Greens: You should harvest mustard greens while they’re still young and tender. Older leaves will get tough and increasingly bitter as they get older. Discard any yellow leaves that may appear on the plant. Mustard greens are harvested one of two ways. You can either pick individual leaves and leave the plant to grow more, or the entire plant can be cut down to harvest all the leaves at once. As a rule of thumb, it is important to never harvest more than 1/3 to 2/3 of the plant at one time, so that it will continue to grow. If you want to continue growing leaves longer, pluck off the flowers as they come in. Once the flowers blossom, the leaves will no longer grow. Seeds: A popular way of harvesting mustard seeds from their pods is by pulling up the plant and hanging it upside down in a paper bag and allowing it to dry out. After two weeks, shake the bag and the seeds should fall out of its pods and drop to the bottom. This method is the hang dry method. Another convenient way of drying them is to pluck the individual pods and dry them on a screen for two weeks in a single layer as well. When harvesting mustard, if the seeds are already not dry enough, these methods work nicely along with other alternative methods: oven drying and using a food dehydrator.