The Large-leaved Avens flowers don't look much different from those of Yellow Avens, which, although it has yet to be documented for Saratoga County, is nevertheless quite common across the state (ranked as Demonstrably Secure). But it's true that the Yellow Avens does have much narrower leaves than those of Large-leaved Avens. Here's what Yellow Avens looks like:
It is definitely the size and shape of the leaves that distinguish G. macrophyllum var. macrophyllum, a very aptly-named plant (macrophyllum means large-leaved). This is a photo I took of another patch of Large-leaved Avens I know about in Essex County, taken in late May before the flowers opened but the basal leaves were well developed. That distinctive broad orbic or kidney-shape of the terminal leaflet of those compound basal leaves is the clincher for this species, and obvious even at this early stage of emergence.
It's that distinctive leaf shape that will help me find additional specimens of Large-leaved Avens, if there are any, when I go looking for them this weekend along that trail in Moreau Lake State Park. The flowers will probably be spent, having doubtless produced by now the burry seed-head all Avens species produce. This next photo shows the typical Avens seed head, this one from the Yellow Avens:
So wish me luck. The last time I found an Endangered species of wildflower at Moreau Lake State Park (that one was Whorled Mountain Mint, Pycnanthemum verticillatum var. verticillatum), it turned out to be the largest and healthiest population in the state. Let's hope that might also be the case for Geum macrophyllum var. macrophyllum, the aptly-named Large-leaved Avens.
UPDATE: My eagle-eyed pal Sue Pierce and I returned to where I first found this G. macrophyllum and by diligent searching found 13 more plants, in addition to the one I first found (which continues to thrive). This particular species of Avens appears to prefer an edge habitat, one that is neither in deep shade nor full sun. Of the 14 plants, 5 were mature plants with flowering stems (now gone to seed), while 9 of them possessed basal leaves only. There was no mistaking those big round basal leaves! So at least we know we have a population of this Endangered plant, and not just a single anomaly. Here's what the mature plant looks like now, with those distinctive burry seed heads: