14 September 2023

Kentucky Coffee-tree (Gymnocladus dioicus) is a moderate-sized canopy tree and belongs to the pea or legume family. Its scientific name is from Greek: gumnos means naked, klados means branch and dioeciou means having separate male and female plants. The tree grows moderately fast to 18-24 m high with a 12-15 m spread. The tree has scaly tan to dark grey bark and has stout and widely-spaced branches and large leaves.

In the United States, the tree grows from the southern Great Lakes region east to New York State, south to Oklahoma and Arkansas with scattered populations as far south as Texas and north to North Dakota. In Canada, it is found only in southern Ontario with an estimated total population of fewer than 500 mature native trees; the uncommon and rare tree is designated Threatened in Canada under the Species at Risk Act. This tough species can stand to a variety of natural and human-influenced stressors. Nevertheless, its distribution is limited due to the separate sexual characteristics of the species – the seeds are produced only by female trees in a population. The seeds are also protected by a hard bitter pod that is rarely consumed by animals, and the heavy pod cannot be carried by winds. The fallen seeds also not germinate frequently on unfair grounds. The tree is often found in floodplains and river valleys, where moving water can carry the seeds. The tree often propagates vegetatively by extending its roots and making ramets.

Some conservatory agencies occasionally planted young trees on streets and parks across cities in Ontario. Figure 1 shown here is from such a tree in Scarborough, Ontario.

Fig.1 Kentucky Coffee Tree

Compared to other deciduous trees, it has a short growing period in a year. Its leaves fall in autumn and emerge in early spring. In May and June, greenish white flower clusters appear; male flowers and female flowers grow on separate trees. Male flowers do not produce seeds, but female flowers produce flat, green hard pods. The bean-like pods become yellowish brown and to dark brown in the fall (Fig 2 & 3).

There are four to eight seeds in each pod. The seeds are initially green and starchy; as the colour changes to greenish yellow, a tough skin will develope to enclose the now yellow and harder nut. At the end, the pod and seeds become dark. The dry dark brown seeds give the coffee-tree its name (Fig 4 & 5). Indians made a drink from the roasted seeds and the settlers had learned to make a coffee substitute. Seeds of the tree can be bought on-line (e.g. Amazon.ca) and can be planted in private gardens as an ornamental tree. The tree grows in sandy loam under full sun, but can tolerate dry and alkaline soils under partially shade. The leaves and the pods contain alkaloid cystisine that is bitter and toxic to humans and animals. A tea made from the leaves or pods was used as a laxative. The green seeds are not bitter and somewhat sweet when eating raw or in boiled water. A small amount of eating these seeds or drinking a coffee from the roasted seeds might likely not bring a “heavy” adverse effect, although cautions must be taken.

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