Central Arizona Highlands: Climate and Plant Growth of the Verde Valley

Climate, or long-term weather, influences what plants naturally grow in an area. The Central Arizona Highlands is a distinct area between the extensive Colorado Plateau to the north and the Sonoran Desert to the south. The climate of this area is a cycle of winter precipitation, spring drought, summer precipitation, and fall drought. The Verde Valley is the lower elevation region of the Central Arizona Highlands, where the towns of Sedona and Cottonwood are located. Historical weather records are one way to learn about the area. The book Arizona Climate (The University of Arizona Press, Tucson, AZ) gives a summary of records at various stations throughout Arizona from 1931–1972.

Many climatic factors influence what kinds of plants will grow in your yard. Minimum winter temperature and frost occurrence, maximum summer temperatures, rainfall amount and distribution, humidity, day length, and light intensity influence all plant growth. Each garden, field, and yard has its own microclimate that is largely influenced by topography. Slopes are usually more frost free than flat land. Beware of narrow washes that carry cold air from higher ground. Structures, paved areas, hedges and windbreaks may change airflow patterns, alter day length or light intensities, trap heat during the day and slowly release it during the night, or in other ways modify local climate.

Minimum winter temperature is a major limitation. The best way to keep track of air temperature for your landscape is a maximum-minimum thermometer. This is a U-shaped tube that records the highest and lowest reaches of the mercury in 24 hours. The thermometer should be read and reset at the same time each day.

Contrary to popular belief, the air does not absorb heat directly from the sun. In fact, on a clear day, much of the sun's energy passes through the air unscathed. Upon reaching the surface, the sun's radiant energy is absorbed, providing energy to heat air and soil, and to evaporate water. Bare soil absorbs more heat than soil covered by grass or a ground cover. Temperatures fluctuate widely in the air and soil region closest to the surface. Have you ever seen frost on the grass when the official temperatures is 34 or 36 degrees F? Temperatures at the grass surface are likely to be colder than the surface temperatures of dry soil because the grass and thatch insulate the surface from heat flowing upward from the soil at night. You can take advantage of this soil heating to protect fruit trees in bloom on a cold night. The soil absorbs the sun's energy during the day and radiates it back at night, increasing slightly the air temperature around the fruit tree. And a bare soil will radiate back more heat than a soil with a plant cover.

Soil temperature plays an important role in how seeds germinate when planted. The soil temperature on the surface can change by 50 degrees F and more, but at depth of 3 feet, soil temperatures change little on a daily basis. You can use mulch to modify the soil temperatures. The mulch acts like insulation in a house, greatly retarding heat flow into and out of the soil. During the day, the solar energy is absorbed at the mulch surface. Because the mulch is typically porous (like insulation), heat transfers poorly from the mulch-air interface to the soil below (or vice versa at night). The result is that most of the sun's energy is dissipated to the air as heat. In contrast, if you want to heat up the soil, don't use mulch. Remember that bare soil absorbs solar energy which heats the soil.

Spring is known for its drying winds. Typically, winds are light during nighttime hours. Gardeners can take advantage of this by setting out new transplants in the late afternoon and early evening. About mid-morning, the winds begin to rise quickly. This sudden jump in wind speed coincides with the lifting of the nighttime inversion. From this time on until sunset, winds can be high and gusty. Near sunset, the surface cools and the inversion reforms, resulting in lower wind speeds. When you set out new plants, be sure to protect them for the first several days from winds.

With a range of 140 to 200 frost-free days in the Verde Valley, gardeners grow a wide variety of crops. A good weather-watcher can extend the season and make the difference between a fruitful harvest and no harvest at all.