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Getting in Touch with Our Roots


Getting in Touch with Our Roots

By Robert Kourik © 2015

They're rather ugly. Yet they're mostly “invisible.” Nobody really understands them. “They” are the roots which provide sustenance and support for all plants. Because roots lie beyond our normal view, most gardeners have conjured up some pretty fanciful assumptions about how roots grow. Thus, misconceptions about the growth of tree roots—both shade trees and fruit trees—permeate and distort how almost every tree in America is watered, mulched and fertilized.

If you have a watering moat within two feet of an established tree’s trunk than you may be unwittingly wasting water, constricting root growth, reducing the tree’s sturdiness to wind and possibly threatening the tree with various root rots. Adding moisture and, especially, fertilizers well beyond the two-foot radius can make for a bigger, healthier tree and one less prone to blowing over. Let’s look at the common myths pertaining to watering, feeding and mulching trees:

The Roots are as Wide as the Canopy

This old adage is as common as it is wrong. In heavy-clay soils, roots may be one-half again as wide as the foliage—still well beyond the foliage’s edge (also called the dripline). In sandy soils the roots will often grow up to three times wider than the dripline.

Taproots Anchor the Tree

Some trees are anchored by the deep, vertical taproot. These include: walnuts (Juglens sp.), pecan (Carya illinoensis), many Eastern pines (Pinus sp.) and oaks (Quercus sp.), Scotch pine (Pinus sylvestris) and some Western oaks (Quercus sp.). (However, any naturally-taprooted tree planted over a shallow soil with a barrier of clay, hardpan or rock will not grow a characteristic taproot.) But taprooted trees, aren’t always the rule. Most fruit trees and many shade trees have a fibrous root system composed of a number of horizontal roots radiating out from the trunk and vertical roots—called sinker roots—at various intervals along the horizontal roots.

With horizontal-and-sinker roots, watering at the very base of the tree won’t help most of the roots. Many yards sit on top of less than ideal and shallow soils, any taproots will then travel far to the side to gather enough water and nutrients and to support the tree. Confining the root zone with water and fertilizer close to the trunk diminishes the tripod effect of the horizontal-and-sinker root system. 

Trees Feed Deep in the Soil

Trees do have deep roots, up to dozens of feet below the surface in loamy, deep soils. But, the deepest roots are primarily for anchorage and emergency water and nutrients during extreme droughts. The upper layers of the soil are the most aerobic and this is where the natural processes of the soil can release the most nutrients. With trees, the upper one to two feet of the soil provides over 50% of the nutrients required for good and productive growth—whether it’s flower, leaf or fruit production. And the top foot of soil is almost always has the highest percentage of uptake of minerals and sometimes for water absorption compared to any other foot of soil

Root-Guided Tree Care

All this “new,” really rather old, information about root growth points to some important new ways to treat established shade and fruit trees: No watering moats next to the tree’s trunk—although freshly transplanted trees can tolerate this. Add any required fertilizer starting at least two feet away from the trunk and extending up to 1.5 times the width of the canopy. No mulch is needed in the first six- to 12-inches from the trunk. Place mulch out to the dripline and well beyond. Water to replenish moisture lost in the top one- to two-feet of soil. Experiment with your trees, try these new guidelines on one or two trees and you’ll probably see improved growth and maybe even reduced pests and disease.



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The Roots of Root Studies

The domain of the root is the dark, therefor mysterious, layers of earth beneath our feet. The root-curious gardener can glimpse some root growth by viewing road cuts and peering into construction sites for basements. Fortunately, we have more ready and accurate assistance from soil scientists who have actually excavated entire tree root systems and made literal maps of their growth patterns. This obscure science of root mapping is both old and contemporary. In 1727, the French were studying the root systems of herbaceous and forest plants. Root mapping picked up in the 1920s and the 1930s. One of the most important researchers during this period was John E. Weaver, formerly Professor of Plant Ecology with the University of Nebraska, his books and papers on vegetable, native prairie and tree crop root systems are still some of the most extensive in the literature. The best fruit tree root maps come from an rare translation entitled “The Root System of Fruit Plants“ by V. A. Kolesnikov, published in Moscow, USSR in 1962. At the University of Florida’s Environmental Horticulture Department, Associate Professor Edward Gilman is one of a few researchers carrying on important root studies in the 1980s and 1990s. 

Roots Sense Water and Fertility

Roots aren’t psychic, they can’t sense where water and fertility might lie. But when they stumble across good feeding, they branch-out to colonize the best soils. Roots don’t actually expend energy to grow through a hostile and infertile soil zone to search out “greener pastures.” Roots are inherently lazy and will proliferate were the feeding is the easiest. Roots are mostly opportunistic. Roots randomly in many directions. During times of adequate moisture they may grow far-and-wide. Roots don’t “smell” water and fertility, but when they stumble across fertility and proper moisture levels, they proliferate by branching and making more feeding root hairs. This is like a salad bar where one tends to forage on the crab salad first before going to the corn-on-the-cob or the salad greens.

Adding all the water and nutrients to a moat surrounding the tree’s trunk will tend to centralize or localize the roots. This can make the tree more dependent on your care as a root system large enough to support the tree doesn’t develop. However, Gilman warns that this is not a cut-and-dry area. He maintains that concentrating all the water and nutrients near the trunk will not constrict all the roots and there’s no reduction in the fine feeding roots—in fact, the addition of mulch and fertilizer will increase the fine feeding roots.