It's true that you can't make an ID from just the flower, however there aren't that many yellow composites blooming this early in the year around here. I'll bet it's one of the Groundsels (Packera spp, they just changed the name recently so it's listed as Senecio in older books.) Scroll down for some pics very like yours. There are a bunch of different species, so yours might not be this exact one. WW Here is a link that might be useful: Ragwort, Packera (Senecio)

I don't know what it is but it is a gorgeous pic. Blue Ridge Mountain Girl

WELL...last night someone on name that plant forum indicated that it was called Yellowrocket, which led to finding that it is probably a very close relative of Yellowrocket: Wild Radish. Whis is a very close relative to Wild Mustard :-) The only reason it can't be Yellowrocket is because the stems and leaves have tiny hairs, which indicates it is Wild Radish. However. I have seen pics of Wild Radish now on Google Images that don't look like this plant, and some that show Yellowrocket looking dead-on accurate, save for the leaves. Ha. At any rate, I'm pulling them up...tis' not to be... Thanks so much for your help, timbu and fammsimm. Now I am also familiar with two other kinds of weeds, so if I spot them, I'll know right away :-) Joanna

Not sure about the second pic, but the first kind of looks like one of the native hydrophyllums, probably virginianum. If the younger leaves have any splotchy white markings that would cinch the deal.