It’s best to let seeds ripen on plants until they are almost ready to disperse on their own. Any seed harvested too early will not be viable. Seeds usually change color (from light to dark) when they are ready to be harvested. Cut off the entire stem containing the spent flower head or the entire pod containing the seed. Take them indoors to a cool, dry spot to let them completely finish ripening inside a closed paper bag. Mature seeds are happiest when they are stored in a cool, dark, dry place in an airtight container until it’s time to replant them.