Answer: If the perennials have been forced in a greenhouse and are already actively growing way ahead of the normal stage they would be if growing in the garden, then you need to wait until after frost because they are too tender.
If the perennials have been grown outdoors and are fully acclimated and conditioned to the weather, then you could plant without waiting.
If you already have them and they are tender and you still expect frost, you can set them outdoors in a sheltered area with morning sun to acclimate. Bring them into a covered place such as a garage if temperatures drop below freezing.
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