Gardening in Tucson
and Southeastern Arizona

Microclimates and Plant Placement

Definition
Each wild plant community has a set of factors which cause that group of plants to be found together. These include sunlight and shade, slope of ground, wind exposure, moisture, soil quality and temperature. Taken together, these factors are called a microclimate.

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 southeastern Arizona, 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. In summer the angle of the sun is much higher, 81 degrees, nearly vertical. A second factor is that the sun rises and sets on the north side of southern Arizona six months of the year. From March 20 to September 21, 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.

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. Plants in the following list can be grown or sheared to make good hedges. All except Silver Buffalo Berry need good drainage.

• Atriplex canescens: Four-wing Saltbush evergreen
• Foresteria neomexicana: New Mexican Privet deciduous
• Leucophyllum frutescens: Texas Sage deciduous
• Rhus virens: Evergreen Sumac evergreen
• Shepherdia argentea: Silver Buffalo Berry deciduous

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.

A backhoe can be used to dig a hole 8' by 8' by 6' deep at the bottom of a slope. The hole can be filled with cactus or brush and then topped with the entire quantity of original dirt to form a mound. This hole then becomes an underground cistern to trap rainwater which nearby trees will exploit. As vegetation in the hole rots, the mound above it will subside to become flat. As long as the rotted vegetation in the hole is less dense than the surrounding dirt it will continue to store water.

Soil Moisture
Plants compete for water, some with shallow roots and some with deep roots. A few plants will dry out the soil around them, leaving no moisture for neighbors. Know which plants are moisture hogs and what distance is needed between the various plants in your garden.

Soil
Modifying pH-neutral to alkaline soil for acid-loving plants requires a lot of material, work and money. If you modify the soil for acid-loving roses, you will not be able to grow native desert plants in that location later without replacing the soil. To adequately control soil acidity and moisture for plants such as blueberries, very large containers with sides protected from the sun and holes in the bottom may be an effective solution.

Heat Control
Plants do best in a certain temperature range. Cold temperatures slow or stop growth and inhibit seed germination and pollen development. Warm temperatures speed growth but also increase plant respiration, loss of moisture, food consumption and reduce sugar content of fruit. The best temperature range, cooler or warmer, depends upon each species of plant. Fruit trees such as apple and plum are cold-climate crops that need winter chill hours. Other trees, such as Vitex, prefer to stay relatively warm all year.

Color influences microclimates because dark-colors absorb light and retain heat and white reflects heat and light. Dark mulch will warm the soil earlier in the spring, and very light mulch and surfaces are great for those plants like lavender that love reflected heat in the summer. Rock mulch, however, is not recommended in the desert because it significantly increases ground-level temperatures and increases the water requirements and stress levels of landscape plants. Only some desert plants can handle that heat and even they are stressed.

The effectiveness of rocks and stone walls in absorbing heat in the day and releasing it at night depends upon their size. Small dark rocks will heat to their cores quickly and start radiating heat back in all directions during the day. Very large dark rocks can absorb heat during the day and not radiate it until the evening because their core temperatures do not reach their surface temperature until late in the day, if at all. Large boulders can keep an area warmer and extend a growing season by several weeks in the spring and fall.

A white wall reflects sunlight and heat in summer and can get too hot for many plants, especially on the south and west sides of the wall. In general, white walls are attractive but stressful for nearby plants in our climate.

North-facing walls create shade and cooler temperatures on their north sides in the daytime. A south-facing, medium to dark stone wall absorbs the heat and light of the sun during the day and then slowly releases it at night on all sides. If this same area is protected from winds it can actually be a microclimate as much as 8 to 10°F warmer than in other spots in your yard.

The more paved area you have -- especially with light-colored paving -- the more heat will be reflected onto nearby plants. Reducing paving and adding more ground cover or other types of plants will help reduce temperatures.

Assessing Your Microclimate
Map out your property and identify those elements that affect the surrounding microclimate. Be aware that some microclimate elements may cancel each other out. For example, if one area has shade from east and west, but a south-facing wall on the north side is white, any plants in that location will be hit with high temperatures in the middle of the day. Also, slopes may exaggerate or cancel temperature and moisture situations.

Plant Categories
There is a difference between desert (native) plants that need full sun and plants from other climate zones. Outside the monsoon season, Arizona seldom has clouds and the sun is intense. Full sun for non-desert plants means part shade in Arizona, especially afternoon shade. Most successful gardens have some part shade from tall trees. Some native desert plants, however, do need full sun to protect against fungal infections during the rainy season.

Plants with aggressive roots do better in very dry situations but must be kept away from septic fields, sewer lines and water lines. Acacia, Cottonwood, Mesquite, Robinia (Locust), and Texas Mountain Laurel can be placed on hilltops or slopes where rain runs off rapidly.

Septic (leach) field areas cannot have deep-rooted plants. Plant those with drought-tolerant, rabbit-resistant ground covers such as

• Achillea ageratifolia: Greek Yarrow perennial
• Thymus 'Reiter': Reiter Creeping Thyme perennial
• Also try self-seeding annuals.

Plants that need good drainage, often desert-adapted plants, will not do well in a low area where water collects or drains poorly. Place these on slopes, ridges or areas that drain well.