Often when we refer to a microclimate we are talking about a small area, possibly as small as a few square feet. In other words, a microclimate is simply the local climate on a small scale. This small area has elements that cause it to be slightly different than the overall climate of a city or county. In the mountains of the desert southwest, gardeners experience a tremendous diversity of natural microclimates.

It is possible to create a microclimate to improve the growth and success of one or more plants. However, whether you simply want to make the best use of the natural microclimates of your property or create a special situation for a selected plant, you should know how to recognize and modify the elements that create one.

Sunlight and Shade
Temperature is affected by the amount of sunlight an area receives. Areas that get little or no sun tend to be cooler than those that receive a great deal of sun. The physical elements that affect the amount of sun an area receives include houses, walls, fences, mounded earth works, slopes and even where you park your car. Reflective surfaces such as windows and pools can add extra light and heat to a plant. Biological elements that provide shade include trees, shrubs, ground cover, grass and mulch.

The amount of shade an area receives depends upon the season. In winter the angle of the sun is lower, to about 34 degrees in Tucson and 33 degrees in Phoenix. In summer the angle of the sun is much higher, 81 degrees for Tucson, 80 degrees for Phoenix. A second factor is that the sun rises and sets on the north side of Tucson and Phoenix nearly six months of the year. From the start of spring to the start of fall, sunrise and sunset provide sun on the north side of every house. Seasonal shade is also provided by deciduous trees that provide shade in the summer and allow winter sun to warm the ground and nearby structures.

Eastern Exposures
Receive sun in the morning and shade or reduced sunlight in the afternoon all year. These areas are cooler and lose less moisture from the soil. Plants that like part shade and moister soil do well there. However, a light-colored wall or reflective surface turns this area into a stressful hot zone and negates the eastern exposure effect.

Western Exposures
Provide shade in the morning with sun in the afternoon. In summer these areas are hotter and their soil is drier. In the spring and fall, however, hot afternoon temperatures and cold nighttime temperatures can damage sensitive plants. The temperature extremes of western exposures are best for hardy plants that like dry soil, hot temperatures and part shade. Deciduous trees on a west side can be used to shade a house in summer yet allow sunlight to warm it in winter.

Northern Exposures
Have full shade most of the day with the coolest temperatures and most moisture year round. Plants that prefer cooler temperatures, higher humidity and less light do well there.

Southern Exposures
Are hotter, brighter and drier in the summer, especially if next to a light-colored structure. Southern and south-western exposures provide the most warmth in the winter. Plants that prefer warm temperatures do better there.

Slopes
The governing principle of slope temperature is that warm air rises and cool air sinks. The tops of slopes are warmer, the bottoms cooler. Slope moisture and soil structure varies with location and steepness. On the top of a slope the soil is usually rockier and courser. This type of soil drains well and holds less moisture. At the bottom of a slope, because of rainwater runoff and erosion, the soil is moister, finer and higher in nutrients. Further, the steeper the slope in the middle, the drier the soil.

Slopes that face south or west are warmer. Plants on these slopes often bloom and leaf out first in the spring. Plants that prefer elevations that are lower and warmer than your location will do better on south- and west-facing slopes. Slopes that face north are colder and moister. This is the location for plants that prefer higher elevations and cooler temperatures than your location.

Installing retaining walls or rock dikes crosswise to a slope, creating terraces, helps reduce runoff. Wildflowers are known to spontaneously appear on these terraces.

Air Circulation
Sinking cold air can form frost pockets in basin areas in the wintertime. Look around your yard on an early morning when the temperature is below freezing. Where there is frost on the vegetation there is a low spot in your yard. If frost is a problem, provide air drainage just as you would water drainage.

Wind
Wind affects a microclimate by drying and heating or cooling plants depending on the relative air temperature. Be aware of trees, hedges, walls, paving and dirt roads that channel wind. Groves of trees or shrubs create "outdoor rooms" with reduced wind velocity. Stone walls and tall evergreen hedges can protect from wind year round. In areas with no prevailing wind direction, a circular or rectangular hedge can be grown to protect sensitive plants from wind.

Rainwater Harvesting
A major purpose of rainwater harvesting is to spread rainfall water out over a larger area and slow it down so that it sinks into the soil, becomes available to plants and does not cause erosion. Roofs, patios, sidewalks and paving create areas of high water runoff that can be collected in large containers for later reuse. In addition to collected rainwater, channels can be dug on slopes to manage rainwater flow and divert it near trees and shrubs needing water. Rock beds in channel bottoms, rock dikes and rock edging can be used to slow and direct water flow and give it more time to sink into the soil. Trees and shrubs that do not tolerate flooding can be placed above low areas where rainfall is allowed to collect. Plants that tolerate periodic flooding such as Desert Willow and Mesquite can have rainwater pool around their roots and be slowly absorbed into the ground.