The Hardiness Zone is the standard by which gardeners and plant growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones. Ohio is divided into 3 zones with most of the state in zone 6A. Check on the plant you're thinking of growing and it will include a Zone number, this tells you which area that plant can reasonably be expected to survive. However, Ohio is Ohio and the weather here can be highly unpredictible. One year we can have extremely mild winters and other years, we might as well be living near the arctic circle.