Question: I see catalogs listing OzoneO ratings. What does this mean, and how do I know if a plant will grow in my region?

Answer: Most catalog listings refer to the USDA Zone map, which was created in 1960 and updated in 1990. These zones are based on average winter minimum temperatures from 125,000 weather stations across North America. The USDA Zone rating provides general guidance, but it isnOt foolproof, because it doesnOt take into account other factors that influence plant hardiness, such as rainfall, summer high temperatures, snow cover, etc. Also, gardens can have their own Omicroclimate.O For example, a sheltered spot up against the house may be warmer than a windy low area or Ofrost pocket.O Most plants are rated with a minimum zone of cold hardiness. These are the numbers you see in catalogs or books: "hardy to zone 3," or sometimes simply, "zone 3." A plant rated hardy to zone 3 can withstand temperatures as low as 40 below zero. In contrast, a plant rated to zone 8 will only tolerate lows of ten degrees above zero. Sometimes you will find a listing such as "zones 3 to 8." This means the plant is cold-hardy all the way to zone 3 but does not perform well in zones warmer than zone 8. The USDA zone map works best in the eastern half of the country. In the west, factors such as elevation and rainfall can vary dramatically within a region. Therefore, many western gardeners use the Sunset Western Garden Book's zone system because it takes a variety of factors into account, not just minimum winter temperatures. And last but not least, for gardeners in hot regions, whose plant choices are limited by intense summer heat, the Heat Zone Map was developed.