A non-native annual, Field Mustard, Brassica campestris (also called Rape [from the Latin for turnip], Brassica rapa), grows all along the northwest coast of the United States and Canada, frequenting low-elevation disturbed sites. (Varieties grow all over the United States.)
I’ve seen plants blooming in late January at a flat, sandy river bank near the mouth of the Elwha River in Northwest Washington State. In the Dungeness Wildlife Refuge, mustard plants are much larger and more exuberant, blooming in May.
Mustards are members of the family Cruciferae. The four-petaled flowers are diagnostic of the family. The lower leaves of Field Mustard are long and somewhat lyre-shaped, while upper leaves wrap around the stem at their bases. One specimen, growing in loose sand, had several vertical stalks connected by under-sand horizontal stems. (6) I didn’t see any evidence that these stems were rooting.
Pojar and Mackinnon write, in Plants of the Pacific Northwest Coast, that Field Mustard was cooked and eaten by northwest tribes, including the Stl’atl’imx. Cultivated plants such as turnip, rapes, mustards and Chinese cabbages all belong to the same species, which was domesticated as far back as 1500.