I was happy last year to see the early blooming, cheerful marsh marigold, Caltha palustris, coming up in my garden. I remembered it with pleasure from my years living and hiking in Michigan, where it was one of the earliest wildflowers to appear in the woods, along with skunk cabbage. My new plant was next to a stepping stone, which must have kept the ground moist enough for the marigold.
Except it wasn't marsh marigold, it turns out, but a look-alike invasive known as lesser celandine, Ranunculus ficaria. I got the bad news via a short article in the Washington Post. I then googled lesser celandine, and came upon a great article that helped confirm what I now suspected: http://www.earthcaretaker.com/alienplants/mondaygarden/159/SS159lessercelandine.html
In addition to photos, the article lists some ways to distinguish the two plants. I dug up a bit of my plant, and sure enough, it had underground tubers, characteristic of lesser celandine.
The name celandine also applies to another cheerful yellow flowered plant, celandine poppy, Stylophorum diphyllum. Some of my master gardener friends suggest referring to it by its other common name, wood poppy, to avoid confusing it with the invader. Celandine poppy, though native, is considered by some to be somewhat invasive. It has been well-behaved for me, and I have it mixed in with the lovely native Virginia bluebells, Mertensia virginica.