How Reliable Are the USDA Hardiness Zones?

The USDA hardiness zone map has a total of 11 zones. Each zone, from zone 1 to zone 11, is on average, 10 degrees colder than the next lower one in the winter. This works very well in flat country, but in areas where variables come into play such as the mountains and plateaus which are colder than the adjacent low-lying regions and cities which generate extra heat, gardeners have to be a bit more careful. Although, if you look very carefully the newest maps are much more accurate in indicating mountains and plateaus.

Another factor is global warming. The last time a map was released was in 1990. A revision, due in 2003, which showed that the temperatures was rising, was never published. The latest version was released in 2009 and there is a big difference. Take a look at this version of the 1990 map-.arborday.org. Click on the play button to see the 2009 map and watch how the hotter zones are moving ever further north. For instance, in the old map, the Florida Keys are in zone 10. Now they are in zone 11. Take a look at the top of the map and watch zone 3 start to disappear into Canada. By the time the next map is created, hopefully in less time than it took to make this one, zone 3 could be completely in Canada.

Another thing to consider is the fact that the map only rates plants by their cold hardiness. Plants can also be damaged by summers that are too hot. Be careful what version of the map you are referencing. If possible, pick plants that are hardy a couple of zones away from yours. For instance lets say you are in zone 4. Try for plants that are hardy from 3 to at least zone 5. That way you will not be at the coldest or hottest extremes. If your zone is the coldest zone the plant is hardy in, plant it where it will be protected from winter winds if the temperatures get lower than usual. A good mulch is also helpful in keeping the roots of the plant from freezing. Putting a burlap fence around the plant is another idea. Simply put a stake as tall as the plant at four corners around the plant to make a square and attach burlap around the outsides. It will help keep the wind away and still give the plant room to breath. Your local garden center will also have some ideas.

If your zone is the hottest one a plant is hardy in, plant it where it gets the sun in the morning, not at the hottest time of the day. If the plant can take full shade, plant it in the shadow of a taller one and be sure to keep the soil the way the plant likes it.