When do you pick mustard greens, and what do you do about the yellow flowers?
Harvesting Mustard Greens
Certified GKH Gardening Expert
The yellow flowers indicate that the plant is bolting. Bolting will decrease the growth of the leaves and make them more bitter. You can cut the flowers off to slow this, but they normally mark the beginning of the end of the harvest season for the greens.
Here is an article that may be helpful: