Harvest fennel seeds in late summer when they ripen on the stalks. Observe the seeds as they ripen. When they turn from green to brown, harvest immediately. Fennel seeds left on the stalks too long will be scattered by the wind.
Carefully clip the fennel seed head from the stalk, careful to not shake off the seeds. Place the entire seed head into a paper bag.
Hold the seed head upside down over a large bowl and work the seeds free with your finger tips. Pick out stray plant material that makes its way into the bowl.
Lay the seeds out on a drying screen in a dry area for two to three days to dry completely.
Place the seeds in a glass jar and fit with an airtight lid. Label the jar of fennel seeds.