If the ground is frozen, or very wet, simply put them outside because in the vast majority of cases the plants are very hardy.
How hardy are perennials?
The vast majority of perennials will survive even the most severe frosts, including those that go down to -10 degrees. However if you cannot plant your perennials for a few weeks and a hard frost is threatening plants with thick roots such as Anchusa and Papaver, and more tender perennials including Penstemon should be kept somewhere out of the cold especially if the pots are very wet. This is because the roots can ‘explode’. We suggest that you put the plants under cover such as a cold green house, but not for long. Plants them as soon as you can.
Should I put them in a green house?
Except for the perennials mentioned about, DO NOT PUT THEM IN A GREEN HOUSE. If you put the plants in a green house they will grow and the new growth will be soft enough to be damaged by frost once placed outside.