Dr. Kim D. Coder
Warnell School of Forest Resources
The University of Georgia
Trees constantly lose water to the atmosphere. Water is the single
most limiting essential resource for tree survival and growth.
Water shortages severely damage young and old trees alike, and
set-up healthy trees for other problems. Drought conditions can
lead to tree decline, pest problems, and non-recoverable damage.
Supplemental watering can greatly assist in maintaining tree health
during droughts - both during the growing season or during the
Trees can be old and valuable. They are usually considered non-replaceable
beyond 10 inches in diameter. Many associated landscape plants
are low cost and easily replaceable. If these plants are damaged
or lost to drought, the landscape can be corrected quickly and
relatively cheaply. Large, drought-killed trees can not be replaced
in two human generations. Please emphasize watering trees during
Ideally, irrigation should automatically begin when soil moisture
reaches some critical measure determined by a moisture probe or
soil tensiometers. Trees should be zoned apart from turf and other
landscape plants. Careful tuning of irrigation systems are needed
to prevent over-watering trees.
Manually, the best ways to water trees are by soaker hose or
trickle (drip) irrigation which you turn on and off. Sprinklers
are less efficient for applying water to trees than soaker hoses
or drip irrigation, but are easy to use. Even a garden hose, moved
often, can provide a good soil soaking. Use a light organic mulch
to conserve moisture and apply water over the top of the mulch.
Do not concentrate water at the base of the trunk as this can
lead to pest problems.
Deep watering a tree with a pipe or wand stuck into the soil
12-24 inches is not as good for trees as surface applications.
Most of the tree's absorbing roots are in the top foot of soil.
Applying water deeper than this level misses the active roots
and allows water to drain away from the roots, wasting efforts
and water. Apply water across the soil surface and let it soak
into the soil. Surface soaking allows tree roots more chances
to absorb any water, helps maintain soil health, and helps maintain
essential element cycling and transformations in the soil. ===================================================
Lay-out water hoses or applicators out to the tree crown edge
(drip-line). Try to water the soil areas directly beneath the
foliage and shaded by the tree. Do not water beyond the drip-line
and do not water closer than 3 feet to the trunk base on established
trees. Be sure the water soaks in well. Use mulch and slow application
rates on slopes, heavy soils (clays), and compacted soils to assure
water is soaking-in and not running-off. If the tree is surrounded
with other landscape plants, or by turf, deep soaking water applications
will benefit all. Do not spray tree foliage when applying water.
Water droplets on tree leaves can lead to pest problems and destruction
of leaf tissue through sun damage. Try not to wet the trunk if
Young, newly planted trees need additional watering care. Water
does not move sideways in a soil. You must apply water directly
over where you need water in a soil. For new trees, concentrate
water over the root ball, as well as the planting area, to assure
Old, large trees can be extensively watered over the entire area
under their foliage. Another method in watering large trees is
to select roughly 1/3 of the area whin the drip-line for concentrated
water applications. The whole area below the foliage can be watered
The best time to water is at night from 10 pm to 8 am. Trees
relieve water deficits (refill) over the night time hours. Watering
at night allows effective use of applied water and less evaporative
loss, assuring more water moves into the soil and tree. Night
time application hours, when dew is already present, does not
expand the foliage wetting period for understory plants. This
watering cycle minimizes pest problems.
The next best time to water is when foliage is dry and evaporation
potential is not at its daily peak. This watering period is late
afternoons. Be sure to allow applied water to dry-off of foliage
surfaces before the evening dew appears. This dry gap between
watering and atmospheric condensation helps minimize pests which
require longer wetting periods. This is especially critical where
turf surrounds a tree.
Because trees lose water from day to day, month to month, and
season to season - dormant season watering during winter drought
is important, especially for evergreen trees and juvenile hardwood
trees that have not lost their leaves. Because of temperature
and relative humidity interactions, much less water is required
in the dormant season, but water is still needed. Do not water
when the soil surface is less than 40°F.
For every 18°F increase in temperature, the amount of water
lost by a tree and the site around it almost doubles. This feature
of water loss must be factored into applying supplemental water
to a tree. Trees surrounded by pavement and other hot, hard surfaces
can be 20-30°F warmer than a tree in a protected, landscaped
backyard. Water use rapidly climbs with increasing temperatures,
and so should water application volumes. ================================================
Depending upon soil texture, bulk density, daily temperatures,
and rainfall amounts, 1-3 inches of water per week should keep
a tree healthy. Trees in limited rooting areas, in containers
or pots, or on major slopes, need additional care to assure water
is reaching the root system in adequate amounts and not suffocating
roots from lack of drainage. Five gallons per square yard is about
1 inch of water.
Fine soils (clays) require careful attention to prevent over-watering
and root death. Sandy soils can be severely droughty because water
runs out of the rooting zone quickly. There are some water holding
compounds that are commercially available for keeping water near
roots. In addition, composted organic material additions and organic
mulch covers on the soil surface can help hold and prevent rapid
loss of applied water. =======================================================
Trees should be watered once or twice a week in the growing season
if there is no rainfall in that particular week. A few heavy (high
volume) waterings are much better than many light, shallow waterings.
A greater proportion of the applied water is utilized by the tree
with heavy watering. Also, light waterings encourage shallow rooting
which can lead to more severe drought damage. Once you begin watering
you should continue to water until rain comes. ===========================================
Many plants in a small area can effectively compete within the
soil to use available water. This water competition can be severe.
Remove excess plant competition from around any tree to decrease
water stress. Use mulch to conserve water and prevent weed competition.
Careful applications of herbicides can also reduce weed competition
for water, but severe drought conditions can lead to unexpected
When landscape watering is not allowed because of water-use restrictions,
"gray water" could be used. Gray water is waste water
from household bathtub, shower, sink, dishwasher, and/or washing
machine. Gray water use is approved in only a limited number of
counties. You must check to see if it is legal in your county
or city. Gray water will play a greater role in water conservation
in the future. Sodium-based exchange-softened water should not
be applied to soils.
Xeriscaping, or developing water-efficient landscapes, is becoming
more important. There are a number of concepts involved in developing
a water-efficient landscapes, when integrated wisely, will conserve
water while providing a functional and aesthetically pleasing
landscape. Trees are a critical part of any water-efficient landscape.