Zone 4 is for -20 and that's what I usually go by. Most of the maps for this area of PA say 6, but the maps don't take into account the Pocono Mts and other hill top areas. Most plants rated for 5 and 6, just can't survive the winters.
This spring was warmer, but in 09, the spring was so cool, the trees didn't leaf out until the first of June. We had a hard freeze May 22.
