If you're planting a spring garden in the U.S. this year, you may want to set aside some extra seed money. The U.S. Department of Agriculture has updated its plant hardiness zone map for the first time since 1990, reflecting how some crops are moving north as winter grows warmer.

Despite all the long-term dangers associated with global warming, it does have a few short-term perks, such as helping some plant and animal species expand their range. And when life gives you lemons — which, incidentally, may now be easier to grow in Northern states — you make lemonade.

The new map was revealed this week at the National Arboretum in Washington, just in time for millions of home gardeners who are still thumbing through seed catalogs. It's a big change from the 1990 version, which many growers have long considered outdated since it's based on temperature data from 1974 to 1986. As the Associated Press points out, 18 of 34 cities listed on the old map are now in new zones, as are large swaths of some states, including Ohio, Nebraska and Texas.

"It is a good thing the government updated the map," Woodrow Nelson of the Arbor Day Foundation tells USA Today. "Our members have been noticing these climate changes for years and have been successfully growing new kinds of trees in places they wouldn't grow before."

Southern magnolias can increasingly survive in Pennsylvania, for example, while Iowa winters are less deadly to passion flowers, Japanese maples and Fraser firs. "There's a lot of things you can grow now that you couldn't grow before," Boston University biologist Richard Primack tells the AP. "People don't think of figs as a crop you can grow in the Boston area. You can do it now."

Here's what the 2012 plant hardiness zone map looks like (click to enlarge):

The version above uses temperature data from 1976 to 2005, but that's not the only reason it's more accurate. The USDA also factored in several other climate variables for the first time, including prevailing winds, slope of terrain, proximity to bodies of water and proximity to urban "heat islands." Plus, while the 1990 map was static, this year's update adds an interactive online version, letting users type in a ZIP code to get more precise averages of the coldest annual temperature in that area.

But even if the new map is a symptom of global warming, the USDA is quick to point out it shouldn't be taken as evidence. "Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years," the USDA website explains. "Because the [new map] represents 30-year averages of what are essentially extreme weather events, changes in zones are not reliable evidence of whether there has been global warming."

Still, many farmers and gardeners say they already have all the evidence they need. "If you want to look at what might be the most politically correct thing, you can say 'something's happening,'" Charlie Nardozzi, a gardening consultant in Vermont, tells USA Today. "But the climate is changing. Spring is coming sooner and lasting longer. Fall lasts longer, and overall the weather is so much more erratic now." And George Ball, chairman and CEO of the W. Atlee Burpee seed company, tells the AP that climate change "is not big news to gardeners."

2012 may be a good year to challenge your green thumb, then, and try out some crops that would've been impossible to grow a few decades ago. Growers might as well make the most of climate change, even if its benefits can never match its overall toll. But since weather trends are expected to grow less predictable and more extreme as the planet heats up — and many experts believe they already have — it's better not to invest too heavily in any one crop, old or new.

"There's definitely a changing climate," Nardozzi says. "But that doesn't mean we won't have a harsh winter again that could kill all their plants."

Visit the USDA's Agricultural Research Service website to see its interactive version of the new map, and check out the AP video below for more information:

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The USDA has updated its planting-zones map for the first time in 22 years, reflecting a warmer U.S. where many southern crops are expanding north.