NH *does* go lower than -20°C. -20°C is -4°F. Many winters, NH goes to -20°F - which is -28°C. That might not seem like a huge difference, but trust me - it is. And there's probably an occasional -30°F.
I still remember a California winter where the temperature fell to 19°F... and remained below freezing for ten days straight. A *huge* number of trees died - though I acknowledge that my cherry tree survived it. So I'm wary of anyone who assumes that trees can necessarily handle New Hampshire's much, MUCH colder weather.
I figure if all these trees can live there, I'd probably be SEEING them all there. But no, I'm just seeing apples, peaches, cherries and nectarines, and no nut trees at all.
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