The critical balance between landscape trees and irrigation
8 May, 2007 By: Ron Hall LM Direct!
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CHARLESTON, SC — Green Industry professionals, including land care contractors, golf course superintendents and on-site grounds managers, plant and maintain trees. Irrigation and proper soil moisture is critical to the establishment and long-term health of trees, explained Dr. Thomas Smiley at the recent conference of the American Society of Irrigation Consultants Conference (ASIC) here.
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Smiley focused on soil characteristics, planting depth and mulch in his presentation to about 150 ASIC members and guests. Each of these factors affect or is affected by soil moisture. ASIC members, of course, design and develop specifications for major irrigation projects, including golf courses, city parks, residential and commercial developments and college campuses, to name a few sites where they ply their trade.
Unkind urban soils
Often Green Industry pros must plant and maintain trees in urban soils. While the characteristics of these soils can vary dramatically from site to site, rarely are they ideal for tree growth and health. In many cases they exhibit a high bulk density, are hydrophophic (resisting water) and are alkaline. Because the soils are modified and not impacted by the nutrient cycling process of, say, a forest (the native habitat of many species of trees used in urban settings), urban soil temperatures can be variable, as well, said Smiley, arboriculture researcher at the Bartlett Tree Research Laboratory, Charlotte, NC.
Soil, of course, is the one constant in plant health care. Good soils produce healthier, easier-to-maintain trees. And good soils, for most trees and landscape plants anyway, mean soils that are not compacted, drain well and have a percentage of organic matter — in a word, topsoil.
“High soil density will reduce root growth and limit the movement of water and limit plant potential,” he emphasized.
But topsoil is not the whole answer to tree establishment and ongoing tree health, he pointed out. For one thing, some species of trees do well in wet or soils that are heavy and don’t drain well. The key is to determine the characteristics of the soil and then select the tree best suited for the site. Examining the soil structure is as easy as using a simple soil sampler, taking up cores and drying them out to determine bulk density and following up with a percolation test, which is as simple as digging a hole in the soil, filling it with water and timing how long it takes the water to drain.
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Different types of roots
Trees draw their nourishment and moisture from a variety of root systems but generally only pines and some species of nut trees develop what is generally regarded as “tap roots,” and these types of trees aren’t commonly used as urban trees. Urban trees contain a combination of lateral roots, fibrous roots and, in the case of mature trees, deep roots, said Smiley. The deep roots, which occur under the drip line, provide stability for the tree. The lateral and fibrous roots can grow some distance beyond a tree’s drip line, generally more than twice the distance of the drip line as they seek moisture and nutrients necessary for the tree’s survival, he said. Most fibrous roots grow within the top six inches of the soil, the same rootzone for turfgrass. But they can (and often do) grow deeper, depending upon soil and environmental conditions.
While most of the nutrients and irrigation water to sustain a tree should be applied nearer the trunk (but not on the trunk to lessen the chance of phytophthora root rot, the #1 killer of landscape plants), root growth to provide extra stability for a tree can be encouraged by fertilizing and watering beyond the drip line.
The right amount of irrigation
Whether replanting a small or large tree, providing the right amount of supplemental water is essential to the tree’s survival and health. For instance, trees that are planted too deep and are over-watered have a difficult time getting established.
“A healthy tree has a normal drying cycle,” said Smiley, who pointed out that trees suffering from drought stress and trees that are over-watered often show the same symptoms, such as wilting. He urged the use of electronic soil moisture monitoring on those trees that absolutely must be kept healthy. “These are great products. They shut your (irrigation) systems off when the soil is too wet,” he said.
Smiley said a basic rule of thumb for irrigation newly planted trees (in the Eastern part of the United States anyway) is 10 gallons of water per caliper inch per week.
Finally, Smiley offered basic information about using mulch around trees. As simple as it seems, many people, including so-called professionals, still get it wrong, he said.
Avoid the “volcano” mulching, he stressed. In other words, two to four inches of mulch applied around the tree’s trunk is sufficient to retain moisture, moderate soil temperature and discourage competing plants. But don’t put the mulch up against the trunk of the tree. This could encourage the development of phytophthora root rot, as could constantly watering the trunk and foliage of the tree.
It’s better to use drip irrigation and bubblers to water trees rather than sprinklers, and to water deeply but infrequently, he advised.
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