My zone 6 has not been - 0 F since the 80's. The ULTIMATE lows have been 6 F to 8 F above zero. That is the rare LOW temps. Our normal lows are 13 F to the 20's. The normal regular temps are 20's and low 30's. Those are in January pretty much. I have had good luck with some Live oaks this winter SO FAR. I mention this because it has been mentioned that zones are judges on the average lows. If that were STILL true Pa would be zone 8. Our AVERAGE lows are in the 20's and ULTIMATE lows would in the teens_ 7F. At the most conservative we would be zone 7. We are colder for longer than more SOUTHERN states too.