The Challenge for Increasing Rainfall Infiltration in Urban Landscapes
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By George Ware, Ph.D.
Dendrologist Emeritus
The Morton Arboretum
Established home landscapes offer opportunities for changes that increase rainfall interception and absorption for both trees and lawns., here are some cultural considerations.
A Tree-mulch System as Means of Improving Rainfall Insoak
A generous area of double-ground mulch around a tree acts as a sponge, detaining rainfall very effectively. In an area of double-ground mulch, four inches thick, little rainfall leaves the tree-mulch detention system, even on gentle slopes. For a parkway tree, there is a substantial reduction in amount of run-off reaching fast-flowing street gutters. Mulching lone open-grown trees produces discrete detention units. The presence of mulch induces earthworm activity in a year or two after application, improving soil permeability and facilitating the mixing of mulch and underlying soil. Increasing soil porosity means increasing insoak. The mulch decomposes rapidly and must be replaced every year or every other year. The double-ground mulch ends up as humus, producing a long-lasting sequestration of carbon. Soil porosity brings about vast proliferation of fine rootlets and more rapid growth of the whole tree. Rapid growth means rapid addition of carbon in roots, trunk, and branches. The tree-mulch system produces a somewhat "forest-like" soil environment and reduces energy costs associated with maintenance of replaced lawn.
Improving Soil Porosity in Lawns
Poorly permeable soil is widely prevalent in the Chicago region. Rainfall insoak is often slow, leading to rapid run-off and flooding. Good soil porosity plays an important role toward keeping arriving raindrops in place. Increased depth of topsoil would not only detain more rainfall but would also facilitate the planting, maintenance and awareness of healthier landscapes. In new neighborhoods, trees and shrubs are often planted in difficult impermeable substrate. Increasing the width of a planting hole would introduce backfill, substantially increasing soil porosity around young trees. Mowing less frequently and at higher grass-blade length would provide more rainfall receptive grass blades. Studies have shown that during heavy rain, closely mowed lawn is little better than black-top in detaining raindrops.
One of the techniques for solving the problem of exposed surface tree roots (sometimes called ‘elbows') in a lawn is to add around the area a thin veneer (1/2 inch) of organic topsoil. Thoroughly wash it into the grass. In a few weeks repeat the procedure, never obscuring the green grass. This incremental thickening of topsoil improves porosity and rainfall insoak. With this method, health and greenness of the lawn are maintained, and the elbow is no longer visible. The mounding effect is minimal.
Bluegrass lawn has deep-seated homeowner acceptance but has some serious drawbacks with respect to its independent sustainability. Many people do not understand that properly functioning roots depend upon photosynthetic contributions made by the green grass blades. Mowing too early in the spring may induce a surge of green that robs the root system of nutrients and vigor. Generous early watering may favor top growth over root growth, leaving the lawn vulnerable to summer heat. Excessive fertilizer may also be a part of this scenario in which nitrogen becomes a water-borne pollutant, leaving the property. Widespread clayey lawn soil in our region presents significant limitations that seem to encourage current procedures for lawn care that are competitive with trees and detrimental to tree health. Proper use of mulch is a way of imitating the environment of the forest floor. Widely accepted is the cautionary warning that more than 85 percent of urban tree problems begin underground.