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 broadleaf 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 the 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 stems.
Keep watering in fall as long as you can, until the ground freezes. Newly planted trees may need supplemental watering for the first two to three years until their roots get established. Water according to how much a plant may be getting naturally, or need, and don’t wait to see signs of stress such as wilting and yellowing leaves. For newly planted trees and shrubs, proper watering is the single most important maintenance activity to help ensure their survival and long life.
WATERING TREES AND SHRUBS
Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont