When a large, established tree starts looking sick and begins to decline, the cause is often hidden from view. Although we see the symptoms in the branches and leaves of the tree, the real problem is often in the root zone.

Most people think that the root zone of a tree forms a mirror image of the canopy. If the tree is 40 feet tall, a thick "tap" root must go 40 feet down into the ground. Also, it is common belief that roots only extend away from the trunk to the "drip line" formed by the overhang of the tree's canopy. Wrong on both counts!

The roots of plants have two major functions. One is to provide an anchor for the plant so that it does not fall over in the wind. This is accomplished by deep roots and may be the primary reason that some plants form a tap root. The other reason that plants need roots is to absorb water and nutrients. Tiny root hairs do most of this job.

If you think of roots in terms of these two functions, it explains why the vast majority of a tree’s roots are actually located in the top 12 to 24 inches of soil. That is where the water and nutrients are to be found.

A few thick roots may run deep into the ground for anchorage but these are a minor part of the overall root system mass. Not all trees have a true tap root. Some plants such as the nut trees, sassafras and some oaks do and this makes them more difficult to transplant successfully.

Also, the horizontal range of roots varies widely from species to species. Some trees send their roots out to a distance roughly twice their height. A 100 foot tall tree may have roots out to over 200 feet away from the trunk. Other trees may only reach out to about half to two thirds their height. A 60 foot tall spruce may send roots to only 30 or 40 feet away from the trunk.

Understanding the range and form of a tree's root system can be very helpful in maintaining tree health. The root system of a tree must be in "balance" with its canopy. A certain number of roots are needed to provide water for a certain number of branches and leaves. Damage to the root system will result in a corresponding loss of leaves, branches or, in severe cases, the entire tree may die.

Roots may be damaged in many ways. They may be physically damaged during construction such as digging for sewer lines or they may be crushed when heavy equipment runs over them. Roots may be damaged by chemicals such water softener backwash, swimming pool chlorine or certain herbicides used on the ground above the root zone. Misused fertilizers may also cause damage.

Roots may also be damaged in less dramatic ways. To be healthy, roots need free oxygen in order to grow and to be able to absorb water from the soil. Thirty years of foot traffic and rain pounding on the soil around a tree’s roots may cause soil compaction which squeezes all the oxygen out. Planting a tree too close to houses, sidewalks, driveways or other obstructions will limit the expansion of the root zone needed to support a mature size tree.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


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