Tree roots: a little understanding of them goes a long way

Dr. Thomas O. Perry of the North Carolina School of Forest Resources likes to tell the tale of the insurance company that wanted to use a tree as its logo. The tree, securely anchored by its deep, probing roots, would symbolize the support and security clients would get by buying the company's insurance.

That logo and the accompanying advertising program never got off the ground. The artist, instructed to be as realistic as possible, quickly realized that to convey a tree, including its root system, with even remote accuracy was impossible in the space available.

''The problems of scale,'' says Dr. Perry, ''were overwhelming.''

The insurance company and the commissioned artist are not alone. Most people know very little about tree roots, and what they don't know is likely to hurt them - the trees, that is. In particular, people have little appreciation of how widespread the root system of a tree is or where the bulk of the feeder roots are likely to be.

With just a little basic understanding of tree roots, Dr. Perry feels, homeowners would readily avoid damaging trees, often very expensive ones, at the rate they do each year. In particular he emphasizes these two facts:

* Typically, tree roots extend well beyond the leaf area. A healthy, open- grown, 40-year-old oak, for example, might have a leaf area extending 70 feet across and feeder roots that extend 30 feet beyond that.

* A significant portion of the roots of all trees is concentrated in the topmost surface of the soil, extending only a few millimeters deep. Under natural conditions feeder roots also grow above the soil in the litter of their own fallen leaves.

Knowing these facts, then, we know better how to take care of our prize trees , or, better still, how to turn that struggling specimen into the superb tree we thought it would be when we bought it from the nursery.

Feeding. Because of the surface-spreading nature of the feeder roots, trees respond readily to fertilizer that is broadcast on the surface of the soil.

Surface applications make nutrients ''immediately available to the tree roots ,'' Dr. Perry says. ''It does not have to move down into the soil,'' he adds. ''Even the reportedly immobile phosphates are immediately available to the tree.''

As a result, ''tree spikes'' (blocks of fertilizer driven into the ground around trees) are an unnecessary expense, according to Dr. Perry, who points out that ''foresters routinely broadcast fertilizer on millions of acres of land and achieve rapid and large increases in growth rates.''

Herbicides. When chemicals are used to destroy weeds, extreme care should be taken. Remember, says Dr. Perry, ''a tree is really a broad-leafed weed'' when it grows in your lawn.

Some herbicides are formulated so that they are rendered relatively harmless when they hit the soil. But because so many tree roots grow on the surface of the soil, these poisons, in effect, strike the tree roots before they get to the soil. This way leaf curl and crown dieback can occur in dogwoods and other trees even when no herbicide strikes the leaves or the trunk of the tree directly.

Soil compaction. The ''largest single killer of trees,'' says Dr. Perry, is soil compaction. When flattened and hardened like pavement, the soil surface is devoid of pore space that holds both oxygen and water. Denied air and moisture in the very surface area that is so important to the majority of feeder roots, trees are easily set back and may eventually die.

Constant foot traffic will compact the soil so that it sheds rain like a roof. Even hundreds of tiny pigeon feet, gathering every day to eat the corn scattered by well-meaning bird lovers, have been known to compact the soil beyond a tree's ability to endure.

Mulches. Organic mulches, up to 6 inches deep, are extremely beneficial to tree roots, which will grow into and feed directly on the decaying mulch. But Dr. Perry warns that excessively thick mulches ''can induce fermentation, immobilize nutrients, and cut off the supply of oxygen.'' This, in turn, can kill trees or stunt them severely.

Plastic mulches (under an organic mulch or decorative pebbles) are also bad for trees unless the plastic is ''perforated to allow gas exchange and the underlying soil is well drained and porous.''

Flowers and lawns. Flower beds or lawns under trees are a fine way of landscaping. But biological compromises must be made to achieve proper balance between trees and plants.

The inescapable fact is that tree and flower roots compete for nutrients, moisture, and space. When the soil under a tree is dug up to begin a garden, ''it involves tearing out tree feeder roots, which will produce a corresponding death of twigs and branches in the crown of the tree,'' Dr. Perry concludes.