The recent hot, dry weather is causing many lawns to turn brown earlier than usual. Watering decisions are often complicated because most landscapes contain shallow-rooted lawns and flower beds as well as more deeply-rooted trees and shrubs.

The following information may help homeowners make watering decisions and clear up a few misconceptions about root systems.

How long should sprinklers run?

Sprinkling systems that are scheduled for 10 to 15 minutes every morning may only wet the top inch or two of soil. This may be adequate for most lawns, but deeper-rooted plants may begin to suffer stress under such watering schemes.

Less frequent, deep watering for 30 minutes or longer every three to five days during the hot summer months will allow water to reach the thirsty roots of most shrubs and trees.

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.

How deep do tree roots grow?

Generally, tree root systems are quite shallow and rarely develop tap roots. Their growth depends upon the soil conditions, size of plant and species.

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

Soils that are rocky, sandy and well drained may have deeper root systems because the trees have to worker harder to reach adequate moisture. Soils with a layer of clay within a few inches or feet of the surface tend to have shallower and wider-spreading root systems. High water tables and impenetrable soil also create such root systems.

How far out do roots 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 several feet beyond a tree’s drip line.  Consequently, a root system can spread far beyond the edge of a tree’s crown.

I once discovered an exposed root following a flood that measured more than 100 feet long from the trunk of a large cottonwood tree.

Do dropping leaves or needles mean a tree is dying?

Many conifers like western red cedars, pines and fir trees naturally shed interior needles during the summer and fall. These are needles that are no longer functional. The shedding may be more prevalent during dry summers as the trees attempt to reduce transpiration in order 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.

Flowering plums, various cherry species and other leafy ornamentals often suffer from diseases like the brown rot fungus, “shothole” fungus and from aphid attacks.

These diseases usually reoccur each year and are more serious during wet, cool springs. Controls are usually available and professionals can be consulted for advice.

Master Gardener clinics are also excellent sources of information and possible diagnosis of problems.

It is critical that when considering treatments, a problem must be properly identified and the appropriate chemical applied at the right time. Many treatments are best applied in the spring when trees blossom or when the new growth is emerging.

Dennis Tompkins is an ISA certified arborist, ISA qualified tree risk assessor and Master Gardener from the Bonney Lake-Sumner area. He provides pest diagnosis, hazardous tree evaluations, tree appraisals, small-tree pruning and other services for homeowners and businesses. Contact him at 253 863-7469 or email at dlt@blarg.net. Website: evergreen-arborist.com.