple act of grafting was widely discredited. Even with the nascent understanding of genetics in the early 19th century, horticulturists knew the principles of applied botany very well—it was impossible to create an actual hybrid without intervention from the birds and the bees.
After Adam had propagated and introduced his tree into the European horticultural trade, an even more curious trait began to manifest itself. In addition to the supposedly “hybrid” flowers, flowers of both the rootstock and the original scion also began to appear along the tree's branches.
We now know that Adam's original claim was not entirely off mark. His tree was indeed an amalgamation of both plants' genetic material, without, however, actual fusion of their cells. Known as a graft chimera, a plant of this kind exhibits a rare phenomenon in which the cells of the rootstock extend upward and into the scion while retaining their genetic identity. A cross section of a stem under the microscope looks like a jigsaw puzzle, with each piece of the puzzle representing the cells of either the laburnum or the chamaecytisus. Flower buds arising from tissue of both species appear to be “hybrids,” while those arising from homogenous tissue of either “parent” appear true to form.
Graft chimeras have been reported from a handful of other species, notably between the hawthorn (Crataegus) and medlar (Mespilus). The correct way to denote a graft chimera is by use of the + symbol preceding a combination of the two genus names: +Laburnocytisus, +Crataegomespilus, and so on.
Propagation of +Laburnocytisus adamii is not difficult yet comes with caveats. Hardwood cuttings taken from November through January root readily. However, one can't be certain that one has “captured” the chimera—that is, a piece of stem containing both species—until it blossoms two or three years later. Luckily, I haven't encountered any difficulties thus far in propagating the plant, though the possibility remains.
This small tree will ultimately create a rounded crown rising to 25 by 25 feet. As both Laburnum and Chamaecytisus are signature Mediterranean plants, +Laburnocytisus is best grown in full sun and sharply draining soils. It is dependably hardy anywhere that laburnums can be successfully grown.
Admittedly, the pink and yellow flowers of +Laburnocytisus adamii are not to every taste. But this tree has garnered a permanent place in my garden for other reasons: it is emblematic of the mysterious ways of the natural world and the lessons to be learned from the garden. I wouldn't be without it.
TYPE OF PLANT: small deciduous tree
FAMILY: Fabaceae (pea family)
ORIGIN: graft chimera
HARDINESS: USDA Zones 5–8; Sunset Zones 3–9, 14–17, 34, 36–39
HEIGHT/SPREAD: 25 ft./25 ft.
LEAVES: alternate, trifoliolate, leaflets elliptic to elliptic ovate, to 2 1/2 in. across, medium green
FLOWERS: typical pea shape, held in racemes to 7 in. long, yellow tinged purple; yellow laburnum and pink chamaecytisus flowers may also appear
SITE REQUIREMENTS: full sun in well-drained soil
PROPAGATION: by hardwood cuttings taken November-January