Wait until the poppies start to die. The stems will turn brown and feel fragile to the touch. Check the plants for falling petals and a hard, dry, gray-green bulb in the center. This means the seeds are ready for harvesting.
Pull off the gray-green bulb portion of the poppy flower. Be careful not to rip the bulb or crush it.
Place the bulbs in a brown paper bag and tape the bag closed.
Let the bag sit in a dry, cool room for three weeks to let the bulbs dry out completely.
Take out the bulbs one by one. Snip off the top portion of each bulb and shake out the seeds into another container. There should be more than 100 seeds in each bulb. This is the most effective way to harvest the most seeds.