Last week in Virginia, sugar maples were pumping sap like crazy – three to four week early. On the Delmarva and in southern Pennsylvania this week, cereal crops were warming up hit Feek's growth stage 4 and 5 – nitrogen application time.
But whoa! Isn't it way too early? The answer is simple: It happens when it happens, and you need to prepare for it. Warmer, longer, more southerly growing seasons have been inching northerly across much of the country for decades.
It's been well documented by the National Oceanic and Atmospheric Administration's Northeast Regional Climate Center at Cornell University. Growing degree days have begun accelerating earlier and earlier over the last 30 years, spurring fruit trees to bud earlier and earlier, causing insect populations to hit threshold levels earlier, and pushing weed emergence earlier.
January's USDA release of its updated "Plant Hardiness Zone Map" was final confirmation of the climate changes occurring between 1976 and 2005. Keep in mind that climatic changes are continual, and the map is based on data already seven years old.
"The northward march of the hardiness zones illustrates the continued warming that has occurred across the United States and around the globe in recent decades, particularly in winter," says Art DeGaetano, Cornell University climatologist and director of NOAA's Northeast Regional Climate Center.
Climate model projections indicate that winter temperatures will continue to warm through the 21st century. "By 2080," he predicts, "the hardiness zones that currently cover the area from southern Virginia to northern Georgia may replace those that we see across New York in the current update."
The ramifications are many, suggests David Wolfe, chair of Cornell University's Climate Change Focus Group at the Atkinson Center for a Sustainable Future. In 2006, the Arbor Day Foundation published an updated plant hardiness map using USDA's methodology, which showed a dramatic 'zone creep' northward throughout the country.
That helps explain why invasive weedy plants like kudzu are expanding northward. It also explains why Brown Marmorated Stink Bugs have become a major crop pest in the Northeast.
New and more detailed PHZM
USDA's 2012 Plant Hardiness Zone Map (PHZM) is GIS (Geographic Information System)-based for the first time, and designed for the Internet. Using low-temperature values from actual weather reporting station data, you can see the zone map for your ZIP code by computer.
As noted earlier, the zones are based on 1976–2005 weather data. A trial check didn't find that adding more recent years of data made a significant difference.
Each zone represents the mean (average) extreme minimum temperature for an area, calculated from the lowest daily minimum temperature recorded for each of the years. It doesn't represent the coldest it has ever been for that location.
Because this map was created digitally with GIS technology, it can show smaller areas of zone delineations than ever before. For example, cities tend to hold more heat because they have large amounts of concrete and blacktop, so a city or town may be assigned to a zone warmer than the surrounding countryside.
Higher elevations tend to be colder than surrounding lower areas, so the top of a mountain may be an area of cooler zones. A location near a large body of unfrozen water may provide milder winter weather and be in a warmer zone.
Global warming disclaimer
Climate changes are usually based on trends in overall average temperatures recorded over 50 to 100 years. Because the PHZM represents 30-year averages of the coldest temperature of the year, changes in zones aren't reliable evidence of whether there has been global warming, according to USDA.
Compared with the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new PHZM is generally one half-zone warmer than the 1990 version throughout much of the United States, as a result of a more recent averaging period.
Some zone changes, though, are the results of the new, more sophisticated mapping methods and more station observations. That has greatly improved accuracy, especially in mountainous regions. These changes are sometimes to a cooler, rather than warmer, zone.
To access national, regional, state and ZIP code maps, click on http://planthardiness.ars.usda.gov/ .