Answer: It depends in part on what you're transplanting. I would wait until early spring to transplant fall-bloomers like asters and sedum. Plants that have begun to show signs of entering dormancy--browning foliage--can be moved in late summer/early fall. In cold regions like yours, it's best to divide and transplant no later than early fall, so that the plants have time to develop roots and get established before the cold weather and snow hits. Don't worry, though. Anything you don't get to this fall can always be transplanted in early spring. Be sure to water new transplants frequently during dry spells, since you'll invariably damage some of the roots during the transplanting process.
Congratulations on your success with your first perennial garden!
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