New USDA Plant Hardiness Zone Map for gardeners shows a warming climate

Wintertime minimum temperatures in the U.S. have risen so much in recent decades that the United States Department of Agriculture decided last week to update their Plant Hardiness Zone Map for gardeners for the first time since 1990. The Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones. Compared to the 1990 version, zone boundaries in the new 2012 edition of the map have generally shifted one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period. The old 1990 map was based on temperature data from only a 13-year period of 1974-1986, while the new map uses data from the 30-year period 1976-2005.


Figure 1. Comparison of the 1990 and 2012 USDA Plant Hardiness Zone Maps. Image credit: USDA and Arbor Day Foundation

Northwards, ho!
While humans are generally not attuned enough to nature's rhythms to tell if the climate is changing, plants and animals know the climate is changing. Many species of animals, insects, and plants have shifted their ranges poleward and to higher elevations in recent decades because of global warming. The 2007 IPCC report stated that "numerous studies document a progressively earlier spring by about 2.3 to 5.2 days per decade in the last 30 years in response to climate warming. That report also documented over 400 species that have moved their ranges poleward or to higher elevations because of climate change. For example, conifer trees expanded northwards into former tundra areas at a rate of 12 km per year between 1982 - 2000 in portions of Canada (Fillol and Royer, 2003.) Holly plants moved northwards by several hundred kilometers in recent decades into coastal Norway, Northeast Germany, Denmark, and coastal Sweden in response to warming temperatures (Walther et al., 2005.) As the climate continues to warm, plant and animal species previously unknown in many regions will appear, and will disappear from places they used to inhabit.


Figure 2. Change in the boundary line between conifer forest (taiga) and tundra between 1982 (grey line) and 2000 (white line) over Canada. In the grey box marked "Transect", the rate of northwards migration was 12 km per year, or 228 km (142 miles) in nineteen years. Image credit: Fillol and Royer, 2003, "Variability analysis of the transitory climate regime as defined by the NDVI/Ts relationship derived from NOAA-AVHRR over Canada", Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International.

Jeff Masters