Black mustard. Non-native, an invader from Europe. Found along roadsides, on our mesas, and on the lower reaches of most trails.
Easily confused with field mustard (Brassica rapa), the best way to distinguish is to look at the upper leaves, in B. nigra these leaves donot clasp the stem, while in B. rapa they do. Also this plant is much taller (anything above 4 feet is probably black mustard), and is more common when it blooms (Apr-May).
Also easily confused with Summer mustard (Hirschfeldia incana). Summer mustard is a smaller, bushier plant, its stems are rough and the backs of the veins of its leaves are hairy.
Black mustard is tall, not-bushy, with non-clasping leaves, smooth stems and non-hairy leaf veins.
Blooms Jan-June but is at its peak April-May. This is the species which covers our hillsides.
These leaves do not clasp the stem.
While those of B. rapa do.