I probably first encountered the cute little Geranium robertianum on the shores of Lake Huron at Drummond Island. It's a small-flowered cranesbill, known 'commonly' as herb-robert. Historically it was first collected in Michigan on Mackinac Island, and then later along the shores of the Great Lakes. It is a circumpolar species, found in parts of northern North America, Europe and Asia.
What gives? It is a plant of tremendous fecundity and it has been in Michigan for thousands of years. Why haven't I encountered it locally before?
I e-mailed Tony Reznicek of the University of Michigan Herbarium. His reply: "Well, you have the same problem that I have. I think I got my Geranium robertianum from H. E., and am pretty sure it is the European form. It is somewhat invasive for me, but not so much in woodland shade. I doubt it will be another garlic mustard.
"I should note that farther north in Michigan, Geranium robertianum appears to be a native of rich woods, presumably a different form that your plant and mine."
So that's the likely story: it may be a native species, but there are a variety of forms found in its huge range. It seems we've introduced an aggressive genotype from overseas into the local flora. And now I can't decide how aggressive I should be about eradicating it from my property. How native is it? How important is it to protect the woodlands from this "newcomer"? Is it inevitable? Does it matter? If it gets out it will change the character of my woods, but it is not at all clear that it will outcompete anything native. After all, other than Virginia creeper, mayapple and the native oaks, hickories, cherries, etc., what I gave growing in my woods consists of Asian bittersweet, garlic mustard, weedy celandine, buckthorns, autumn olive, burning bush, japanese barberry, etc. -- all things that I wish would just disappear. I'm tempted to say "Go for it, little geranium." I don't know if I can stop it. I pulled up one large clump, threw it over my shoulder, came back a week later and found that it had re-rooted itself.
Wikipedia tells me that in the state of Washington, it is known as Stinky Bob and classified as a noxious week.