The recent storms were welcomed by our thirsty landscapes. However, homeowners who water throughout the summer often do not leave the sprinklers on long enough to benefit nearby tree and shrub roots.

Watering decisions are sometimes complicated when landscapes contain lawns, shallow-rooted plants and shrubs as well as more deeply-rooted trees. Sprinkling systems that are scheduled for 10 to 15 minutes every few mornings may only wet the top few inches 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 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.

The following information may help homeowners make some watering decisions for next year and to clear up a few misconceptions about root systems.

How deep do roots grow?

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

Most moisture and nutrient uptake occurs in the tiny root hairs at the outer edges of a root system. These small roots are usually in the upper 12 to 18 inches of soil. Larger, anchoring roots closer to a tree 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 worker harder to reach adequate moisture. Soils with a shallow layer of clay that may cause poor drainage tend to have shallower 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 more than 100 feet from the trunk of a large cottonwood tree.

Some trees are dropping needles. Do they need more water?

Many conifers – like western red cedars, pines and fir trees – naturally shed interior needles during the late summer and fall that are no longer functional. The shedding may be more prevalent during dry summers.

If the newest or outer growth is dying, 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, an inspection by a qualified professional may help determine if it can be treated, may spread to other trees or can be ignored.

Why are some trees losing their leaves early?

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 suffer from the brown rot fungus, “shot hole” fungus and aphid attacks. Some trees may experience complete defoliation and sometimes will grow 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, the above mentioned diseases usually reoccur each year. When considering treatments, it is absolutely critical to properly identify a problem and to apply the appropriate treatment at the right time.

Professionals can be consulted for advice. Another excellent source of information is the WSU “Hortsense” web site. It describes many problems and their treatments. Master Gardener clinics and programs offer diagnostic services and control recommendations.

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 email at dlt@blarg.net. Website: evergreen-arborist.com.