Friday, February 10, 2012

USDA Plant Hardiness Zone Map

A couple of weeks ago the USDA published the latest edition of the Plant Hardiness Zone Map.  This is the map that tells us what plants are most likely to thrive in a certain location.  The big news about the 2012 map is that the zones have changed by about a half a zone and the USDA has added two new zones—12 and 13.  The map is also interactive.  You can put in your zip code and it will tell you what zone you are in.
New hardiness zone map
This year’s map is based on weather data from 1976-2005.  Each zone represents the average extreme minimum temperature for that zone.  The previous map, published in 1990, was based on data from 1974-1986.  Of course, the media immediately jumped on this as proof of climate change and global warming.  The USDA cautions us not to read too much into these changes, that they are not a reliable indicator of climate change or global warming but that some of the change is simply due to better mapping technology that has greatly improved the accuracy of the map.

So what does all this mean for us folk here in Northern Illinois?  Well, don’t plan on planting crepe myrtle or camellias anytime soon.  We are still in Zone 5, though we are in subzone 5B.  We also should keep in mind that the zones are just a guideline.  Each individual garden has its own micro-climates.  The spot by the garage that is protected by the wind and gets radiant heat  off the exposed foundation might even be a Zone 6 while the low spot toward the back of the garden where cold are can sink could be a Zone 5A.   And  there are many more factors that impact how well a plant grows in our area including: light exposure, soil fertility and pH, temperatures at both ends of the thermometer, the duration of any particular cold snap (my rosemary held on until just last week in its whiskey barrel by the garage) and the general humidity level.  Many plants can adapt to varying conditions but it is important to have the proper conditions and the proper plant for those conditions. 
Cold hardiness zones only tell a part of the story.  Many plants, especially annuals, also have a hard time dealing with heat.  The American Horticultural Society publishes a Heat Zone Map and many plant tags are beginning to carry this designation.  The Zone tolerance measures a plant’s ability to withstand some number of “heat days,” or days the temperature goes above 86.  According to the AHS we are in heat zone 5, which means we get between 30-45 days above 86.