Originally Published In The Deseret News -- Wednesday, November 16, 1994
Foliage Remains, But Older Needles Change Color And Drop Off In Varying Cycles.
Forlorn gardens are now in a state of transition, changing from beautiful fall colors but not yet assuming their winter character. While contemplating your garden, check your trees for potential problems.
Evergreens keep their foliage throughout the winter. Contrary to popular belief, the foliage does not last indefinitely. Older needles turn color and drop off in varying cycles, ranging from one to 17 years, depending on the species of trees. This needle drop is genetic and part of natural growth. Needle drop usually goes unnoticed because it occurs gradually and is concealed by newer foliage. Older needles near the center of the tree are not everlasting, but they are shed with maturity. It is natural and nothing to worry about.
Determine the age of the needles by counting the annual growth rings. These are dark rings that go around the shoots on the bark. Annual growth rings are formed as the terminal bud is set each year.
Some evergreens, such as larches, shed their needles every year. They are deciduous needle-type conifers and drop all needles each fall. This is alarming to someone who has never owned a larch tree.
White pines shed last year's needles. Each fall, these needles turn brown and start dropping. They make a dramatic contrast to the healthy needles that have grown this year. Arborvitaes also shed last year's foliage.
Austrian and Scotch pines are the most common landscape trees that show sudden needle drop. They retain their needles for three years and then drop them. Spruce trees and fir trees retain their needles for several years. Their needle drop may not be visible except on inner branches. They often drop their needles in the spring - not the fall. Large spruce or fir trees are almost devoid of needles near the trunk.
There are other causes of needle drop in evergreens. Many are serious and need attention. Spider mites cause needles to turn off-color - generally a stippled or gray-green color. Small webbing is often associated with mite infestations. Mites are not feeding now, as they are warm-weather pests, but severe damage causes needle drop and makes trees less resistant to other maladies.
Evergreens are also subject to drought. Insufficient water causes needles to drop earlier and more severely than normal. Trees in wet or poorly drained soils show the same symptoms. Evergreens do best when watered deeply and regularly. Inadequate water in the fall causes needle drop, as does winter drought from low humidity and low soil moisture.
Improperly planted evergreens show considerable leaf drop. Broken root balls or strings or other materials left around the roots may cause premature needle drop. Construction near established trees also destroys the roots, causing needle drop.
Trees need adequate nitrogen and iron in the spring to maintain a healthy green color on their needles. Herbicide damage can also cause major problems for evergreen trees. As the roots pick up herbicide, yellow needles and non-seasonal needle drop may occur.
Do not confuse natural seasonal drop of evergreens with insect or disease problems. Normal needle drop occurs seasonally only on older needles near the interior of the tree. If there are questions about a diagnosis, examine the needles carefully. Needles that yellow and drop normally from age may have small spots or blemishes or brown, mottled discolorations. Spots or blemishes on current season needles, may be caused by insects or diseases.
Contrary to reports that needle diseases are damaging evergreens, no evergreens in the lower mountain valleys have needle blight. Needle cast fungi and other pathogens only attack needles in higher mountain valleys in Utah.
As always, the best control for any problem is good preventive care. Older needles drop normally, but symptoms on the newest needles generally indicate something is drastically wrong with the trees. The best control in these situations is proper irrigation, proper fertilization and a good, deep mulch of needles. This helps maintain soil moisture and protects the shallow roots from excessive heat.
Don't be confused by every brown needle on evergreens. Remember the needles fall off naturally in cycles ranging from one to 17 years. Concern is justified only when new needles grow abnormally.
Evergreens are the backbone of the summer and winter landscape. Their beauty, protection and wildlife habitat make them valuable. They will survive natural needle drop and many other maladies and grace the garden for many years.
© 1998 Deseret News Publishing Co.