Climate change impacts on gardening

New Plant Hardiness Zones for our state

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For many of us, winter is a time to pore over new seed catalogues: Montanans love to garden - despite the many challenges of growing produce in one of the colder, more northern climates in the Lower 48.

Understanding which kinds of fruits and vegetables grow best here and figuring out exactly when we should plant tomato seedlings, herbs, and root vegetables, is the key to ending up with piles of good food from our own gardens each year.

The plant hardiness zones map from the U.S. Department of Agriculture helps answer these important questions, and the big news this past week was that the U.S.D.A. has updated this map for the first time since 1990 - although it now looks surprisingly different.

Photo by iStock.

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So what is a plant hardiness zone map anyway?

This color-coded map, which appears on the back of seed packets, provides information about which kinds of plants are best suited to specific regions of the United States and suggests planting dates based on average freeze patterns each year.

The U.S.D.A. plant hardiness zone map from 1990 is shown above, with different colors representing different zones of average minimum winter temperatures each year.

The higher the zone number, the warmer the climate.  For example, most of Montana's Flathead Valley is in a much warmer zone than the area in and around Bozeman.

Map courtesy of the U.S. Department of Agriculture.

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The new plant hardiness zone map, released on January 26, 2012, is interactive and contains microclimate information down to the scale of individual counties, which you can access by typing your zip code into the map (link at the top of the list of Quick Links at right).

The big surprise is a major shift northward in the zones due to warmer winter temperatures, pointing to the fact that some plants and trees can survive further north than ever before, with spring planting occurring sooner in the spring as well.

In some cases - like Ohio, Nebraska, and Texas - nearly the entire state is now in a warmer zone, and the U.S.D.A. has had to introduce two new, warmer zones in the southern U.S. for the first time.

In Montana, much of the fertile Flathead Valley is now in Zone 6a, with minimum temperatures each winter expected to be no lower than -10 to -5 degrees F.  Compare this with the 1990 Zone of 5a, which indicated minimum winter temperatures of -15 to -20 degrees Fahrenheit.

Map courtesy of the U.S. Department of Agriculture.

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The new plant hardiness zones will not find their way onto the backs of seed packets until next year, so the new interactive U.S.D.A. map is the only source for this new information until then.

So make sure to check out this new information as you thumb through your seed catalogues this year, as you may be finding out for the first time that some new kinds of plants just may grow in your garden after all.

Photo:  iStock.