Harvest seeds in early August when the plants just start to yellow, but before the pods get too brittle. Cut the stalks just below the first stem branches.
Hang the plants upside down to dry in a cool, dry area for at least two weeks. After they are dried, separate the seeds from the plant by beating them over a screen and catching any seeds that fall.
Make mustard out of the seeds. Yellow mustard plant seeds produce a mild mustard, while the brown and oriental seeds will give you a stronger, spicier mustard. The flavor is at its strongest when mustard is made fresh, so only make enough for immediate consumption.
Return any leftover seeds to storage. Be careful to avoid cracking the seeds while moving them. Store all seeds in a tightly closed plastic bin.
Save some mustard seeds for planting. Mustard is an annual plant and therefore must be replanted each year. To grow mustard plants, push seeds about ½ inch to 1 ½ inches into moist, fertile, well-drained soil. Keep seeds wet until germination, which happens about three to seven days later. Plants should mature in 80 to 90 days and will once again be ready for harvest.