Right now there is a lot of wild mustard in bloom around the Central Valley. It's a pretty plant but it's also a bit of a nuisance for anybody growing cabbage, collards, bok choi, or other cole crops. It's a nuisance not so much because it's hard to get rid of. The bigger problem is that it's a great over-wintering host for cabbage aphids.
I recently found one plant with over a thousand aphids on it. This image will give you an idea of what to look for. Compare this with what you can expect on your cabbage plants if you don't take care of the problem. Cabbage aphids are not just another aphid species that can be treated with the standard aphicides. They're a lot harder to get rid of than say green peach aphids or melon aphids. It's definitely worthwhile taking precautions to avoid this kind of damage. This is particularly true for growers that have their cole crops in the ground early.
This is the time of year when the aphids can jump from wild mustard to your crops. Some of the worst damage can happen early in the year before the aphids' natural enemies become effective. So enjoy the pretty yellow flowers - but preferably at a distance (and down-wind) from your farm.