The potential for damage certainly exists but really only when planting. Once in the ground, the roots of the geranium will co-mingle quite peaceably with the JM.

So try to minimize the disruption as much as you can. Pre-plan the placement to avoid any major roots, plant small if you can (4" or quart much better than a gallon) and as small a quantity as you feel you can get away with. Don't opt for full coverage immediately - the geranium does spread and quite handily :-) And doing this planting in winter while the tree is still dormant will present the least disruption to the roots.

oh that's such good news. i will get my plants from a local woman and they're all in 4" pots. perfect!

Hardy Geraniums do just fine under my J. maples.

Thank you for your comment, Mike. That's an impressive, healthy sweep of ground cover on your property. Beautiful landscape!!!

I use hardy Geraniums for weed control. Shotweed and chickweed are NOT problems. Of course, I don't grow many small plants in the ground. There is a trade off.