When we talk about weather as it pertains to gardening, we often reference political regions and the familiar USDA Winter Hardiness Zones, which each cover quite a bit of territory. But weather and its synthesis, climate, can vary on a much smaller scale. There can be major differences within a county, a neighborhood or even a yard or garden. These differences may be such that relying on large-scale climate zones is not entirely appropriate. We must enter the realm of microclimates.
City plants are subject to the "urban heat island" phenomenon in which paved surfaces and structures limit the cooling of the air at night. Surfaces can create a similar situation in suburban or rural gardens.
A microclimate is defined as the climate of a small area that differs from that of the general surrounding region. The area can be as large as a city or as small as a plot of land; it may be measured in square miles or just a few square feet. Large-scale and small-scale microclimates share some causes in common, and they can also be caused by different factors.
Large-scale microclimates
Topography is a major influence on large-scale microclimates. Changes in elevation affects temperature. On average, temperature decreases 3.5˚F per 1,000 feet change in elevation; at times this change can be greater. But on clear, calm nights, the heavier cold air sinks into valleys and makes them colder than the surrounding higher elevations. Frost may occur in the valley while the upper lands remain frost free.
A body of water also affect temperature. Since water tends to warm or cool slower than land, its presence tends to moderate temperature extremes. It's not as hot during the day or as cold at night near the water as it is in areas farther away. It's overall cooler in the summer and warmer in the winter, too.
People influence large-scale microclimates, too. The term "urban heat island" refers to abnormal warmth in cities at night. It happens because building materials such as concrete and asphalt absorb the sun's heating rays during the day and radiate this heat through the night. On clear, calm nights, cities can be 10˚F warmer than the surrounding suburbs, or even more.
Small-scale microclimates
Some the above factors can create small-scale microclimates, as well.
Topographic differences within your yard or garden can make changes. Low spots can accumulate cold air and be more prone to frost, similar to valleys. Although elevation changes within a small area are typically not enough to cause significant temperature differences, there are always exceptions, especially in hilly or mountainous locales.
Your yard likely contains small-scale heat islands. Any surface paved with concrete or black asphalt will absorb heat during the day and emit it at night. Depending on the air movement, areas surrounding a paved surface can be warmer. Structures such as your house also radiate heat at night.
Other influences make a major difference in small microclimates. Sunlight is crucial for plant development and it affects air and soil temperatures. Keeping the sun's position in mind, we can see how other factors interact with sunlight to produce small-scale but significant variations in weather and climate.
A major element tied into how much solar energy reaches the ground is the slope of the land and its aspect, or what direction it faces. The actual degree of slope will also determine the magnitude of the impacts. In North America, an east-facing garden will get morning sunlight and tend to be cooler than a west-facing garden, which falls under stronger solar rays. A south-facing slope will receive the most sunlight. Sun-loving plants would flourish there. South-facing slopes also tend to be warmer and drier. North-facing land receives less solar energy and can be considerably colder and often wetter.
Shading is a major factor that affects the amount of sunlight reaching the soil surface. Shading from trees or structures must be allowed for. Knowing the sun's path and where the shading object sits in relation to the garden will tell you where and when shading will occur. Shadows are longest when the sun is low in the sky and minimal at noon.
These two factors, slope and shading, are critical to soil temperature in spring. South-facing, open areas will more quickly reach temperatures necessary for seed germination. North-facing or shaded areas will take more time to warm up.
Creating microclimates
To some extent, you can manipulate a microclimate to your gardening advantage. Any type of covering can protect young plants from cold temperatures; these are particularly handy on clear, calm nights that maximize radiational cooling. Watering your plants is a manipulation of their microclimate. The irrigation is providing additional and consistent moisture. You can help plants cope with wind, which can be very drying, by building or planting a windbreak, be it a mound of dirt or a row of dense shrubs. But above all, when you identify natural microclimates on your property you can choose plants that are best suited to those particular spots.