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Features in your yard, both natural and human-made, often modify the overall climate and create small areas with distinctly different environmental conditions (including hardiness zones). Here, your zone rating may go up or down by one or possibly even two levels, changing your planting options.

A microclimate (a small, usually isolated area that is warmer, cooler, drier, or wetter that most of its surroundings) can be anywhere from a few feet wide to a few hundred feet wide. Examples of a microclimate include a low area, a south-facing area, the north side of your house or other structure, an exposed hilltop, a slope, any enclosed and sheltered area, a spot close to the foundation of your heated basement, and so on.

Look for marked differences in these areas:

 Water: Proximity to a pond, stream, wet ditch, or the ocean can make temperature fluctuations less dramatic.

 Soil: Different types of soil can create protective or stressful growing conditions. For instance, clay soils hold moisture and heat and thus can reduce stress in dry or very cold conditions. Very sandy soils drain well and are great where excessive water is a problem, but in hot and dry conditions, they can put plants under severe water stress.

 Wind: See how strong the winds are and how often you get air movement in a particular area. Winds are very drying.

 Temperature: One spot may be significantly hotter or colder than its surroundings. Note that cold air often flows over a landscape like water, settling in low areas and creating cold pockets.A useful tool for determining temperature variation is the maximum-minimum thermometer (also known as a Six’s thermometer). This thermometer can measure the high and low temperature during a given time and can measure the extremes of temperature in a location. See Figure 3-1. This instrument is also available in a digital form. Recording thermometers can store temperatures and humidity levels over longer periods of time, and some can be synchronized to a spreadsheet on your computer. Apps come with some of these digital, remote-sensing devices and can track temperatures on you phone.

 Light: Note dramatic differences in amount of daily sun and shade. Human-made structures (yours or a neighbor’s) as well as trees can contribute to these changing conditions.

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FIGURE 3-1: A maximum-minimum thermometer gives you an idea of the outdoor temperature ranges in your area.