The USDA Plant Hardiness Zone map is a widely used reference that helps gardeners and other growers to choose plants that are suitable for their local climate. Each zone is defined by the average minimum annual temperature; in other words, how cold it gets in a typical year. The term hardiness refers to a plant's likeliness to survive the coldest time of year. For example, if a plant is said to be "hardy to zone 5," it means it can likely survive in temperatures down to -20 degrees F, the minimum annual temperature in zone-5 climates.
Hardiness zones, commonly called growing zones or planting zones, are intended to be taken as advice, not gospel. What matters most is the precise microclimate where a plant resides. For example, even if you live in zone 5, there may be areas on your property that stay warmer than the average zone-5 location, so that it might be possible to grow a zone-6 plant in these warmer areas. Ultimately, the plants will tell you what works and what doesn't, and you never know until you try.
Northeastern U.S. Growing Zones
For purposes of the USDA planting zones, the meaning of "Northeastern" is somewhat expanded beyond how it is used in everyday speech. It does not consist simply of the New England states and New York. Rather, it extends south to Virginia and West to Michigan.
Massachusetts alone (a relatively small state, in terms of territory) contains five different growing zones (3 through 7). The northern reaches of Michigan, New York, Vermont, New Hampshire, and Maine include some of the coldest patches in the continental U.S.
Southeastern U.S. Growing Zones
Within the confines of the Southeast region, gardeners will encounter primarily growing zones 6 through 10. The extreme southern tip of Florida, however, is one rank higher. As is the case with the other regions, rank shoots down precipitously as elevation rises--a highly relevant factor, since this region is home to the Smoky Mountains.
Gardeners in the Southeast are often challenged by soils with excessive clay. If you face this challenge, in addition to applying soil amendments, you may wish to consider selecting plants that tolerate clayey soil.
South-Central U.S. Growing Zones
Dominated geographically by Texas, this region is subject to extremes in climate. Areas along the Gulf Coast tend to be hot and humid. The Texas Panhandle is hot and dry, as anyone who has journeyed through the area on a historic Route 66 trip well knows.
Southwestern U.S. Growing Zones
California alone has such a diverse climate that people refer to "Northern California" and "Southern California" as if they were separate states. But it does not end there. The Sierra Nevada mountains and the Mojave Desert are almost like different planets.
Stark contrasts in climate are the story line for the Southwest overall, ranging from the peaks of Colorado (zone 3) to the burning deserts of southern New Mexico, Arizona, and California (zones 10 and 11). Besides the extremely high temperatures in these southern areas, plant-growers may be faced with the challenge posed by caliche soil.
Northwestern U.S. Growing Zones
Like the Northeast, the Northwest assumes a somewhat expanded meaning when used in connection with USDA growing zones. So if hearing the term conjures up images of the storied rainy weather of the Pacific-Coast segment (Seattle, say), you will have to adjust your thinking a bit. In terms of growing zones, this region extends east to Montana and Wyoming -- hardly known as prime territory for "singing in the rain."
North-Central U.S. Growing Zones
The North-Central region is dominated by cold-weather growing zones (3 to 5), with only the extreme southern fourth of it offering a more hospitable clime (zones 6 to 7). The Northeastern part of the area is contiguous with the western Great Lakes.
Using Microclimates to Expand Your Growing
By definition, a microclimate is the climate of a small, specific place within an area as contrasted with the climate of the general area, or the macroclimate. The climate of the general area is indicated by the USDA plant hardiness zone.
Beginning gardeners typically should plant according to the standard USDA zone. However, it is sometimes possible to grow plants from the next-warmest zone if you know how to exploit microclimates. This is experimental and isn't always successful, but if you play your cards right, you may be able to expand on the standard regional choices in plant selection.
For example, a sunny nook in a yard that is sheltered from harsh winds and frosts is a microclimate: The temperature there will sometimes remain higher than it does elsewhere in your yard. Such microclimates are excellent for experimenting with plants otherwise considered too tender for your region. Let's say you are gardening in zone 5, and the plant that you would like to grow is supposedly hardy only to zone 6. Try growing it in the microclimate of a sunny, sheltered nook. Such a microclimate can also be a great place in which to harden off plants.
Microclimates can also be colder than the general area. For example, if a piece of land sits down in a low spot (at the foot of hills), cold air may settle there. On nights when you have a frost warning in your area, these are often the spots that get hit worst with the frost. Any tender plants that you have growing there may be damaged by the frost. This is why -- much to the dismay of many a new gardener -- the plants in one area of a landscape may be killed by the very same frost that other plants (in other areas of the yard) survive just fine. Frost damage can be a very localized phenomenon.
Note that microclimates are about more than just ambient temperature and atmospheric conditions. Technically, an area of your yard that enjoys full sun could be considered a microclimate, as could one where full shade reigns. Likewise, spots in the landscape that have a dry soil are treated differently (in terms of what you would grow there) than spots that have a wet soil.
It is possible to create a microclimate with a strategically placed structure or planting. For example, building a masonry wall around a garden space is an excellent way to make a microclimate. Not only does the wall supply shelter, but it also retains heat during the day and radiates the stored heat at night, keeping the immediate area warmer. Other structures commonly used to create microclimates include outbuildings (such as outdoor storage sheds), solid fences, and hedges.