The best way to handle this is to wait until fall, dig up the bulbs, split them and replant. It may take a few years for them to get large again, but they will never be the same size as they were when you first planted them because those bulbs were grown in Holland where growing conditions are ideal. Few places in the United States can mirror those growing conditions. But, they should grow bigger as you divide them. You can't grow these from seed. It's not worth the time to try this, especially it would take many years to get a suitable size for bloom and Globemaster is a hybrid so you don't know what you'll get. Remove the heads so they don't develop seeds. You want the plants to put their energies into building up the bulbs, not producing seeds.