On Hardiness Zones and Geography
April 15th, 2005,What a difference a week makes at this time of year. Last week our southern Minnesota landscape seemed barren, with few flowers blooming and the grass still dormant. This week the scene here in Northfield has changed greatly. Magnolia trees, forsythia bushes, and daffodils are blooming, and the grass has greened up with some recent rain. In our own garden, we have an encouraging number of tulips rising, though none have flowered yet. The trees are gradually beginning to come into leaf.
Of course, the threat of frost is still real through May in our area and into June for northern parts of the region, reminding us that cold temperatures are the major limiting factor for plant life in the Upper Midwest. Anything planted here now had better be truly hardy.
Most plants are sold with an indication of their “hardiness zones”–the areas in which they can survive. The most widely used zones are those developed by the U.S. Department of Agriculture (USDA). The USDA Plant Hardiness Zone Map is a thing of beauty and is also quite geographically instructive. It covers Canada and Mexico as well as the United States–a fact I appreciate, since it gets those of us in the U.S. out of our all too nationalistic mindset. Looking at such a map we can see patterns of climate that extend across national boundaries.
The plant hardiness zones are numbered one through eleven and are based on average annual minimum temperature. Why is annual minimum temperature so important? Because low temperature is a primary source of stress for plants. (See the “Hardiness Zone Details” page for a list of zones.) Zone 1, for example, has an average annual minimum temperature below ?50 F. In other words, on average its lowest temperature of the year (its annual minimum) is lower than ?50 F. Sitting for the most part on the northernmost edges of the continent, Zone 1 is truly arctic; it includes Fairbanks, Alaska and Resolute, Northwest Territories.
Each zone has an annual minimum temperature range ten degrees higher (F) than the last. So, Zone 2 has an average annual minimum temperature range between ?50 and ?40 F, Zone 3 is ?40 to ?30 F, etc. At the other end of the scale, Zone 11 has an average annual minimum temperature above 40 F–making it frost-free. In North America, that gentle climate exists only in Mexico. The same zone also covers most of the Hawaiian Islands.
Zones 2 through 10 are further subdivided into “a” and “b” ranges–2a, 2b, 3a, 3b, etc. These a and b designations represent ranges of 5 degrees F in the annual minimum temperature. Zone 2b, for example, has an average annual minimum that is five degrees warmer than Zone 2a. Zone 10b, which includes southern Florida, has an average annual minimum temperature ranging from 35 to 40 F. So it, like Zone 11, is frost-free.
What about the Upper Midwest? Looking at the map of the continent as a whole, you can see the cold, low-numbered blue zones reaching down from Canada into the U.S. The coldest zone that touches the lower 48 states is Zone 2b, which reaches only into northern Minnesota; its average annual minimum is ?45 to ?40 F.
Looking at the “North-Midwest US” hardiness zone map for a more close-up view, the mildest parts of the region are in southern Iowa and eastern Wisconsin, which are in Zones 5a and 5b. In Zone 5b, located around Milwaukee, the average annual minimum temperature is ?15 to ?10 F, 30 degrees warmer than the coldest parts of northern Minnesota. (Urban areas tend to be warmer.)
Jumping back to the continental map, we can get some idea of how the Upper Midwestern minimum temperatures compare to those in other parts of the continent. The coldest parts of northern Minnesota have lows that are on par with those on the southern shore of Hudson Bay, the central parts of Canada’s prairie provinces, parts of interior Alaska, and the southern portion of Baffin Island–the latter two being very northern places indeed. Those parts of the region in Zones 3 and 4 are comparable to northeastern Montana, southern Ontario and Quebec, northernmost New York state and northern New England. Finally, the warmest parts of the region share their zone–Zone 5–with states stretching from Nebraska and Kansas to Pennsylvania.
What we have here in the Upper Midwest, therefore, is a great deal of climatic diversity, and we must choose our plants carefully.