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.
. . . And then, there is was, growing in profusion along the side of the driveway at a home on E Delhi in Ann Arbor. Ignoring the fact that it appeared to be quite rambunctious, I brought one home. Now I am chasing it down the hillside with a backpack full of Roundup, and pulling out specimens hidden under the leaves of mayapple and wild ginger.
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 have 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.”
A couple comments from Wikipedia round out the picture: “It has been introduced into many other temperate parts of the world, probably through its use as an ornamental plant, such as in the San Francisco Bay Area in California . . . In the state of Washington, it is known as Stinky Bob and classified as a noxious week.”