Cold wet winters, hot humid summers and the best fall foliage in the country New England ranges from zone 6 down in Connecticut to zone 3 in parts of Maine and New Hampshire. In the right conditions, you might even be able to get away with a plant better suited to higher zones.
The last frosts can be any time from late April to early June. Memorial Day is a good bet for planting in most areas. The cold, snowy winters can be very good for many perennials but don't forget that snow and ice can rip limbs from trees and shrubs.