A microclimate is the climate of a small (or sometimes fairly large) area that is different from the area around it. It may be warmer or colder, wetter or drier, or more or less prone to frosts. Microclimates are created by the environment, but we can help nature along by some of the things we do and where we do them.
As gardeners, one of the first things we learn is that some plants just won’t live in certain climates. Thus, the USDA developed a cold hardiness zone map (http://planthardiness.ars.usda.gov/PHZMWeb/) to help us know what will survive and thrive in certain areas, and that map is a truly good guideline to set parameters for us to follow. We can stretch the limits of the cold hardiness zones by using, to our advantage, the microclimates that are naturally occurring and also creating new ones ourselves.
MicroNaturally occurring microclimates are usually not changeable, so we learn to work with them. Examples are lakes and streams (ever heard of lake effect weather?), slopes and their directions, boulders, elevation, and wind velocity, just to name a few. Lakes and streams are naturally at lower elevations and are wetter; therefore, the area around them will be cooler and more prone to frost, thus a microclimate. Remember, cooler air slides downhill. South facing slopes will get more sun in the winter and a microclimate is created. Large boulders (also brick, stone walls, etc.) absorb the sun’s heat in the daytime and radiate that heat out as temperatures drop, so again, a warmer microclimate is produced. Higher elevations not only mean lower ambient temperatures but also heavier wind. Wind will dry out vegetation, especially evergreens and woody plants.
Creating our own microclimates will involve planning and analyzing the landscape around us. Planting questionable hardiness vegetation should be done after careful thought of the environment. Remember that the unobstructed south side of your home will naturally get more sun and less wind, so less hardy plants will be more likely to survive than on the north side. If that south side of the house is stone and/or brick, that vegetation will be even more likely to survive. As hardscaping is planned, keep in mind that a light-colored object, i.e. fencing, will reflect more heat than a darker object. Obstructions, like fencing, will also form microclimates behind them. Remember cooler air slides downhill and will puddle behind the fence on a downhill slope.
For further thoughts and ideas on microclimates, visit Cornell University’s http://www.gardening.cornell.edu/weather/microcli.html .