Climate change in the USA and the new plant growers’ map

The US government’s map of planting zones, usually seen on the back of seed packets, has changed. An update of the official guide for gardeners reflects a new reality, that of climate change and shifting meteorological zones. Some plants and trees that once seemed too vulnerable to cold can now survive farther north than they used to.

As this report on Yahoo News pointed out, it’s the first time since 1990 that the US Department of Agriculture (USDA) has updated the map and much has changed. Nearly entire states, such as Ohio, Nebraska and Texas, are in warmer zones. The new guide, unveiled this week, also uses better weather data and offers more interactive technology. For the first time it takes into factors such as how cities are hotter than suburbs and rural areas, nearby large bodies of water, prevailing winds, and the slope of land.

The 2012 USDA 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. For the first time, the map is available as an interactive GIS-based map, for which a broadband Internet connection is recommended, and as static images for those with slower Internet access. Users may also simply type in a ZIP Code and find the hardiness zone for that area.

The 26 zones, however, are based on five degree increments. In the old 1990 map, the USDA mentions 34 different US cities on its key. Eighteen of those, including Honolulu, St. Louis, Des Moines, St. Paul and Fairbanks, are in newer warmer zones. Agriculture officials said they didn’t examine the map to see how much of the map has changed for the hotter. However, the Yahoo News report said Mark Kaplan, the New York meteorologist who co-created the 1990 map and a 2003 update that the USDA didn’t use, said the latest version clearly shows warmer zones migrating north. [See the USDA Plant Hardiness Zone Map here, with zip code form, interactive mapping and downloads.]

Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future.

The USDA has said gardeners should keep that in mind when selecting plants, especially if they choose to “push” their hardiness zone by growing plants not rated for their zone. In addition, although this edition of the USDA PHZM is drawn in the most detailed scale to date, there might still be microclimates that are too small to show up on the map.

Microclimates, which are fine-scale climate variations, can be small heat islands – such as those caused by blacktop and concrete – or cool spots caused by small hills and valleys. Individual gardens also may have very localised microclimates (your entire yard could be somewhat warmer or cooler than the surrounding area, the USDA explained, because it is sheltered or exposed).

The 1990 map was based on temperatures from 1974 to 1986; the new map from 1976 to 2005. The nation’s average temperature from 1976 to 2005 was two-thirds of a degree warmer than for the old time period, according to statistics at the National Climatic Data Center. So far, according to the reports on the new zones map, the USDA is not actively associating its map with the effects of climate change on the USA.

Many species of plants gradually acquire cold hardiness in the fall when they experience shorter days and cooler temperatures. This hardiness is normally lost gradually in late winter as temperatures warm and days become longer. A bout of extremely cold weather early in the fall may injure plants even though the temperatures may not reach the average lowest temperature for your zone. Similarly, exceptionally warm weather in midwinter followed by a sharp change to seasonably cold weather may cause injury to plants as well. Such factors are not taken into account in the USDA PHZM.

David W. Wolfe, professor of plant and soil ecology in Cornell University’s Department of Horticulture said the USDA is being too cautious and has disagreed about the Agency ignoring the climate change connection. “At a time when the ‘normal’ climate has become a moving target, this revision of the hardiness zone map gives us a clear picture of the ‘new normal,’ and will be an essential tool for gardeners, farmers, and natural resource managers as they begin to cope with rapid climate change,” Wolfe has said.

Still, the USDA has emphasised that all PHZMs are guides. They are based on the average lowest temperatures, not the lowest ever. Growing plants at the extreme of the coldest zone where they are adapted means that they could experience a year with a rare, extreme cold snap that lasts just a day or two, and plants that have thrived happily for several years could be lost. Gardeners need to keep that in mind and understand that past weather records cannot be a guarantee for future variation in weather.