I have given up on zones long ago. Trial and error is what I have used for the last 15 yrs.
All my zone 6 are not created equal. Temp alone does not help but the amount of snow cover that could insulate vs a zone 6 that get less snow cover. Without the snow to insulate some plants the cold get to the plant.
A zone 6 like mine with more rain then freeze instead of snow and then cold kill even zone 5 plants that get snow. So instead of cold killing the plant but the block of ice that the rain caused before the cold came in along.
