A few species of mustard came to naturalize in California, and for a few distinct reasons. Some likely grew first as greens in the gardens of the original Spanish Missions. At least one species also provided culinary and medicinal mustard seed. The more prolific types became cover crops and fodder for livestock. Later in history, mustard seed provided oil.
According to legend, Spanish Missionaries established the route of El Camino Real with mustard. After sporadically dropping seed as they traveled between Missions, they could follow the bright yellow bloom by the next winter, and find greens to eat. El Camino Real became worn enough to navigate before the mustard dispersed too extensively to assist.
Most but not all species of mustard that are naturalized locally are of the genus Brassica. Wild turnip and wild radish provide similar greens, but are generally more likely to bloom pale pink or pale white, rather than bright yellow. Garden varieties of mustard, which are available as seed or in cell packs, provide delightfully tender new leaves with mild flavor.