Kentucky coffee-tree (Gymnocladus dioicus) is a native tree of Michigan and is known to reproduce clonally by root suckers especially when the main tree is damaged or removed. Eventually the root system will depleted of resources, so if you keep cutting them back it will help, but it could take some time.
In a lawn setting the active ingredient triclopyr would work well to control the tree shoots while not harming the grass. This can be found in products such as BioAdvanced Brush Killer Plus. Triclopyr is active on broadleaf plants and it can persist in the soil for up to several months, so it is not recommended for ornamental or vegetable garden areas. The seedlings can be cut near the soil surface and the concentrated herbicide can be painted on the cut (within 5 minutes). You could also spray it on the foliage without cutting them, though you will need to follow the directions for mixing if you have a concentrated product versus a ready-to-use product (they usually have two versions, so pay attention to the concentration and directions). The herbicide will be taken up into the root and rhizome system to help achieve a complete kill. Depending on your infestation level you may need to repeat this over time on other newly emerging seedlings to fully kill the underground system. Remember with any pesticide application it is critical to read and follow all labeled instructions for maximum effectiveness and environmental and personal safety.
Another option would be to use a glyphosate product (such as Roundup Weed & Grass Killer, among others), though this will harm the grass nearby. Similar to the triclopyr you can utilize a “cut stump” treatment. Instructions for this are explicitly stated on the product Roundup Super Concentrate. When using products containing glyphosate there are a few important points to consider. First, as with the triclopyr, remember to read and follow all labeled instructions. Second, glyphosate is a broad-spectrum herbicide, meaning it will injure or kill other plants contacted during the application, so care is needed to avoid green plant material, exposed roots, and injured bark of desired plants. Third, glyphosate is relatively safe in the environment when used as labeled. It adsorbs strongly to soil in most cases (i.e. clay and organic matter), allowing even sensitive crops to be planted shortly after application; meaning no carry-over issues are expected. Retreatment of the area may be needed depending on the degree of infestation. Finally, be sure that the product you choose has only the active ingredient glyphosate or glyphosate + pelargonic acid. Products with additional active ingredients may have other unwanted effects and may delay the planting of other plants in the coming season(s).
Both triclopyr and glyphosate are most effective for perennial control in the fall but can be applied anytime the plants are actively growing (temperatures consistently above 50F).
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