Can't vouch for its accuracy but the tale has the Padres scattering mustard seed like so many bread crumbs marking the passage north as they explored the wilds that were old California. Imagine the bewilderment they felt on the return trip south when they discovered the profusion of mustard blooms across every valley in every direction. Impossible to eradicate, you'll even find stragglers in vacant city lots and the wildlands of California are covered each winter and spring with wild mustard.
Incidentally, by July this will all be a brittle, golden, explosive fire hazard. The upside is that once dead, the stalks can be easily snapped off, stripped of their side shoots, and VOILA, a virtually unlimited supply of perfectly-sized, make-believe swords for little boys to fight daddy with.
(click the photo to enlarge)