Poppies usually flower in late spring to early summer. When the petals begin to drop, the ovary develops into a chubby seed capsule. These capsules contain hundreds of tiny black seeds, which are edible [ and very delicious!] in the case of Papaver somniferum.
The seed capsules are green when young [see today’s photograph]. Near the end of the growing season, they begin to turn brown and develop a hard carapace. They will eventually crack open, releasing the small seeds.
For a poppy seed harvest, one must wait until the capsules are fully dry. You can tell when they are ripe by shaking the stem. If the capsules rattle, it is a good indicator that it is time to harvest. Collect them now, otherwise they will spread themselves!