Hardiness zones are typically used to determine which plants will grow in a given area, and are displayed on the map of North America as broad colorful strokes depicting the average lowest temperature likely to be experienced each winter. Each zone is given a number, with lower digits representing colder regions, and vice versa, and the zones are in 10 degree F. increments. Each is further broken down into subranges “a” or “b” – thus, “USDA Zone 5” indicates average minimum winter temps of -10 to -20 degrees F; “5a” is -20 to -15, and “5b” being -15 to -10 degrees F. In the warmest parts of the U.S., it’s not the cold that is a limiting factor: it’s the heat, and heat zone maps have been created for those areas that will never experience a snowflake, or black ice.
The last version of the Plant Hardiness Zone Map was issued in 1990, and was based upon 12 years of meteorological data. Creators of the 2012 map were able to use the latest in computing technology to compile 30 years of readings from more measurement sites into an interactive tool that is much more detailed. When compared to the 1990 map, it is clear that warmer zones have been creeping northward, and that is also true in New Hampshire. However, some of the changes can be attributed to the additional data and computational ability, and the USDA makes no claims about the influence of planetary warming leading to climate change.
Gardeners can be a quirky bunch, often conversing in botanical Latin and using the Zone Map as part of their identity, as in, “I’m a solid Zone 5b” (meaning average minimum temperatures of -10 to -15 degrees F.) Importantly, these temperature ranges refer to ambient air and not to wind chill temperatures, which only apply to exposed human skin. I am often asked to offer plant recommendations for “my Zone 4a (-25 to -30 degrees F.) garden in Amherst”, or some other southern New Hampshire location. In fact, most of the lower half of the state lies in Zone 5a or 5b, with the extreme southeasterly portion enjoying Zone 6 (0 to -10 degrees F.) winters. That the Seacoast is milder is unsurprising, but the new map shows large scattered blocks of western New Hampshire, from south of Keene to north of New London as also being in Zone 6a. This should come as welcome news to gardeners in those areas, with a larger palette of plants they can now try.
Gardening books, plant tags and catalogs typically provide the hardiness range of a given plant, assisting the reader when deciding whether she can grow it. However, these zones, while extremely helpful, are more of a guide than a rule – there are other factors which come into play: winter sun exposure (although counterintuitive, less winter sun, especially in the afternoon, is often better for some evergreens), the amount of snow cover (more is usually better, as snow is an insulator,) plant vigor, and the microclimate – or the immediate vicinity of the plant’s intended location – is it sheltered? Does it retain the sun’s warmth into the evening? And lastly, how wet the soil is in winter – plants that do quite well in a cold yet arid climate for instance, the high desert – often perish in our icy mud.
Part of the fun in gardening is pushing the limits of what plants can be grown successfully. Northern gardeners are eager to try more tender species, while those in the south – especially snowbirds, may attempt plants that require more cold to survive. Despite our typically cold winters, we in central New England can grow an astonishing variety of plants, but our warm and humid summers often prevent success with those species happier in the more temperate climes of England or maritime Canada. Which means that we are far more likely to succeed with growing ornamental bananas and gingers rather than the holy grail of many local gardeners: stunningly perfect spires of 6-foot tall delphiniums.
Not that the bananas and gingers will make it through our winters, at least according to the new hardiness zone map.
