Acetylcholine. in CNS and PNS: generally excitatory but is inhibitory to some organs and glands; an organic, polyatomic ion that acts as a neurotransmitter in both the peripheral nervous system (PNS) and central nervous system (CNS) in many organisms including humans.
So Zone 3 is more typical of North Dakota, while Zone 9 is lovely Florida weather. Each hardiness zone differs by 10 degrees F, so the difference in low winter temperatures between zones 6 and 7 is 10 degrees; between zones 6 and 8, 20 degrees; between zones 6 and 9, 30 degrees, and so on.
Plants are classified by the coldest temperature they can endure, using a system developed by the United States Department of Agriculture. Regions of North America have been divided into Zones based on the lowest recorded temperatures, from the coldest, Zone 1, to the warmest, Zone 11. See our Zone map.