Central Arizona Highlands: Climate and Plant Growth of the Verde
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 19311972.
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
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.
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