Watering trees in a dry year
In today's "Growing Together" column, Don Kinzler explains how to best help out our plants during a severe drought — including how much water we should give a tree, and how often.
We could use a good rain, to put it mildly. Much of the region is in severe drought.
Trees are deeply affected during drought years. Have you ever seen the cut-down stump of a very old tree, and observed the appearance of the annual growth rings? Each ring represents one year of a tree’s growth, and they vary in size.
Annual rings produced during years of plentiful moisture are wide, indicating generous growth. Rings produced during drought years are small, tight and narrow, showing the tree produced very little increase in trunk diameter during that dry year. On tree stumps dating back a century, you can clearly see the tight, narrow rings that were produced during the decadelong drought of the 1930s, when trees were greatly stressed.
Hopefully, the current drought will be short-lived. In the meantime, we can prevent our trees from becoming dangerously stressed by providing extra moisture.
How much water should we give a tree, and how often? The answer depends on the tree’s age and whether the soil is heavy clay or light sand.
Old, established shade trees have extensive root systems that can weather dry spells more easily than a young tree whose root system isn’t fully developed. Most large trees will persist through a dry year, but watering them can reduce stress. Stressed trees are more prone to attack by insects, disease and winter injury.
When watering a large tree, let the hose slowly trickle in the area under the tree’s canopy, instead of next to the trunk. North Dakota Extension Forester Joe Zeleznik recommends this: “Water every 10 to 14 days, but only if there’s been no rainfall in that time. A long and slow soaking at the outer edge of the drip line (the area shadowed by the tree crown) is better than multiple short doses of water. Water until the soil is moist, not saturated.”
Younger trees planted within the past five years or so will greatly benefit from watering during dry years. As mentioned for older, mature trees, watering deeply every 10 to 14 days is more productive than frequent, light sprinklings.
To decide how much water to apply, we can follow an old rule that still has merit: trees need 10 gallons of water for every inch of trunk caliper, which is the diameter measured 6 inches above soil line, applied every seven to 14 days. When watering with a hand-held hose, it takes about 35 seconds to fill a 5-gallon pail, by counting "one thousand one, one thousand two," and so on. It’s easy to become impatient, and not provide young trees with a thorough soaking.
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Trees that are planted during the current growing season require greater attention. Immediately after planting, water the tree to remove air pockets from the soil and improve root-to-soil contact. Water twice a week during the first month after transplanting and once a week during the second month, and then cut back to once every 10 to 14 days.
Younger trees can be killed by overwatering. Don’t keep the soil continually soggy with daily watering. If the soil looks dark-moist on the surface, don’t water. If the surface appears dry, scrape aside the top inch of soil, and if the soil still looks moist below, delay watering for a day or two.
Watering frequency also depends on soil type. In areas where the soil is light and sandy, the recommended watering amounts can be divided in two, and applied twice as often. In areas having heavy clay soil, proper watering intervals should be observed to prevent waterlogged soil from killing the trees we were trying to help.
NDSU Extension Forester Zeleznik continues with recommendations: “Another approach to managing drought stress in trees is to reduce competition for water. Mulch around the trees using wood chips, tree bark or other organic mulch. Go as far as you are comfortable with — even as far as the drip line. Keep mulch 3 to 6 inches away from the base of the tree. Weed barrier fabric can be used as a non-organic mulch.”
Mulches around young trees greatly enhance their health. Shredded wood products minimize competition from weeds and grass, retain soil moisture, prevent extreme soil temperatures and prevent lawn mower and trimmer damage.
An easy-to-remember rule is 5-5-5: mulch should be applied 5 inches thick in a circle 5 feet in diameter, and kept 5 inches away from the tree’s trunk. As an underlayment to reduce weed and grass emergence, I often use flattened cardboard beneath the mulch.
Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at email@example.com.