The recent rains have been a relief to trees and shrubs following a very dry summer. While many homeowners water their landscapes throughout the summer, they often do not leave the sprinklers on long enough to benefit tree and shrub roots.

Sprinkling systems that are scheduled for 10 to 15 minutes every morning may only wet the top inch or two of soil. Less frequent, deep watering for 30 minutes or longer every five to seven days during the hot summer months will allow water to reach the thirsty roots of most trees and plants.

Checking soil moisture can be done by digging a small hole with a trowel before watering. Allow a few hours after the sprinklers have finished for water to soak in and then check again to determine how deep the moisture has penetrated.

Watering decisions are sometimes complicated when landscapes contain shallow rooted plants and shrubs as well as more deeply rooted trees. The following information may help homeowners make some watering decisions and to clear up a few misconceptions about root systems.

How deep do roots grow?

Generally tree root are quite shallow. It depends upon the soil conditions, size of plant and species.

Roots usually grow within the top 18 to 24 inches of the soil. Most moisture and nutrient uptake occurs in the tiny root hairs located at the outer edges of a root system. These small roots are usually located in the upper 12 inches of the soil. Larger anchoring roots closer to and under the trunk may grow to depths of a few feet.

Rocky, sandy and well drained soils may have deeper root systems because the trees have to work harder to reach adequate moisture. Soils with a shallow layer of clay tend to have less deep and wider spreading root systems.

Rarely do trees develop tap roots. Again it depends upon soil conditions and the species of trees.

How far do root systems spread?

Generally conifer trees that have needles develop root systems that may extend to or slightly beyond the “drip line” or the outer edge of a tree’s crown.

Deciduous trees may have root systems that extend dozens of feet beyond the edge of a tree’s crown. I once discovered an exposed root that measured over 100 feet from the trunk of a large cottonwood tree.

Some of my trees are dropping leaves or needles. Do they need more water?

Many conifers such as western red cedars, pines and fir trees naturally shed interior needles during the summer and fall because they are no longer functional. The shedding may be more prevalent during dry summers as the trees attempt to reduce transpiration to preserve moisture.

If the newest or outer growth is dying, then some other problem may be involved. Causes can range from hot or freezing weather conditions, a needle disease, insect attacks, a root disease or a combination of several factors. If a problem appears to be severely stressing a tree, it should be inspected by a qualified professional. He or she can help to determine if it can be treated, may spread to other trees or can be ignored. Oftentimes, such trees have normal growth the following year.

Why did so many deciduous trees lose their leaves in the late spring?

The cool, moist spring created ideal conditions for the spread of various leaf diseases. Many ornamental and fruit trees suffered from minor to severe problems that caused leaves to fall prematurely. Flowering plums, various cherry species and others suffered from the brown rot fungus, “shot hole” fungus and aphid attacks. In some instances, trees suffered complete defoliation. However, most now have grown a new crop of leaves that are disease free because the spore spreading season has passed.

What about next spring?

Many trees that suffered this year may have normal growth next spring. However, many diseases usually reoccur each year. Controls are available and professionals can be consulted for advice. Most pesticides are best applied in the spring when trees form blossoms or when the new growth is emerging.

When considering treatments, it is absolutely critical to properly identify a problem and to apply the appropriate chemical at the right time.

Dennis Tompkins is a Certified Arborist, Certified Hazard Tree Risk Assessor, Master Gardener and Urban Forester from the Bonney Lake-Sumner area. He provides small tree pruning, pest diagnosis, hazardous tree evaluations, tree appraisals and other services for homeowners and businesses. Contact him at 253 863-7469 or e-mail at dlt@blarg.net. or website: evergreenarborist.com.