What's Your Zone?
"What do those zone numbers mean?" and "What zone am I in?" are two questions I frequently get from folks who've been reading signs at garden centers and nurseries or perusing plant catalogs. Well, here's the scoop.
The zones that you often see referred to in garden books and catalogs are usually USDA Hardiness Zones, based on the average minimum winter temperature range in a particular area. There are 11 different zones in the US, with the lower numbers corresponding to the colder areas. The zones are also sometimes broken down further into "a" and "b", with "a" denoting the colder part of the zone.
To find out what zone you're in, you look at the Hardiness Zone map, compiled by the USDA. Many gardening books also feature copies of the hardiness zone map.
Say, for example, you live, as I do, in northwestern Vermont. If I look at the map, I see that I am located in Zone 4b, where the average annual lowest temperature is between -20 and -25 degrees F. (Yes, it can get cold up here!) Often, the zone ratings you see for plants don't bother with the "a" and "b" divisions; in this case, Zone 4 temperatures go from -20 degrees to -30 degrees F. The zones are useful as a guide for selecting plants that will make it through the winter in your area. But it's important to remember that the zones are just a broad guideline; within any zone there will variations depending on elevation (higher elevations will be colder), how close you are to a large body of water (which moderates temperatures), amount of snow cover (this protects plants), wind exposure, etc. What this means for me is that plants listed as hardy to Zone 4 or lower are probably good choices for my area, but a plant listed as hardy to Zone 5 or higher would be less likely to make it through the winter where I am. But if I lived at a high elevation, I might find plants listed as hardy to Zone 3 to be more reliable choices. Conversely, I might be able to grow Zone 5 or higher plants if I have a protected spot on my property.
Sometimes you'll see an upper zone listing as well. So, for example, when I look up paper birch (Betula papyrifera) in one of my reference books, it is listed as suitable for Zones 2-6. What this means is that in Zones 7 and higher, which are the warmer parts of the country, this northern tree won't thrive - the climate is just too hot for this particular tree.
Just to make things more confusing, there are other zone maps in use, although the USDA map is most widely used. Garden books published by Sunset and Southern Living have their own climate zone maps that pertain specifically to the western and southern regions of the country, respectively. And the American Horticultural Society has developed a plant heat-zone map that is helpful in assessing the suitability of plants to summer heat, rather than winter cold, and is based on the average number of days per year when the temperature is above 86 degrees F; the higher the number, the greater the number of warm days. For example, the white fir (Abies concolor) is rated for AHS heat zones 7-2, which means it is best suited for parts of the country that have at most between 61 and 90 days above 86 degrees.
So consider zone designations when deciding on the best plants for your garden, but remember to check which map they refer to and keep in mind the particular conditions in your own landscape.
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