First, some facts about transplanting:
In USDA Zone 4, the best time to transplant perennials is early spring (as soon as the ground can be worked) or late summer to early fall (late August through September), roughly 4 to 6 weeks before the ground freezes.
Ok, this is a good basic answer, BUT I have a modified one.
AI’s answer is because plants need to develop a root system. Why? The ground freezes and thaws from late fall to early spring. If the plant doesn’t have an established root system, the ground can push it out.
Now, how do you know when the first ground freeze is? Generally, averaging September 21 to October 7, no! This is the first freeze of the above-ground vegetation. What does that mean? Everything above ground freezes, and the root system, tubers, and bulbs enter winter dormancy.
When transplanting, you need to know when the undersurface freezes. Plant them before this so the thaw and freeze don’t push the entire plant to the surface.
Soil retains summer heat. It takes weeks of persistent cold, day and night, before the freeze penetrates down to a depth of 1 inch (often happening after a few nights when temperatures drop to 25°F or lower)
That’s true, but perennials are very durable. I’ve done this many times, even when I’m busy. Trust me, they will survive, and you can replant them as soon as you can in the spring.
I have photos of the process. This page will have links to every post related to transplanting. Click here!
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