Be a Mastodon today, grow Kentucky Coffee Trees for tomorrow: A homeowner’s guide to germinating Kentucky Coffee tree seeds
by Kristen Sandvall
Kentucky Coffee Tree (Gymnocladus dioicus) is a Species At Risk in Ontario, with only a handful of natural populations left across its natural range. There are various theories as to why this tree is so rare today. Despite its rarity Kentucky coffee tree, has gained popularity in urban landscape due to its tolerance to insects, disease and drought.
One of the most common theories is that the Kentucky coffee tree needed megafauna, large extinct mammals, to eat and disperse the seeds. By ingesting the seeds, megafauna such as mastodons and giant ground sloths, broke down the hard seed coat with their stomach acid, allowing them to absorb water and germinate after being dispersed through the megafauna droppings! With human intervention we can mimic the seed-dispersal abilities of megafauna and help these seeds grow!
These trees belong to the Legume family and are actually related to Cover, Beans and Peas! Their seeds are contained within a large, brown pod, resembling a stout, wavy bean, 4-8 inches long, containing 1-5 large round seeds, about 1 inch in diameter. **Do not confuse these pods with the small, more twisted bods of Black Locust or Honey Locust (seeds less than half an inch in diameter), or Catalpa (pods are longer than 8 inches, and perfectly straight and not flattened).**
Coffee Tree pods hold onto female trees typically throughout the winter months and begin to drop to the ground in the early spring. You may find these pods under trees planted along boulevards, in municipal parks, or even in your neighbor’s yard (make sure to ask for permission to collect seed!)
Extracting the seeds from the pods can be a bit sticky, so gloves are recommended. Once you have your seeds, you will need to scarify them. This just means removing some of the hard seed coat to allow water inside. Nurseries that scarify thousands of seeds at a time use a concentrated acid soak (up to 24 hours in 98% sulphuric acid!), as this mimics digestion by a large mammal.
For propagation at home, there are other, less hazardous methods of scarification. One of the quickest ways is by carefully filing or sanding off a patch of the seed coat. If you have a small rotary tool for sanding, these work well! If you don’t have access to such a tool, try regular hand file, or some sandpaper (with a little extra elbow grease). Whichever method you choose it’s very important to do this carefully as to prevent injury to your hands. For added safety if when using mechanical methods, you can hold the seed with vise-grips while grinding the seed coat down. You only need to file down a small patch of the seed coat enough to let water in.
After scarifying your seeds, soak them for 24 hours in water (pseudo-stratification). At the end of the soak your seeds should have expanded to 2-3 times their size. The seed coat should be much softer now and you will see some of the outer, clear seed-coat peeling off. If your seeds are still very hard and did not expand you can repeat the filing step a bit more and repeat the soak process. Once you have nice plump, expanded seeds plant them in your favourite potting soil about 1 inch below the surface. Seeds will germinate in full sun within 2 weeks. With continued care, you can accomplish the work of a mastodon, and grow a rare, Carolinian shade tree for your yard!