Mustard seeds come from the edible mustard plant, typically grown in home gardens as an edible green. As the weather heats up in summer, the mustard plant sends up a tall seed stalk. Stalk production, or bolting, ruins the flavor of the leaves but provides the promise of seeds for culinary use as a spice. The seeds form in long, skinny pods after the bright yellow flowers on the stalk wither and fall off.
Cut off the seed stalk in late summer after it has yellowed but before the seedpods begin to split open. Cut the stalk beneath the lowest seedpod on the stem.
Place the seed stalks in a paper bag as you collect them. Mustard pods break open easily when disturbed so the bag catches any seeds that drop out.
Set the open bag in an 80 F or warmer location, placing it away from direct sunlight. Dry the stalks for one to two weeks or until the pods begin splitting and spilling the seeds.
Remove the stems and empty pods from the paper bag. Pour the seeds into a bowl and separate out any nonseed plant material. Shake into the bowl any seeds remaining in the pods.
Store the dried mustard seed in an airtight container in a cool, dry place.