We’ve been hearing for several years that global warming is causing a northward shift in USDA cold hardiness zones (see Climate Change and Your Garden). Now, the Environmental Protection Agency has produced a chart that illustrates just how dramatically global warming is affecting the overall length of our growing season (above). Printed in an April 2010 report titled Climate Change Indicators in the United States and based on a century’s worth of data, the chart shows a substantial lengthening of the growing season. Here’s some background on the issue, taken from the report.
The average length of the growing season is the number of days between the average last spring frost and first fall frost, when temperatures remain above freezing. The growing season often determines which crops can be grown in an area, as some require long growing seasons while others mature rapidly.
A longer growing season may sound like a win for everyone, but in fact changes in the season’s length can have both positive and negative effects. Moderate warming can benefit crop and pasture yields in mid- to high-latitude regions, yet even slight warming can decrease yields in seasonally dry and low-latitude regions. A longer growing season can allow gardeners and farmers to diversify crops or have multiple harvests from the same plot, but it can also limit the types of crops that can grow, encourage invasive species or weed growth, and strain water supplies. A longer growing season may also disrupt the function and structure of a region’s ecosystems — for example, altering the range and types of animal species in the area.
A couple of additional factoids from the report:
- The length of the growing season has increased more rapidly in the West than in the East. In the West, the length of the growing season has increased at an average rate of about 20 days per century since 1900, compared with a rate of about six days per century in the East.
- The final spring frost is now occurring earlier than at any point since 1900, and the first fall frost is arriving later. Since 1985, the last spring frost has arrived an average of about four days earlier than the long-term average, and the first fall frost has arrived about three days later.