US Dept. of Agriculture New Hardiness Zones Show Impact of Global Warming

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Gardeners need to plan accordingly.

USDA Hardiness Zone Map 2012

Global warming is changing the relationship between our garden plants and the climatic variables they face.

Gardeners who have noticed some unusual goings-on in their yards had their suspicions confirmed this week when the US Dept. of Agriculture released its new map of plant hardiness zones. The map confirms what many gardeners have already figured out about global warming and the impact it is having on plants. Hardiness zones, based on minimum winter temperatures, are marching northward. This means that plants that wouldn’t have survived through winter in some regions just 20 years ago are now making it.

Comparing the new 2012 map to the last map published in 1990 shows some significant shifts, especially across the Great Plains. Though the USDA cautions that not all the changes to the map can be attributed to climate change, it is clear that the major shifts are related to warmer temperatures. For example, Iowa and Nebraska used to straddle Zones 4 and 5, but now fall almost entirely within the warmer Zone 5.

Most areas in the continental US have warmed by 1-2 degrees F during the last 50 years. This warming has brought shorter and less severe winters. In fact the country has seen many fewer record lows each winter.

“Changes in climate due to carbon pollution will create some enormous new challenges for gardeners,” said Patti Glick, Senior Climate Change Specialist with the National Wildlife Federation... “Global warming is changing the relationship between our garden plants and the climatic variables they face.”

While some gardeners may welcome the opportunity to experiment with new plants, these shifting zones caused by climate change can be problematic. The country is seeing more weather and climate extremes that can be quite challenging for plants. Summertime heat and humidity have been increasing. Droughts are becoming more severe leaving plants more susceptible to disease. Heavy rainfall events are getting even heavier, increasing the likelihood of devastating floods. Gardners may be able to grow some new plants given the warmer winter temperatures, but some old favorites may have a harder time as summer temperatures increase and precipitation patterns change.

Shifting plant hardiness zones also opens the door for harmful invasive species, pests and diseases. Unfortunately, research is showing that climate change favors the spread of these less desirable species. Gardeners in the Midwest and Northeast will not be happy to hear that the northernmost range of kudzu- the “plant that ate the South” – has already moved northward, with sightings from Southeastern Nebraska to Western Pennsylvania. This latitudinal shift has been attributed to increases in winter temperatures. Other plants moving into new areas include garlic mustard, purple loosestrife and Japanese honeysuckle, none of which are a gardener’s friend.

Pests that used to be kept in check by hard winter freezes are also expanding their range northward. For example, the range of the red imported fire ant in the US has historically been limited by cold temperatures and winterkill. Now scientists project that the fire ant range could expand northward by about 80 miles and grow in total area by 21 percent as climate change makes new areas suitable for their survival. Anyone who has encountered fire ants in their garden can tell of the painful consequences! Global warming is also welcoming unwanted pests like the black vine weevil, gypsy moth, bagworm and the mountain pine beetle.

Gardeners Can Help Tackle Global Warming

As guardians and stewards of our environment, gardeners can take many simple and thoughtful steps to work with nature to solve the challenges posed by global warming. NWF’s The Gardener’s Guide to Global Warming: Challenges and Solutions, authored by NWF’s Senior Climate Change Specialist Patti Glick, provides many excellent recommendations, including:

  •     Reduce the use of gasoline-powered yard tools that put carbon pollution, the root cause of global warming, into the atmosphere.
  •     Remove invasive plants from the garden and choose an array of native alternatives.
  •     Reduce water consumption, which will improve the resiliency of your garden during droughts and heat waves, reduce energy consumed to transport water; and reduce runoff of fertilizer into waterways.
  •     Plant lots of native trees and native grasses to absorb carbon dioxide from the atmosphere and serve as long-term carbon storage.

Media Contact: Mary Burnette, Burnette(at)nwf(dot)org, 703-438-6097 – to interview Patti Glick about the new hardiness zones and consequences for gardeners contact Mary.

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