Autumn Needle Drop of Evergreens
Lately, many people have noticed yellowing interior growth of their evergreen trees. We have been fielding lots of calls regarding this phenomenon. In early fall, many homeowners observe a discoloration of the leaves or needles on their evergreens and fear that some insect or disease has affected the plants. Do not be alarmed, this is a natural condition. The drought conditions of late summer have made this normal seasonal needle drop appear more pronounced than normal.
Contrary to popular belief, evergreen foliage does not remain attached indefinitely. Older, inner needles discolor and drop-off after one to several years depending on the species involved. Evergreen shrubs and trees remain green throughout the year because they do not lose all of their foliage at one time. Usually needle drop goes unnoticed because new needles conceal the old, inside needles and foliage that have turned yellow and brown. Sometimes the drop occurs slowly, but on other occasions, many needles discolor and drop simultaneously. Most evergreens drop their needles in the fall.
White pines are the most dramatically affected. This species commonly bears three years' needles in the summer and two in the winter. In October or November of some years, this species may have only one year of needles still attached. Matured white pine needles turn yellow throughout the tree. The tree will appear unhealthy when the yellowed needles outnumber green ones of the current season.
Austrian and Scotch pine usually retain their needles for three years. Spruce and fir trees retain their needles for several years. Needle drop may not be visible unless one looks for it on the inner branches. Few needles turn yellow and drop in late spring or early summer of their third year
Other causes of yellowing needles.
Seasonal needle drop can be confused with needle loss due to spider mite damage. When mites are involved the needles are off color, generally stippled, and gradually turn an off- yellow color. A light webbing is associated with heavy mite infestations. Spruces are particularly susceptible to spider mite injury. If mites are suspected, hold a white sheet of paper under a branch and sharply tap the branch. Look closely for small moving mites on the white paper.
If evergreens are not provided sufficient water during the dry part of the summer, leaf or needle drop may be earlier and more severe than normal.
A lack of nutrition often means short yearly growth and premature leaf or needle drop. Applications of fertilizer are very helpful in maintaining healthy and vigorously growing shrubs and trees. Iron deficiency is common in soils with high pH and more serious with some evergreen species.
Some forms of herbicides applied to the lawn or in the vicinity of evergreens may cause sufficient injury to result in needle yellowing and non-seasonal drop. This damage usually manifests itself at the very tip of the branch, often with a curling and distortion of the very end of the branch.
Wet or poorly drained soils--
Evergreens planted in wet these soils will not be vigorous and will often show an abnormal amount of premature needle drop.
Don't confuse natural seasonal drop of evergreens with various insect disease problems that can reduce the vitality and aesthetic value of shrubs and trees. Normal needle drop is a seasonal occurrence, and the symptoms are distributed generally throughout the interior portion of the plant. If you have doubts about accurate diagnosis, examine the leaves and needles carefully. Needles that yellow and drop normally from age may have occasional spots and blemishes. Old needles sometimes show mottled brown coloration from invasion by nondisease-causing fungi. On the other hand, spots or blemishes on the current season's leaves or needles may be caused by insects or disease. An accurate diagnosis of the condition will determine whether chemical sprays are necessary to arrest the problem.