Last week, for the first time since 1990, the U.S. Department of Agriculture updated its Plant Hardiness Zone Map, one of the tools we gardeners use to decide what will thrive, and what won't, in our personal plots of earth.
The map divides the U.S. into zones based on average extreme low temperatures, and the big news for many gardeners is that they've been promoted -- that is, moved up the zone chart to a higher number, meaning a slightly warmer clime.
In my climactically capricious corner of the Northeast, some of my neighbors have moved from Zone 5a to Zone 5b, meaning that their average extreme low temperature in the winter is now calculated as -15F, not -20. Does half a zone really matter? Yes and no.
If your zone has shifted, your planting calendar has, too, meaning your first hardy plants (such as spinach and kale) can go into the ground a week or two earlier in the season, mid-March, say, instead of early April. But every garden, and every year, will vary. So even if the USDA says you're now in balmy zone 6 (lucky!), if your earth remains frozen on March 15, you still won't be able to plant.
So let’s address the elephant in the room -- or the elephant ear plant in the flower bed, as it were. How much of this is due to climate change? USDA officials insisted that the map isn’t meant to be evidence of global warming. But it’s impossible to ignore (and officials admitted as much in response to questions) that warmer winter temperatures are responsible for many of the zone shifts, which sometimes stretch nearly across entire states (big ones, too, like Texas). “The ‘normal’ climate has become a moving target,” Cornell plant and soil ecology prof David W. Wolfe told the Associated Press, and the new map reflects that.
The USDA says the map also takes into account other, more local factors, such as proximity to urban areas and altitude, both of which can affect temperature. But year-to-year variability is inevitable. This makes some gardeners tear their hair out, and others ignore the USDA maps entirely.
(There are alternatives. Some swear by Sunset magazine's climate system, which is based on a broader range of data, such as summer high temperatures, rainfall, wind, and more. Others say your best bet is to get to know your fellow gardeners. Talk to people who've lived and gardened in your area for a good while. And if you don't know your neighbors, consult your local agricultural extension office.)
Whichever tools you choose to determine what you're going to plant, it's time to get started. Get out your graph paper, make a planting map, and dive into your seed catalogs. Depending upon your zone, you may be ready to start seeds indoors. And even if you're still immersed in those extreme low temperatures, you can order seeds and dream of summer now.
