A friend asked me this so I figured I’d answer it here.
In spring, I wouldn’t start playing things until the end of March at the earliest. The best time is April-May, though we used to sell perennials at Natureworks in June into the summer. In spring, you want to make sure your soil is relatively dry before you dig. If you dig in while it’s wet or soggy, it will make the texture very heavy for the rest of the season.
In the fall, the ideal planting time is mid September through early November. Last year, however, I planted three shrubs at the tail end of November because the folks at Earthtones nursery said I could. I just went out and checked one of them and it’s got little buds coming!