Papaver somniferum. The very largest poppy flowers appear in early summer, followed by the very largest seed heads. The seed head of Giganthemum poppy is the size of a baseball, and chock full of edible poppy seeds for baking. This is an easy plant to grow, but you'll want to give it some room in full sun, in soil that is well drained and not too fertile. Direct sow the tiny seeds on the surface of the soil, and keep the area moist until germination. They need light to germinate. The massive flower stems grow 90-120cm (36-48") tall, topped by flowers around 10cm (4") across. Bees love the dark blue pollen.