Answer: You can plant dormant bare root perennials as soon as the soil can be worked, and you can plant container grown perennials that are acclimated to the weather as soon as the soil can be worked. Perennials coming out of a protected greenhouse may need to be conditioned and if they are very tender, may need to be protected from freezing temperatures. Those that have been growing outdoors can be planted any time in the spring. A year round layer of organic mulch about two inches thick is fine. The mulch would be placed over the root zone and around the perennials but not over top of the foliage or crown.
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