(Reclaimed from Bubblews, where the photo I snapped was destroyed by the system a few months later. Sideshowmom donated this particularly showy Bidens image to Morguefile.com. Virginia's wild species seldom, if ever, have flowers as big, relative to the leaves and stem, as this specimen.)
When the soil is wet, and the little yellow flowers are still showing, it's time to weed out Bidens. (I am not here talking about our Vice-President and others like him, although some people would like to weed those out of the political sphere too...) Bidens is the scientific name of a genus of plants that, according to Peterson's Field Guide, can be positively classified into species only by experts.
A few species of Bidens, found only in Hawaii, are rare. The ones found in the contiguous United States are weeds, and should be pulled up, because those tiny yellow flowers will ripen into larger clusters of annoying burrs. The most common species found in Virginia are B. bipinnata, B. frondosa, and B. polylepis. Some other names by which they are known are Spanish Needles, Sticktights, Beggars' Lice, Beggars' Ticks, and a few unprintable terms.
(B. bipinnata is fairly easy to distinguish from the other two:
Bidens are unusual in having two distinct kinds of leaves on the same plant. There's what scientists call the "basal rosette" of small round leaves, which appear first, and then as the plant matures it forms long, thin leaves above the base.
In spring, on the morning after a wet day, it is possible to enjoy looking at Bidens. The plants are still small and innocent-looking. Small flowers, yellow and white, can actually be pretty. So I like to take a moment to enjoy them before I yank them up. Once the soil dries out, they don't yank up easily, but on mornings like this one they do. There is never any shortage of Bidens that will mature, ripen, and scatter their annoying seeds, no matter how many we yank, so we should always uproot Bidens on sight.
This Bidens was found growing in a crack between slabs of limestone, and was yanked out and laid flat on a piece of limestone for the picture.