Tree root myth-conceptions

Thee are several misconceptions about tree roots that seem to be firmly rooted in the public consciousness. I have heard people refer to them many times over the years. Here are a few:

1. Tree roots spread only as far as the branches (to the drip-line).

2. The root system reflects the shape and volume of the trunk and branches.

3. Trees live on, and are supported primarily by a taproot.

4. If you don't water deeply, roots will rise to the surface.

5. Roots seek fertile soil and water.

Let's look at each of these myths and try to shed some light on the subject of tree roots.

1. The drip-line idea seems to be the result of a rule of thumb that has become enshrined in tree ordinances across the country. For example the City of Napa Tree Protection Ordinance defines it as "the area measured from the trunk of the tree outward to a point at the perimeter of the outermost branch structure of the tree." The drip-line is commonly used as a convenient and easily observed location where protection fencing is installed during construction projects around protected trees.

The term "drip-line" refers to the edge of the canopy where rainwater drips onto the soil. Many people have gotten the idea that roots somehow get a message to grow that far from the trunk and no further.

But what about lop-sided trees, such as those growing on the edge of a grove, with no branches on one side? Have they no roots on one side? Shouldn't they be protected to some extent on that side? That's why I prefer to think of a tree root protection zone in terms of a distance from the trunk proportional to the diameter of the trunk. For example, with one foot of protection zone radius per inch of trunk diameter, a 24 inch diameter tree trunk gets a 24 foot radius of protection. Using this rule, a one-sided tree gets a more realistic protection zone on the side where there are no branches.

(Interestingly, having measured these proportions many times, comparing the drip-line to the one-foot-per-inch-of-trunk-diameter rule, I've found that in many cases, they match!)

But the point is this: tree roots spread far beyond the drip line if they can. Professor Edward Gilman, a leader in modern tree research at Florida State University has found through excavation studies that roots grow, on average, about three times the spread of the canopy. In other words, he says, more than 50 percent of the roots may be outside of the drip-line. Others have found that roots spread to a distance equal to three times the height of the tree.

2. That the root system reflects the shape and volume of the trunk and branches seems to be a pleasant idea of symmetry and balance in nature, with no basis in fact. In one sense, yes, the capacity of the root system to absorb water and minerals must be in balance with the whole tree. But observations have shown that tree root systems are wide and shallow. A more accurate model of the tree is a wine glass with a very, very broad base. Studies have shown that, although trees often have some deep roots, depending on species and soil, the great majority of roots tend to be near the surface of the soil.

3. The taproot idea applies to very young trees more than it does to mature trees. If you've ever tried to pull up an unwanted oak seedling, you've probably experienced the amazing strength and depth of the taproot. The radicle, the initial root that sprouts from an acorn, will grow as far as soil conditions allow. A 2-inch tall oak seedling may have a 6-foot long taproot. It's an adaptation that results in the survival of oak seedlings.

Studies and observations have shown that tree taproots tend to become, as Gilman says "less prominent as the tree ages and develops an extensive lateral root system."

Trees are sometimes classified as taproot trees (hickory, butternut, walnut, pecan), heart root trees (red oak, honey locust, sycamore, pine) or flat root (birch, fir spruce, silver maple). This can be especially useful in growing and transplanting young trees. I recall having particular difficulty getting pecan trees to grow in the nursery, because the dramatically deep roots wouldn't fit into a standard nursery container. Root pruning usually resulted in the trees' failure to thrive. For mature trees, the taproot is less important, or completely gone. Tree culture and protection must allow for a wide-spreading root structure.

4 and 5 are related. The idea that woody roots can relocate themselves, or deliberately go in search of water and minerals does not fit the observed facts. Woody roots expand in diameter as they grow, but they don't relocate themselves, unless the soil anchorage fails and the roots are pulled up by motion in the trunk, induced by weight and wind.

Roots elongate at the tips and respond to favorable growing conditions. They often grow upward in the soil profile, but no studies have shown, as far as I know, that roots have some kind of remote sensing ability that enables them to grow toward a source of water or minerals.

There is much we don't yet know about tree roots, and how they grow. For now, though roots may seem to have a mind of their own, tree roots are simply opportunistic. They grow where they can.

Bill Pramuk is an ASCA registered consulting arborist. Please send questions to bpramuk@pacbell.net.