Cladrastis is a genus of about a half dozen species of pinnate-leaved, deciduous trees in the pea family (Fabaceae), all of them native to Asia except Cladrastis kentukea (American yellowwood), which is native to southeastern North America, and the only widely cultivated species. Though not particularly common or well-known locally, Cladrastis are beautiful trees. For ease of identification, they stand out as among the few alternately arranged, pinnately compound-leaved temperate trees in cultivation with individual pinnae that are often alternately arranged along the rachis of the leaf. Like Cercis (redbud), Maackia, Styphnolobium (scholar tree) and Gleditsia (honey locust), Cladrastis species are rare among the Fabaceae in that they are not associated with root-inhabiting, nitrogen-fixing bacteria. The genus is most closely related to Maackia, but Cladrastis have alternate pinnae and the petiole base encloses an axillary bud.
American yellowwood is a moderate-sized tree with an initially narrow, but broadening crown and usually, stiffly ascending branches. The pinnate leaves may be sizable, especially on rapidly growing shoots. Individual leaflets are more or less ovate or elliptic, 5 to 10 cm long, usually seven to eleven per leaf, and borne on a slender, green, 20- to 30-cm long rachis. The current year’s shoots are surprisingly fragile (Cladrastis is from the Greek klados = branch + thraustos = brittle).
Fragrant white-and-yellow pea-flowers are arranged in showy drooping panicles, somewhat wisteria-like, at the tips of the branches in June; however, flowers are seldom borne on young plants. It may take a decade or more before trees begin blooming and even then, they tend toward alternate-year bearing (i.e., flowering heavily one year, with little or no flowering the following year). Flowers are followed by flattened 10-cm-long legumes, which turn from green to brown in autumn prior to the often sensational, brilliant yellow leaf display. The bark of yellowwood is subtle and attractive—smooth, light grey, becoming darker grey, and somewhat pebbly, but otherwise unadorned. The common name denotes the yellow heartwood of the American species.
Like a number of tree species originating in the American South, yellowwoods often struggle in Vancouver’s shallow soils, particularly when they are saturated in winter and overly dry in summer. Nevertheless, trees do extremely well in full sun, in deep, well-drained soil with summer moisture, and they are otherwise unfussy with respect to soil texture and soil pH. They are capable of surviving drought once established, even displaying some shade tolerance, as well. A network of a few coarse roots will develop at the surface in wet soils and these can be troublesome and eventually push up sidewalks. Root loss, either through root rot in overly wet conditions, mechanical damage or digging, may significantly reduce the vigour or even the viability of trees.
Narrow branch angles are common to all Cladrastis species and indicate congenital weakness and a propensity for stems to tear away from established wood. On top of that, the wood itself is brittle and subject to breakage, in much the same way as occurs with the black locusts (Robinia spp.). In other words, trees should be located away from utilities and vulnerable structures in windy areas. The species is naturally low branched or multi-stemmed–this can compensate for the occasional loss of a branch–and trees have the ability to re-sprout from established roots. Yellowwood forms a rounded crown with outer spreading branches that droop attractively. Suitability for street plantings or other confined spaces usually necessitates crown raising (i.e., limbing up). It is also often recommended that for long term health of trees, they be correctively pruned to encourage wider branch angles and prevent bark inclusions. While this is possible, it goes against the natural inclinations of the species, and the pruning itself is sometimes problematic, as trees tend to “bleed” sap excessively if pruned in winter. Bark is unfortunately thin and subject to considerable damage by sloppy use of line trimmers, mowers and other equipment, as well as by winter sunscald when young stems are left un-shaded. Pests and diseases in Cladrastis are mostly unknown, but the species is susceptible to verticillium wilt, and the fungal species responsible are often present in native soils. The presence of these pathogens does not reflect the probability of infection, but there is a correlation between amount of infective material, root damage, saturated soil, and rates of infection.
(excerpted from the Vancouver Trees App, and yes, we are actively working on an Android version at the moment!)