A microclimate is the climate of a small 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 may be quite small – a protected courtyard next to a building, for example, that is warmer than an exposed field nearby. Or a microclimate may be extensive – a band extending several miles inland from a large body of water that moderates temperatures.
When you use the USDA Hardiness Zone and spring and fall frost-date maps, remember that microclimates can cause your farm to be very different from the hardiness zone listed on the map. If you are in a cold valley, your minimum winter temperatures may be lower than what the map predicts. As a result, some marginal plants may not survive your winters.
Cold valleys may also be prone to late spring and early fall frosts, making your growing season shorter than indicated by the frost-free season map. If you put out tender plants too early, you might lose them. If you plant long-season heat-loving plants, they may not mature before fall frost.
Some microclimates are much larger, extending for miles because of the effects of:
Large bodies of water, such as the Great Lakes, the Finger Lakes, Lake Champlain, Long Island Sound and the Atlantic Ocean, tend to moderate air temperatures of adjacent inland areas. Low temperatures in winter are not as extreme, and these areas are less prone to late spring and early fall frosts. Smaller bodies of water also have the same effect, usually to a lesser extent.
Topography has a profound effect on microclimates. Cold air is heavier than warm air. So on cold winter nights or nights when frost threatens, the cold air flows downhill and collects in low spots — just like water flows down hill and collects in puddles. On winter nights, some valleys may be 10 degrees or more colder than neighboring slopes. These valleys may also be more prone to frost.
Hilltops may not suffer as much from frost or cold temperatures on nights with radiational cooling. But if they are exposed, winter winds can often wreak havoc. Winds dry out plants, and are particularly hard on evergreens, which cannot replace moisture lost through their needles or leaves when the ground is frozen.
The slopes between cold valleys and windy hilltops can have different microclimates depending on their aspect (which direction they slope). North-facing slopes are slow to warm up in spring because they receive less direct sun, compared with south-facing slopes. But gardening on a south-facing slope can be a mixed blessing, especially when early spring warmth causes plants (fruit trees in particular) to begin flowering prematurely, only to have the blossoms killed by a sudden frost.
Fences, walls and large rocks can protect plants from wind and radiate heat, creating sheltered spots. Sometimes, if fences block cold air drainage through your property, the cold air can puddle behind them causing very localized frost damage on near-freezing nights.
Raised beds and terraces — like hillside slopes — can warm and drain earlier in spring, especially if they are oriented toward the south.
Soil types can also affect frost. Heavy clay soils can act much like paved surfaces, moderating the temperature near ground level. Lighter soils that have many air pockets in them can act as an insulating layer on top of warmer subsoils, trapping that heat below ground and are hence more prone to frosts at ground level.