Saturday, January 28, 2012

New Plant Hardiness Zone Maps Are Out

For the agriculturally and garden inclined, knowing what "zone" you are in is sometimes really important when it comes to knowing what you can get away with in your garden, yard, or farm for crops.  The last time the USDA issued zone hardiness maps was in 1990...it took until earlier this week for them to come out with new maps, which do show a number of changes around the Delaware Valley.

A plant hardiness zone, for those who never has gardened before or never want to, basically is an indication of how low temperatures can get in a particular geographic location.  From there, plants and trees are assigned to a zone by their ability to withstand extreme low temperatures.

The last map, in 1990, looked at a twelve year window of low temperatures (1974-1986), with the new map providing for a thirty year window of temperature (1976-2005) and was computer-aided in its design and detail.   More information, better accuracy one could surmise.

Chief among the changes is the introduction of zone 7b into the Delaware Valley.  7b used to be confined to Southern Delaware and generally south of Washington, DC but now encircles the southern fifty or so percent of the city,  Southwest New Jersey, and along the Delaware River to Wilmington, as well as Cape May County.  Zone 7a, which used to be into Philadelphia only, has expanded into Southeastern Pennsylvania and much of South Jersey.   The rest of our immediate area is in zone 6b.

One could make the argument that this is an example of climate change at work.  Yeah, you could and we won't dispute one way or the other the thought of that being a player in this.  One big, concrete factor that gets little mention though -- the urban heat island and increased urbanization in general.  Average low temperatures have increased over the last 20 years thanks to more development around the Airport...we've also had a less favorable pattern for cold in the Eastern US over that time frame.

In Philadelphia, the average low during the 1951-1980 time frame was 45.3 degrees.  By 2011, our average low for the 1981-2010 time frame had increased to 47.0 degrees (total of 1.7 degrees).  Our average highs in those two separate thirty year periods increased at a slower rate, from 63.5 to 64.7 (1.2 degrees).   That half degree difference (the extra nighttime warming) can be attributed in some extent to urbanization.  

Anyways, you can check out the new zones by state and nationally at the USDA's website.