Woodsense: Kentucky Coffeetree

By Robert J. Settich March 4, 2024

Tough wood with an uncertain future

There are only a handful of trees of the Gymnocladus genus worldwide, with all but one in Asia. Kentucky Coffeetree (Gymnocladus dioica) is the only native North American in the group.

The first part of the tree’s scientific name means “naked branch,” an allusion to the fact that it leafs out very late every year and then is also usually the first tree to lose its foliage. As a result, the tree may be bare for up to six months every year. The scarcity of branches, though, makes it a popular ornamental tree for people who want as much light as possible during the fall and winter months.

Making the tree look even more bare is the fact that its compound leaves are huge—12 to 36 inches long and 18 to 24 inches wide—means that there are very few branches. The leaf is in a bipinnate formation that subdivides sets of leaflets into even more leaflets, with the overall form resembling a fern frond.

The tree has a Janka hardness rating of 1390, just a bit tougher than white oak. It is a special type of ring-porous wood where the pores congregate in the latewood in small groups called pore clusters—a feature unique to Kentucky Coffeetree. The color of the heartwood as well as its surface texture reminds me of red oak, though other people think the grain pattern is similar to ash. There’s very little of the yellowish-white sapwood, and it has a sharp boundary. If you have any difficulty distinguishing the wood, shine a UV lamp on it to remove your doubts. (See the box “Glowing ghoulish green.”) The wood has a fairly strong reputation for insect resistance and reportedly withstands ground contact as a fencepost.

Adaptable but stubborn

Kentucky Coffeetree tolerates a wide range of conditions, hardy in zones 3 through 8 in the United States. It survives in dry uplands to damp bottomlands, even those that flood occasionally. The tree shrugs off pollution, deicing salt, and drought, making it tough enough to survive urban environments. Deep roots and a scarcity of branches, makes it resistant to wind damage. The tree can reach higher than 70 feet, with a trunk diameter averaging about 2 feet.

Despite this resistance and adaptability, Kentucky Coffeetree simply isn’t very good at propagating itself. (The International Union for the Conservation of Nature (IUCN) lists the Kentucky Coffeetree as a vulnerable species on their Red List.) Only female trees bear the pods, and for those seeds to be viable, she requires a male tree nearby. Fallen pods need to decompose to expose and then soften the hard shell of the seed. That can take two years and yields only a 5% germination rate. If you want to give nature a hand, you can boil the seeds or file through the hard shell before planting.

An easier solution is to find seedlings ready to plant. As of this writing, the Nursery of the Iowa Department of Natural Resources offers plants for only $1.10 each, though the minimum order is 25 seedlings.

Like the living dead. When fully clothed with its leaves, you can hardly tell that the tree has few branches. When the leaves fall, some people think the tree looks dead.

A poor substitute. The tree’s roasted seeds make a type of coffee that is reportedly both pungent and bitter, making even chickory taste good. That’s saying something.

Coffeetree trivialities

Eponymous tree. The coffeetree was briefly the official state tree of Kentucky, the only species to include its state’s name.

Large leaf. The leaves of the Kentucky Coffeetree are the largest of any native North American species.

With a little help from my friends. The mastodon was a great friend of the Kentucky Coffeetree. That beast’s digestive system was so acidic that it dissolved the hard seed coating so it could germinate. Achieving similar results today requires hours of soaking in concentrated sulfuric acid.

Hardly worth the effort. Extracting seeds from the sticky goo inside the pods is messy, toxic, and labor-intensive. 100 pounds of pods yields only about 30 pounds of seeds.

Inculpability. There are very specific time and temperature guidelines for roasting the beans to make them non-lethal, but I’m not going to repeat them here. If something goes wrong, I don’t want your survivors to come after the magazine.