Zone 4 winter temps

We reached zone 4 average winter low temperatures the week of Jan 17, 2011. It was -24 degrees F at the Horticultural Research Center in Chaska, MN. Although grasses are covered with a thick blanket of snow for insulation, this air temperature is what determines the winter hardiness zones, USDA Zone 4 is -20 to -30 degrees F.  Good news for researchers!