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WATER TREES NOW TO PREVENT LONG-TERM DROUGHT DAMAGE
The concern of many tree lovers these days is drought. What effect will this summer's drought have on trees this winter? Next spring? Five and 10 years down the road?
Many large oaks, maples and other hardwoods that died during the past decade can trace their demise to the droughts of 1987-88. Bone-dry conditions during those summers stressed trees so severely that secondary invaders -- insects and diseases -- found easy going as they invaded their vulnerable prey.
The concern now (despite recent rains) is how our yard trees will fare next growing season as a result of less than three inches of rain between July 4 and September 11.
"The effects of moisture stress on trees and shrubs are not always immediate," explains Patrick Weicherding, University of Minnesota extension horticulturist. "When your lawn is stressed the turf turns brown. When trees and shrubs are stressed the harm may not show up for several years until the tree or shrub begins to decline and die back."
Water stress in trees and shrubs can show itself with a long list of signs: Temporary leaf wilt, scorched leaf margins, reduced annual growth, increased insect and disease problems, loss of vigor, loss of older leaves, defoliation of scorched leaves, premature fall leaf color, stunted leaf size, death of lower, shaded branches, and increased winter injury.
Weicherding says water makes up over 50% of the fresh weight of trees and shrubs and is the single most limiting factor to growth and health. Water plays a critical role in plant physiology: as a solvent; a reactant in photosynthesis; a carrier for nutrients and other materials; and in the maintenance of turgidity.
Drought-stressed trees and shrubs are more prone to winter injury because they do not have the ability to go through the long, slow process of shutting down for winter dormancy. As a result, they are more susceptible to dieback, stem cracking and bark splitting. Woody plants that show fall color in August and early September are shutting down from stress, not winter dormancy.
Water stress can be especially hard on trees that were planted in the wrong place. A species native to a shaded environment -- when placed in an exposed site -- will show more water-stress symptoms than a plant adapted to full sun. This is why sugar maples often do poorly when placed in full sun in the center of a compacted-soil lawn. Maples prefer a cooler, partially shaded environment in soil enriched with decaying plant matter.
You can do your trees and shrubs a favor by watering deeply (6-8 inches deep) at least once a week until the ground freezes. If you haven't been watering this summer, it may require more watering to attain that depth of soil moisture than you would expect, points out Gary Johnson, University of Minnesota urban forester.
For newly planted trees and shrubs, Johnson advises watering every 2 to 3 days. For every inch of stem caliper, apply 1.5 gallons of water and just pour it over the root ball area. So a two- inch tree needs three gallons every two to three days. If the soil is poorly drained, the plants may need water less often. In drier soils, they may need to be watered daily.
Laying a hose against the trunk of the established tree in the front yard doesn't do much good since feeder roots are located away from the trunk. For every inch of trunk diameter (measured 4.5 feet up from the ground), you need to water one to 1.5 feet of root radius. So, for a tree that is five inches in diameter, you need to water 5 to 7 feet out from the tree trunk. Most feeder roots are located within this area.
Johnson prefers to use soaker hoses and says he leaves his run for several hours in his yard. Overhead sprinklers lose a lot of water, but if that's all you have, use them. Right now, the most important thing is to provide water to your woody plants.
Depending on precipitation levels this fall and winter, watering next spring may be critical for tree health. Early in the growing season, trees and shrubs require plentiful water for new growth. Trees that were recently planted or have suffered other root damage require more irrigation during the next two to three years. Trees and shrubs with chronic water-stress symptoms need a few deep soakings in the early part of the growing season, before stress symptoms appear.
Weicherding says mulching is one of the best things you can do to improve plant health and reduce water stress. A generous tree ring mulched to a depth of about 4 inches helps eliminate competition from weeds and grass, helps retain soil moisture, moderates soil temperature, and gradually improve soil structure and fertility as it decays.
Mulch can be applied any time of year and either wood chips or shredded hardwood are great materials to use. Avoid "mulch volcanoes" by pushing mulch away from the trunk or stems.
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