This is the mustard that may be found blooming at any time of the year, and on almost all trails (and in town too).
At first glance it looks like a small version of black mustard (Brassica nigra) (its leaves do not clasp). But its stems are rough and the veins on the backs of the leaves are hairy. It also tends to be smaller and bushier.
(Black Mustard, on the other hand, tends to grow straight and tall (often taller than I) with smooth stems and non-hairy leaves).