Wild radish. Non-native, an invader from Europe. Found along roadsides, and on our mesas.
This is a complex of species (R. sativus, R. raphanistrum) and hybrids which I have not bothered to sort out. They all appear pretty much the same to my eye differing only in flower color. In theory R. sativus has blue blooms, R. raphanistrum has cream blooms, and the white blooms are hybrids.
Blooms year-round. At its peak in Apr-May. The cream coloured variety is fairly rare in Santa Barbara, most plants are blue or white (and shades between).
Seedpods start appearing at the end of April