Gymnocladus dioicus (L.) k. Koch
Kentucky Coffee Tree
Basis for Listing
Gymnocladus dioica (Kentucky coffee tree) is uncommon or rare wherever it occurs (Isely 1998), and it may be the most enigmatic tree in Minnesota. Although there are a number of scattered records in the state (Eastern Broadleaf Forest and Prairie Parkland Provinces), most have fewer than 50 trees and none are known to be reproducing by seed. This is important because even though G. dioica can reproduce by root suckers (a form of vegetative reproduction), sexual reproduction (via seed) may be necessary for the long-term viability of populations. Given its limited distribution in the state and the concerns over reproduction, the species was listed as special concern in 2013.
Gymnocladus dioica is a midsize tree, up to 23 m (75 ft.) tall, and 74 cm (30 in.) in diameter at breast height (dbh), with stout and spreading or weakly ascending branches. It is clonal by root suckering. The bark of mature stems is gray, with shallow fissures and flat plates that tend to curl at the edges. The leaves are bipinnately compound, with numerous leaflets. The inflorescence is a loose terminal raceme, 7-20 cm (3-8 in.) long, bearing 18-50 flowers. The flowers are usually (though not always) unisexual, with male and female flowers on separate trees. The fruit is a thick and heavy pod, yellowish and leathery in summer, turning reddish brown and woody by winter, and persisting on the tree until early spring. The key to identifying this tree is understanding the large compound leaf. It is the largest leaf of any tree in northern climates, nearly 3 ft. (1 m) long, and with as many as 100 separate leaflets. Do not mistake the individual leaflets for the entire leaf. The leaves of honey locust (Gleditsia triacanthos) and black locust (Robinia pseudoacacia) are similar, but the individual leaflets of G. dioica are larger and have pointed tips. The large strangely shaped pods are also unique, if not bizarre, and can usually be found on mature female trees all winter (Smith 2008).
Gymnocladus dioica is found most often in mesic hardwood forests on terraces of the Minnesota River (in the Prairie Parkland Province), the Mississippi River below the Twin Cities (in the Eastern Broadleaf Forest Province), and a few major tributaries. These are raised terraces, well above the reach of normal flood events. Although it is clearly a forest tree, seedlings of G. dioica do not do well in the shade of a dense forest canopy; they generally need open ground and small patches of sun. That said, G. dioica also produces suckers directly from its roots, and the suckers seem to do fairly well in shade.
Biology / Life History
The genders of G. dioica are usually separate, meaning that each tree is either male or female. As a result, some sites are known to consist entirely of single-sex clones derived by root suckers from a single parent. Gymnocladus dioica is notable in Minnesota as the last tree to leaf out in the spring and one of the first to drop its leaves in the fall. In fact, it is seen without leaves for so long (at least 7 months of the year) that healthy trees are often mistaken for dead. The pods are another mystery. No animal that currently shares its habitat is known to eat them or disperse the seeds within. The pods simply fall from the tree and eventually rot where they land. It has been theorized that the animal that evolved to disperse the seeds may have become extinct near the end of the Pleistocene Epoch, about 13,000 years ago, when so many large North American mammal species became extinct (Barlow 2001). This could explain why G. dioica has become so uncommon and the surviving populations so scattered and isolated. There are other theories of how G. dioica seeds were dispersed, some involving the migratory aspect of certain Native American Indian tribes.
Conservation / Management
Since G. dioica reproduces clonally from root suckers, it is possible that each tree in a “stand” originated from one individual and is, therefore, genetically identical to every other tree in the stand; they would even be the same gender. Such a stand would be of less conservation value than a stand with numerous genetically distinct individuals and a relatively equal distribution of genders (Smith 2008). It is not certain whether such a population exists in Minnesota or, if one does exist, how to assure its survival. Population structure of individual stands would not be an issue, except the sites are so far apart there may be no gene flow between them, further complicating conservation efforts.
Best Time to Search
Gymnocladus dioica can be identified any time of the year, but it is easiest to identify when fully developed leaves are present, which in Minnesota is between about June 1 and September 15.
Conservation Efforts in Minnesota
A few publicly-owned sites are known to harbor G. dioica and this may afford adequate protection to the individual trees within their boundaries.
Welby Smith (MNDNR), 2018
(Note: all content ©MNDNR)
References and Additional Information
Barlow, C. 2001. Ghost stories from the ice age: some plants are haunted by large mammals from another era. Natural History 110:62-67.
Isely, D. 1998. Native and naturalized Leguminosae (Fabaceae) of the United States. Brigham Young University Press, Provo, Utah. 1,007 pp.
Minnesota County Biological Survey. 2007. Native plant communities and rare species of the Minnesota River Valley counties. Division of Ecological Resources, Minnesota Department of Natural Resources, St. Paul. 153 pp.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the prairie parkland and tallgrass aspen parklands provinces. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 362 pp.
NatureServe. 2009. NatureServe Explorer: an online encyclopedia of life [web application]. Version 7.1. <http://www.natureserve.org/explorer>. Accessed 26 June 2009.
Smith, W. R. 2008. Trees and shrubs of Minnesota. University of Minnesota Press, Minneapolis. 703 pp.