The beginning of june if you want to be sure about not having any frost problems. You could put them in the ground now but you will need to be close enough to visit them and cover them up at night if it is going to freeze or go down to the low 40s. Cover them the evening before then visit them again in the morning to uncover.
Also if you have the cash you could buy a really small green house and heat it at night.

hope that helps....