MICROCLIMATES: WHAT DO THEY MEAN TO YOU?
Dr. Leonard Perry, Extension Professor
University of Vermont
Microclimates, or the climates in small areas, should mean a lot to you if you're a gardener. As a colleague of mine put it, "In the real world, we garden in microclimates, not hardiness zones."
Most gardeners are familiar with the USDA hardiness zones and maps that divide the country into areas with similar average annual minimum winter temperatures. These may be a good place to start when choosing which plants may grow in your area, but only that-- a place to start. These zones do not take into account the many factors affecting your own property, or microclimate.
If these zones might be termed a "macroclimate", a climate over a large area such as much of a state or part of one, then smaller areas are usually termed "microclimates". I actually like to use another term, "mesoclimate", to refer to areas larger than your own yard or neighborhood. These could be a whole town, a valley or hillside, or an area by a lake where the large body of water moderates the air temperature.
I use microclimate to refer to an area on your own property, but even this can vary widely. Often I find a property, even much less than an acre, can have several hardiness zones due to various factors. I then term these smallest of areas, perhaps along the side of a house, or on the south side of a wall, "miniclimates".
In addition to the sides of homes and walls, particularly south sides, that can radiate heat, other hardscape elements that store and radiate heat might be sidewalks, paved surfaces, patios, even single large rocks. Placing an alpine plant on one side of a rock rather than the other may mean the difference in it living.
In addition to such heat radiation, another main factor that might affect your micro- or miniclimates is shade or sunlight. Receiving less sunlight, shaded areas often remain cooler. I have one garden bed only receiving morning sun, shaded by large evergreen trees from the warmer afternoon sun. This bed is often the last to have the ice and snow melt in the spring, so plants emerge much later here. This bed, in effect, represents a whole hardiness zone colder than a bed only 20 feet away in the sun.
Areas shaded by trees affect temperature in another way. At night, the soil radiates heat away and upward into the atmosphere. Just as clouds help trap this heat, and so there is seldom frost on overcast nights, so do tree branches help keep the ground and plants underneath warmer.
Differing soils may also affect frosts, and so create microclimates. A heavy soil acts similar to pavement, trapping heat and moderating temperatures. A lighter sandy soil has many air pockets, so doesn't trap heat, and in fact helps insulate the warmer subsoils. Such soils may trap this heat below ground, and so may be more prone to frosts at the ground level.
Slopes have a major effect on microclimates, with the colder air draining downward into low areas, basins, or valleys. Slopes may also represent whole different hardiness zones, depending on the direction they are facing. Have you ever noticed along roads that slopes on the north side (facing south) often have no snow in winter and are earlier to green in the spring?
Winds can also create microclimates, especially for woody plants and evergreens. Windy areas may have less frost, but may also have less snow in northern climates, the wind blowing the snow away. Since snow is perhaps the best insulator for plants, holding in the ground heat and protecting buds on plant stems, little snow may mean colder soil temperatures and plant death or no flowering some years. Wind also can dry out the foliage of evergreens in winter, and most plants in summer, meaning they may need more water to prevent drought damage.
So if you try a plant, supposedly hardy to your zone, and it dies, it may not be your fault or that of the nursery. If it is something you really like, try it again in a different year. Also try it in a different part of your property, a different microclimate.
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