Gymnocladus dioicus Seeds | Kentucky Coffeetree
LATIN NAME: Gymnocladus dioicus
COMMON NAME: Kentucky Coffeetree, Kentucky Coffee Tree
Plant Profile
The genus name is Greek for “naked branch”, and a mature specimen earns the description. Kentucky coffeetree leafs out late, often well after every neighbouring tree is in full cover, and it drops its foliage early in autumn, so for five or six months of the year it stands as a bare grey armature on the river bottom. The leafless interval is not a sign of poor health. A tree in a warm garden in England keeps the same rhythm as one growing wild beside the Wabash.
Nothing else on the continent carries a bigger single blade. The leaf is bipinnate, occasionally approaching a metre long and more than half as wide, and it is assembled from forty to seventy oval leaflets set in opposite pairs along branching secondary axes. The leaflets are dull dark green above and paler beneath, with smooth margins, which is unusual in a family that usually shows a terminal leaflet and toothed edges. The rachis is swollen at every junction, so the whole structure can shift with the light.
The species is dioecious, so the odds are even that any seedling you raise will never fruit. Male trees carry dense clusters of small green-white flowers with ten stamens and no working pistil. Female trees carry fewer flowers with a short style and flattened, sterile stamens, and only those trees set pods. The male flowers are slightly larger and carry the heavier scent, and bees work both for pollen and nectar. A scattered perfect flower turns up now and then, which is why a nominally male tree can produce the odd pod.
Legume does not mean nitrogen fixer, and this tree is the proof. Kentucky coffeetree sits in the caesalpinioid line, and almost none of that group forms the root nodules that house rhizobia. Dig beneath a mature specimen and the roots are bare of nodules. Whatever fertility gathers under it comes from leaf litter and shade, not from bacterial capture of atmospheric nitrogen, which is why it behaves in a plantation like a hardwood rather than like a black locust.
Science & Biotechnology
The seed carries quinolizidine alkaloids, cytisine among them, the same class that gives laburnum and the brooms their dangerous reputation. Cytisine is a nicotinic acetylcholine receptor agonist and is the model on which a modern smoking-cessation drug was designed, which makes the seed a material of genuine pharmacological interest rather than a curiosity. Raw, the seed is poisonous to people and to stock, and the pulp does not improve matters.
Uses & Benefits
The timber is heavy, hard, close-grained and durable in the ground, and it polishes to a warm brown. It went into fence posts, rails, tool handles and interior joinery, and remains one of the best native woods in eastern North America for anything that must resist rot in contact with soil. The supply is small because the tree never grows in stands, only as scattered individuals, and the sawyer who finds one usually works it on site rather than hauling it to a mill.
Growing from Seed
Chosen trees are hard to multiply. Cuttings strike poorly and the named selections that nurseries offer are grafted onto seedling rootstock, kept high so the stock can be cut back and does not throw suckers past the union. Because the tree is dioecious, and because seedlings segregate for pod production, thornless vigour and autumn colour, laboratories have worked the alternative route: somatic embryogenesis from immature seed tissue, and axillary shoot multiplication on a cytokinin-rich medium. Plants raised that way need a hardening stage under mist before they tolerate open air.
The seed coat is nearly impermeable, and a lot of seed will sit motionless for a long time if it is left alone. Nick the coat on the side away from the embryo with a file or a sharp knife, or pour water just off the boil over the seed and let it stand overnight. Sow two to three centimetres deep in a free-draining mix of two parts loam-based compost to one part coarse grit, and sow into a deep container, because the seedling makes a thick taproot at once. Space the pots a hand-width apart and water thoroughly, then let the surface dry between waterings. Give the young plants full sun from the start; a shaded seedling grows weak, spindly wood and a crooked base that no later pruning will correct. Move them into open ground once the stem is as thick as a pencil, and stake the first winter.
Our Take
I grow this one for the winter silhouette as much as for the summer leaf, and I would rather have a male clone in a small garden than a fruiting tree dropping pods on a path. If you want the legume habit with a lighter footprint, our notes on honey locust cover the sister genus with its sweet pod, and false indigo bush gives a nitrogen-fixing shrub for rough ground. For a broad temperate shade tree with no pod at all, southern catalpa is worth the space. Give the seedling room from the first season and leave the taproot alone.
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