plant hardiness zones

A New Map

The coldest nights of the year are due to arrive in about a month. These frigid nights might test the hardiness of trees, vines, and shrubs, perhaps nipping back a few branches here or there on some plants, perhaps killing others outright. Though bitter cold is a few weeks away, I have done what I could to prepare plants for the onslaught.

The first step in preparing any plant for the winter is knowing how cold it’s going to get. There’s no crystal ball offering this information, but decades of weather data have been compiled by the U. S. Department of Agriculture to prepare the USDA Hardiness Zone Map, which is helpful in predicting the probable minimum temperature for an area. The map has gone through numerous incarnations of increasing accuracy and . . . drum roll . . . recently this year the latest incarnation was revealed.

So, what’s new? The increasing accuracy reflects minimum temperatures measured at 13,625 weather stations averaged over 30 years. As before the country is divided into zones denoted by color and number, 13 of them, the lower numbers reflecting colder regions. Each zone denotes the average coldest temperature in that time period, not the minimum temperature ever experienced.

You often see this map, the country covered over my squiggly lines filled in with a color. Big change this year: The USDA offers this map only online Read more