a proliferation of Evening Primroses. Two years ago it was Bidens. They're always yellow, that's all I know. OH wait they're not. Two years ago I remember Teucrium canadensis in the Fern Valley Prairie. This year it's buttercups. There are buttercups every year, but this particular portion of the National Grove of State Trees has not looked like this in my memory, and I see similar drifts all around the greater Washington area.
The world of living things is so complex I don't pretend to have any idea why these population explosions happen. In Plant Pathology, decades ago in college, we had to explain, on tests, where a particular disease came from. Often the answer was, "the inoculum is ubiquitous." Which implied that the disease occurred because circumstances became favorable for its development. I imagine it's the same with these plants; the seeds were there, or small plants, and circumstances arranged themselves in such a way that massive number of the plants matured. I wonder if this number of plants exists every year and only rarely develop to maturity. Or did this start more than a year ago with an extreme germination event? Or both?