I failed to get mine in the ground last year, but from what I understand it's "when the ground is workable" because they're shipped bare root and dormant, so you want to get them in the ground before it fully thaws (just like fruit trees). As long as they're covered with a few inches of dirt, another frosts before they finally wake up for good in spring won't hurt them.

Thank you for your help. I planted three plants last week. I found a good spot for the rest of the plants but haven't had time to plant them yet.

Sounds like you have an opportunity to experiment. Some went in before last frost. Put the others in after. See what wins out!

I shall let you know how they are doing. I will try to get out into the yard today. Its been cold and wet last few weekends.