Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont
Trees and shrubs need moist, but not thoroughly wet, soil in order to grow well, resist insects and winter injury, and, in some cases, to produce flowers and fruit. If fruiting plants fail to fruit, or produce few fruit, it may be due to drought during flowering or fruit production.
From early spring until about September 1, apply water adequately
to all woody plants. Keep in mind that some trees, including
beeches, cottonwoods, larches, poplars, aspens, willows, maples,
birch, spruce, mountain ash, will need more water than others.
Hydrangeas and magnolias are shrubs sensitive to drought, and so
needing more water than most shrubs.
Newly planted trees and shrubs—ones planted this season— will
need more water, too, the first year. Except for evergreens, it
is not advisable to wet the leaves, because this can encourage
rust, blight, and mildew diseases.
After that, water less to allow the plant to harden off. This
will reduce chances of damage to wood by early snowstorms and
freezing temperatures. Then, in mid-October when leaves have
fallen, or prior to a ground freeze, apply water liberally several
times to avoid winter drought. If fall, though, is abnormally dry,
you may need to keep sufficient water on evergreen plants and
newly planted trees and shrubs of any sort.
Late season watering is important particularly for broadleaf
evergreens, such as rhododendrons, since their broad leaf surfaces
are exposed to winter cold and winds, which dry leaves out
(“dessication”). With frozen ground and the inability to take up
replacement water until spring thaw, this results in leaves
browning or even dying.
During hot, dry periods, water your plants every six to 10 days.
If the soil is very rocky, gravelly, sandy, or has poor
water-holding capacity, water once every five to seven days
putting on about an inch of water each time. For clay-loam soils,
apply water every 10 days to two weeks, but put on about two
inches of water per watering. Newly planted trees and shrubs need
watering every five to seven days, if not provided by rain. If
plants are in containers, and yet to be planted, keep them watered
very often, perhaps daily with a good soaking.
To gauge how much water is applied, or how much rain plants
actually received (and so how much you need to supply), use a
commonly available rain gauge. You can find them at most hardware
and home stores, or more professional and accurate models (even
wireless remote ones) online from weather supply firms.
Be careful not to overwater (this, too is a leading cause of
plant death, mainly in poorly drained soils), but be sure to put
on enough to wet the ground to a depth of approximately 24 to 30
inches deep for mature plants. This is the zone which contains
most the water-absorbing roots. (Tap and anchor roots are
deeper.) You should wet the entire root area, which extends out
at least as much as the limb spread—the “drip line”.
Water well or not at all. Shallow watering will "starve" the
deeper roots, causing more growth of the surface roots. In causing
more root formation near the surface and less deeper down, you
will predispose those roots to freezing conditions. Roots near the
surface are not protected as well from the cold as are deeper
roots. Also, they will dry out sooner and won’t be able to draw
water from deeper soil levels.
If you have groundcovers or mulch under trees and shrubs, a
soaker hose which slowly emits water along its length would be a
good choice. Otherwise, move a hose at medium water pressure
gradually around under the drip line of a tree (unless the ground
slopes, in which case water may run off the desired area). If
using an overhead sprinkler under a tree or on shrubs, place a
rain gauge underneath to measure water applied. Up to half the
water from an overhead sprinkler may evaporate in hot, dry
weather. Or, you can use a straight-sided small container. One
to two inches collected in such a container means the water should
reach the roots within the top six inches of soil.
Another watering option for trees is watering bags. You can buy
these online or at full-service garden stores and nurseries. You
place them around the tree base, fill them with water, and they
release this water slowly. There are several brands, either
upright or in doughnut shapes, which you see commonly in new
plantings in commercial areas.
Mulch placed around trees and under shrubs will help lessen soil
temperature fluctuations, and conserve water. Don’t apply more
than three to four inches of an organic mulch such as shredded
bark or leaves, and keep mulch away from tree trunks and shrub
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