I guess you mean the Oriental Poppy - Papaver orientale 'Patty's Plum' - in which case you need to wait until the seed capsules are dry and brown, which will vary depending on where you and the plants are, although most will ripen some time in July and August....if you gently shake the seed pods the tiny seeds will rattle inside when they are fully ripe. Don't wait too long though or the pod will split open and the seeds will be lost.