In a recent column I discussed how root cutting and other injuries to tree roots may result in the decline of a tree. There are a number of other practices that injure roots besides cutting, resulting in weak, declining trees. The health of leaves, flowers, branches and all other aboveground parts of any plant is dependent upon root health, yet roots are often neglected because they grow underground and are not seen

Healthy roots are vital to plant survival. Water, nutrients, and oxygen are absorbed by root tips and "mycorrhizae," which are beneficial fungi growing on or in plant roots. Roots eliminate waste carbon dioxide, store food, reduce erosion, produce compounds essential to the plant and anchor the aboveground plant parts.

Quite often, damage appearing on leaves and branches occurs because roots have been cut, smothered by fill soil, overwatered, underwatered or abused in some other way. Too much water and soil compaction are probably the most common problems. Both cause roots to suffocate, and weaken roots so that they become more susceptible to soil borne diseases. Branch and foliage diseases are also more serious when roots are weak. A number of insect pests that attack trunk and limbs, such as bark beetles and borers, can cause more serious damage if roots are unhealthy.

In order to grow healthy plants, you need to provide the roots with proper soil conditions and adequate space. Before you plant, examine the surrounding soil for barriers to root growth. Be sure to find out the moisture requirements for the plants you're growing in order to avoid under or overwatering. If you're planting a new landscape, try to group plants according to their water needs in order to avoid overwatering drought-tolerant plants, and vice versa.

After the first few years of growth, roots of healthy plants grow well beyond the drip line (an imaginary line from the outer edges of the branches to the ground) of the plants. These lateral roots often extend for a distance two to three times the diameter of the drip line. Keep this in mind when you're watering plants, especially mature trees.

About 90 percent of woody plant roots grow in the top 3 feet of soil, most in the top 1 foot. Even so, it is a good practice to break up hardpans or compacted soils deeper than 3 feet before planting in order to improve drainage. In some tree species, deep growing "sinker roots" grow near the trunk and help to anchor the tree and absorb moisture from deep in the soil. Breaking up hardpan layers helps to encourage the growth of these deep roots.

I mentioned earlier that beneficial fungi called mycorrhizae grow in or on the roots of healthy tree roots. Mycorrhizae help tree roots absorb moisture and nutrients, and may also protect plants from harmful nematode and disease causing microorganisms in the soil. The relationship between trees and mycorrhizae is so important that many trees grow poorly and die when their mycorrhizae are absent. Mycorrhizal fungi are already present in most soils, and you can promote their growth and development by providing your plants with the right growing conditions. Avoid overfertilizing, overwatering or underwatering your plants. Apply mulch to moderate soil temperatures around your plants, and to help prevent compaction. Mycorrhizae are favored by the same cultural practices that favor good plant growth.