Goodbye Blueberries. Hello Loquats : USDA Updates its Hardiness Map

Zone 7 Nursery

Here is a picture of one my favorite nurseries.  What is wrong with this picture, besides being way too grainy?

Ok, I’ll give you a hint. The name of the business is no longer as relevant.

Last month the USDA updated their plant hardiness map in which Zone 7 Nursery refers to. This is the map that homeowners, landscapers, and nursery owners use to judge what plants will prosper in what regions. Basically, it uses annual minimum temperatures as a proxy in determining whether plants can survive a frozen cellular assault.

1990 USDA Map (Click to enlarge)

For the 1990 USDA map, the US was divided into 11 zones. While it was better than the 1964 map it replaced, there was still just one problem with this map.  It was weak.

This map used annual minimum temperatures that were averaged over 1974–1986. So, basically, the map relied on only 12 years of data. According to researchers there was actually little documentation on the mapping protocols used in the development of this 1990 map. Even worst, the map was hand drawn. It was an analog cassette still being used in the age of iPods and streaming music.

Finally, after a couple of false attempts in the last ten years, a new plant hardiness map was released by the USDA a month ago. Now this one is compatible with the digital age. It actually uses 30 years of weather data gathered from 1976 to 2005.  Scientists used more sophisticated methods for mapping zones between weather stations that greatly improved the accuracy and detail of the map. Now, it’s GIS friendly. You can actually enter your zip code on the USDA website and see exactly what zone you are living in.

2012 USDA Map (Click to enlarge)

This new map shows in most locations roughly a one 5-degree Fahrenheit half-zone warmer than the previous map throughout the US.  While there were some news stories that jumped to climate change conclusions, I think most of this change is probably due to the fact that this map uses a 30-year period compared to 12-year period in the previous map.

This brings me back to the Zone 7 Nursery picture.  Can you guess what zone I used to live in? I used to be in Zone 7. Now I live amazingly comfortably in Zone 8a. Unfortunate for my local nursery, zones are not set in stone. A quick search on plants and their zones found that I should give up growing blueberries and instead grow a loquat tree.  I wonder how loquat tastes? Ask me in about five years.