Harden off the seedings by reducing the water and gradually expose them to outdoors in May. Then, in May to June, plant them out about 6-12 inches apart.
Mulching can help too because it will retain moisture whilst suppressing weeds at the same time.
When the plants mature, remove the outer leaves so that the crown is exposed and can develop. Remember to remove any side shoots if they grow.
‘Prinz’ is a variety that is slow to bolt and is disease resistant making it a perfect variety to start with.