By Ed
Perry
University of
California Cooperative Extension Farm Advisor,
Modesto
More
garden plants grow poorly or die as a result of poor watering practices than all
other causes combined. While
gardeners often apply too much water, lack of water or “water stress” is also a
major cause of poor plant performance.
As you
plant new trees and shrubs this summer, remember the importance of keeping the
root ball moist for the first few weeks.
Keep in mind that moisture in the soil outside a newly planted root ball
is unavailable to the new plant until roots can grow into the moist soil. It can take up to 13 weeks before
regenerated roots absorb measurable amounts of water from outside the root
ball. In the meantime, the plant is
obtaining most of its moisture from roots growing in the nursery potting soil,
which can dry out rapidly.
Even
mature, well-established trees and shrubs need some irrigation water during the
hot summer months. Woody plants, like landscape trees and
shrubs, usually don’t show water stress as readily as annuals or herbaceous
plants. Drought symptoms on woody
plants may include yellow leaves, “burning” on the edges of leaves, reduced
shoot growth and smaller leaves, wilting and drying leaves and finally dying
branches. Conifers such as cedars
and pines often die from the top down.
By the time you see leaf drop and dieback symptoms, the trees have been
seriously drought stressed.
Drought-stressed trees then become susceptible to attack by bark beetles
and other insect borers.
Other
plants in your garden and landscape will more readily show drought
symptoms. Turfgrasses, for
instance, turn a dark bluish-green color or wilt. Plants with large succulent leaves, like
squash and cucumbers, may wilt during the hottest part of the day, then recover
in the evening as temperatures drop.
This temporary wilting doesn’t necessarily mean the soil is dry. The plants may simply be transpiring
water more rapidly than the roots are absorbing moisture. If the plants are still wilted in the
evening, or wilt early in the day, they definitely need
watering.
You can
get an idea of moisture conditions throughout your garden and landscape by sampling the soil in several locations
and from several depths at each location.
Lawns and leafy vegetables root mainly in the upper 6 inches of
soil. Tomatoes and small shrubs
root mainly in the top 18 to 24 inches of soil. Large shrubs and trees root mainly in
the top 2 feet of soil, with some roots penetrating to 3 feet. When checking the soil beneath large
shade trees, be sure to begin beneath the outermost branches (drip line). The root systems of many common shade
trees may extend up to three times the diameter of the drip line, so that is
where you need to dig. A soil auger
is a good tool to obtain the deeper samples, but a shovel also works well.
You can
make a fairly accurate assessment of your soil’s moisture content with a simple
“hand-feel test.” After taking a
soil sample, try to roll or squeeze it into a ball. If the soil will not form into a ball,
it’s probably too dry to supply water to plants. If the soil forms a ball, rub it with
your thumb. If the ball will not
crumble, the soil is wet enough.
Very sandy soils are the exception, since they will crumble even when
wet. Be sure to sample different
spots in your garden each time.