We live in the area of Erie, PA near the lake. When can we safely transplant old, large primrose plants?
You are in Hardiness Zones 5a-6a I see. Usually it is best to do the dividing and transplanting in the fall when they have gone dormant. However it can be done in late May as well, usually about a week after Mother's Day. Once you have transplanted them, water them in with some water with a product called Super Thrive in it as well as some root stimulator. This will help with transplant shock and get them going well.