Gardeners have always been interested in the weather, and with the advent of major media coverage of climate change, weather is an almost daily concern for people everywhere. Weather is especially critical to those of us who grow plants because the weather affects not only the health and growth rates of our crops but also determines to a large extent which plants we can grow well in the first place.
When you’re choosing plants and designing gardens
, keep in mind that there are climatic variations within a geographic region and even within each garden. Your garden’s immediate climate may be different from that of your region overall. Factors such as altitude, wind exposure, proximity to bodies of water, terrain, and shade can cause variations in growing conditions by as much as two hardiness zones in either direction. It is also important to realize that your area’s climate can change over time.
Cold Hardiness and Heat Tolerance
Hardiness is the ability of a plant to survive in a given climate. In the strictest sense, this includes not only a plant’s capacity to survive through winter, but also its tolerance of all the climatic conditions characteristic of the area in which it grows. Still, most gardeners refer to a hardy plant as one capable of withstanding cold and to a tender plant as one that’s susceptible to low temperatures and frost
In order to help growers determine which plants are best for their regions, in 2006 the National Arbor Day Foundation, using data from 5,000 National Climatic Data Center stations, released an updated version of the 1990 USDA Plant Hardiness Zone Map
. The USDA is also at work on an update to the 1990 map; it was due to be released in 2010, but you know how that goes. Hardiness zone maps divide the United States into 11 climatic zones, based on the average annual minimum temperature for each zone. Zone 1 is the coldest, most northerly region (in Alaska), and Zone 11 is the warmest, most southerly (in Hawaii). If you live somewhere in Zone 6 and a plant is described as “suitable for Zones 5–9” or “hardy to Zone 4” you can expect the plant to do well in your area. If you live in Zone 3, on the other hand, you should select a more cold-tolerant plant. You can find out which zone you live in by referring to the map.
In addition, recognizing that hot weather can also limit plant growth, the American Horticultural Society (AHS) has released its AHS Plant Heat-Zone Map
, based on 12 years of climatic data ending in 1995. On this map, the United States is divided into 12 zones based on the number of days the region experiences temperatures above 86°F (“heat days”). Zone 1 experiences less than one heat day; Zone 12, over 210 heat days. Use this map the same way you would the Hardiness Zone Map.