The new hardiness zone maps are out

By: hcubed , 4:11 PM GMT on February 02, 2012

According to Dr Masters entry, the new 2012 USDA hardiness zone maps have been released.

Already, the CAGW links are spreading like wildfire, so a little history (and more info on other charts).

Info here, including other historical maps: Link

The first USDA Map was published in 1960 (USDA Miscellaneous Publication No. 814 "Plant Hardiness Zone Map.") and was developed by Henry Skinner while he was the Director of the U.S. National Arboretum. The map showed ten broad hardiness zones based on 10 degree F. gradients.

This map was revised in 1965 to add temperature data that had been missing from the original.

Another "hardiness map" was brought out by the Arnold Arboretum (AA), who published their first version in 1938. There are subtle differences between the AA and USDA maps (including the use of different temperature ranges), which lead to some confusion for gardeners. They updated their version in 1978, and by 1990 the AA version had fallen into disuse and the USDA version became the primary source for zone identification.

Back to the USDA map.

"...In 1990 a major overhaul of the USDA map was completed by H. Marc Cathey (USDA Miscellaneous Publication No. 1475) using temperature data from 1974 to 1986. One new zone was added to coincide with adding Mexico and Canada to the map. In addition, the prior 10 degree gradients were broken down into 5 degree a and b zones.

One of the primary reasons given for the update was that, "We have been losing from our landscapes plants that apparently survived the 1940's to the 1960's. Many of the hardiness zone classifications of plants are no longer considered valid. In North America, the ranges of temperature and moisture for the past decade were wider than those recorded for the 1940's through the 1960's." This is an intriguing statement, since it presages the current debate over the proposed update to the 1990 USDA map and the implication that the hardiness zones need to reflect recent global warming..."

Does the term unprecidented warming come to mind?

Oh well, back to the source.

"...In 2002 USDA contracted with the American Horticultural Society (AHS) to revise the 1990 Hardiness Zone Map and "better reflect minor regional variances in temperature that have occurred in the last decade" AHS. The AHS, under the direction of Dr. H. Marc Cathey, who lead the development of the 1990 USDA Map, issued a draft of the update in the May/June 2003 issue of "The American Gardener".

The AHS 2003 Draft differs in several ways from the 1990 version. Mexico and Canada are no longer represented. The number of zones has expanded from 11 to 15 to address ideal growing climates for sub-tropical and tropical plants. The 5 degree a/b zones were dropped in order to make the map easier to read. The AHS version is based on 16 years of data (1986 to 2002), while 1990 Map was based on 13 years of data (1974 to 1986).

The most striking aspect of the AHS Draft is that many of the hardiness zones have moved northward reflecting a general warming trend during the winters over the 16 year measurement period. 40% of the 4,600 weather stations used in developing the map showed a higher zone number compared to the 1990 USDA version..."

So why didn't his joint effort come forward? Who knows. It was rejected by the USDA with no comment.

That didn't stop others from modifying the "official" USDA map.

"...In 2004 the National Arbor Day Foundation released an update to the 1990 USDA Map using 15 years of temperature data. The result is similar to the AHS Draft with many of the hardiness zones shifting northward.

The National Arbor Day Foundation map was revised further in 2006, showing a continuing northward movement of the climate zones even compared to the 2004 version.

In 1995 the Florida Climate Center issued a Hardiness Zone Map for Florida using the same methodology as the 1990 USDA Map. The Florida Climate Center Map was based on temperature readings from 1981 through 1995 and, like the AHS Map and the Arbor Day Foundation Map, showed a northward migration of the hardiness zones..."

But there was more to come: the Heat Zone Maps.

"...The USDA Hardiness Zone Maps are based on the concept that, with respect to temperature, cold will have the greatest impact on plants. The American Horticultural Society (AHS) and other organizations have taken the approach that heat must also be taken into account in estimating whether a plant will thrive in a particular location. In 1997 Dr. H. Marc Cathey, issued the AHS Plant Heat-Zone Map. The Map is divided into 12 zones indicating the average number of days in a particular zone where temperatures in exceed 86 degrees (the point at which heat will cause experience damage to cellular proteins in plants).

More importantly, the AHS has combined the Heat and Hardiness Zone maps to create a four number rating system that indicates a plant's ability to withstand heat and cold. The first two numbers in the system indicate the optimum span of zones for cold hardiness and the second two numbers indicate the optimum span for heat tolerance. For example, a tulip may be rated as 3-8, 8-1. This indicates that, while a tulip can grow in Zones 1 through 8 during the summer, it will only thrive (and bloom), if it receives the average minimum temperatures present in Zones 3 - 8. While this approach is a definite improvement over sole reliance on the 1990 USDA Map, it will require an overhaul of existing hardiness indicators for plants. This will require significant time and resources and it is not clear whether the horticultural community is willing to invest the effort..."

All these maps, and yet there are a few other things the maps don't cover.

1. Snow cover. Snow acts as an insulator against extreme cold, protecting the root system of hibernating plants. If the snow cover is reliable, the actual temperature to which the roots are exposed will not be as low as the hardiness zone number would indicate.

2. Other factors such as soil moisture, humidity, the number of days of frost, and the risk of a rare catastrophic cold snap.

3. Many plants may survive in a locality but will not flower if the day length is insufficient or if they require vernalization (a particular duration of low temperature). With annuals, the time of planting can often be adjusted to allow growth beyond their normal geographical range.

Vernalization (from Latin: vernus, of the spring) is the acquisition of a plant's ability to flower or germinate in the spring by exposure to the prolonged cold of winter. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually flower.

So, do the charts reflect a warming world? Yes.

Consider the fact that Minnesota is now in zone 3. About 12,500 years ago, it was covered by the Laurentide ice sheet.

Ever since that ice sheet melted, plants and animals have been moving back into that area (northward expansion).

Wouldn't it be great if a Plant Hardiness Zone Map existed of that period 12,500 years ago so a comparison could be made to the 2012 map?

I'd bet it would show the Hardiness Zones a little further south than they are now.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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About hcubed

Living in Biloxi MS, have been here since '85 (first Hurricane was Elena).