Late autumn/early winter rains, which persisted until about 10 days ago, have ensured that my particular part of the world is beautifully verdant, perhaps the lushest I have ever seen Northern California's vegetation. This means that most vineyard plants and weeds, (and I'll use the bothersome mustard as an example), seem to be having a unusually long growing season.
Recently, I was told about a neighbourhood vineyard that had purposely planted mustard in several rows of vines on either side of their driveway just to make the approach to the winery look pretty. Well yes, mustard en masse, in full bloom, does indeed look very pretty, but in my mind it is useless as a cover crop. I haven't the foggiest why anybody would go to the expense and bother of sowing mustard in their vineyard when all that effort could be put into a cover crop that would be of benefit to the finished wine (through nourishing the soil). Mustard has a very low biomass, it is not a legume as is often mistakenly thought, and most likely will take more out of the vineyard (in nitrogen - and water) than it would ever put back into the soil after disking-in as a green manure.
To add insult to injury, mustard also provides an overwintering site for the Grape Orange Tortrix (Argyrotaenia franciscana) worm/moth, a vineyard pest that can be every bit as problematic as the Grape Leafhopper.
I really had to look around for a mustard plant to photograph, eventually finding a lone example on my neighbour's property. There is no mustard in Vinoland, and if there was I'd pull it out. Even the aforementioned, mustard-enamored vineyard mowed their sunny, yellow, thigh-high crop down today as a frost damage prevention method...so it doesn't look so pretty anymore.
Weather, weeds, worms - it is a wonder that anyone chooses to be a farmer of anything, never mind wine grapes.