USDA heat and cold zones

If you frequently find yourself shopping for woody plants or herbaceous perennials, you are likely aware of the fact that we live in Zone 6. In case this means nothing to you, the USDA Hardiness Zone Map has long been a guideline for cold hardiness of plants, and since 2003 it has also included heat zones.

The relationship between hot and cold is likely what caused widespread damage for some plants with the December arctic blast. It had been warm, so certain plants had not transitioned to dormancy. The drastic plunge in temperature in a very short period of time spelled doom for some plants.

The current zone map, updated in 2012, is based on temperature information from 1976 through 2005. Data was collected from over 4,600 weather stations across the United States.

Taking the average coldest temperature of a location, they have fine-tuned the zones. Digitally created high resolution map show the microclimates that typically pop up around cities. This is the reason that some of us around Louisville are now in Zone 7 while the rest remain in Zone 6.

The USDA hardiness map has also added four more zones that designate areas that experience average annual minimum temperatures of 50, 60, 70 or 80 degrees Fahrenheit, respectively.

Each Zone has a trendy color attached to it. Our local zones 6 and 7 are buttercup and rose. To check on hardiness zone changes you can plug your zip code into USDA Plant Hardiness Zones. Whether you remain in the old Zone 6 or are in the new Zone 7, don’t worry about changing anything you do in the garden. If anything, take more risks!

The hardiness zone map includes heat considerations. If you have ever tried to grow delphinium in the garden, you know that it is not the cold hardiness that thwarts success. Instead, it is the heat and humidity that makes them melt by mid-June.

In 1997, the American Horticultural Society developed a heat map that sections the United States into 12 different heat zones. They compiled temperature data from 1974 through 1995 from the National Climatic Data center and established zones based on the average number of days a location experienced 86 degrees Fahrenheit or above. Heat Zone 1 has less the 1 day above 86 F and Zone 12 has 210 or more days at or above 86 F.

Why 86 degrees? Well, at 86 F certain plants begin to show heat stress. Leaves may wilt, roots may stop growing, flowers drop, bud initiation ceases, or heat recovery overnight doesn’t occur because temperatures are too high.

The heat zones allow us to be better informed about the plants we buy and what realistic expectations about their summer performance should be. Some nurseries are already including heat hardiness information, but it will likely be several years before it becomes mainstream.

It seems sort of confusing, but here’s how to decipher the four-code system. There will be a set of two numbers, for example, (6-9, 9-1). The first set of numbers recognizes that the plant is cold hardy in zones 6, 7, 8 and 9; the second set of numbers indicates that the plant has heat tolerance from zones 9 down to the coolest of zone 1. Just be sure that zone 6 or zone 7 is included in the number spread of both cold and heat ranges for our area.