Evergreens are trees and shrubs typically prized for year-round, colored foliage. It may be alarming if an evergreen tree's needles drop, turn brown or yellow before dropping or a tree begins to look bare over time. Several factors could be responsible for this needle drop, ranging from natural seasonal conditions to chemical damage, environmental stresses or others.
Natural Seasonal Needle Drop
Evergreen trees retain year-round coloration and foliage because needles typically last for several years while the tree undergoes a gradual, continuous needle drop. Most species see a spike in the amount of needles dropped during a certain season; the majority of evergreens have this peak in fall. Most of the needle loss occurs on the older, innermost sections of branches; white pines typically have the most dramatic seasonal yellowing and drop.
Needle drop may be caused by an infestation of spider mites. Affected needles suffer stippling and eventually turn yellowish. A silky webbing may be present. Test for mites by shaking a branch while holding a white sheet of paper underneath. The mites will appear as small moving dots on the paper.
Inadequate Water or Nutrients
Excessive needle shedding and poor growth may result from drought. Clay soils may break fine roots as the soil dries and shrinks away from roots; well-drained sand and gravel sites can exacerbate drought conditions during hot weather. Insufficient nutrients can also cause needle discoloration and drop. The most prevalent nutrient deficiencies are generally lacks of nitrogen or iron, which first cause the oldest tree needles to turn yellow.
Damage from errant herbicide applications or other chemicals can cause needle distortion. A common symptom of herbicide exposure is tip damage on new growth, needle distortion or browning. Spruce needles will turn purple and drop. Exposure to deicing salt is a common cause of injury to roadside trees. Needle yellowing or browning at the tips precede needle drop. Air pollution can cause narrow yellow bands to develop on needles, tips to turn brown or a severe reduction in growth.
Poorly draining sites with excessive water can reduce the amount of oxygen available to the tree roots. Root suffocation will eventually manifest as needle yellowing and drop and poor growth. Construction or similar activities can compact the soil and lead to poor drainage conditions. Saturated roots are also susceptible to infection by the fungi that cause root rots.
Recently transplanted trees may suffer decline or stress for several years after transplant. If the roots were not properly freed or were disturbed at planting, support wires remain on the tree for too long, the tree is being watered improperly or it was planted too deeply, needle loss can occur.
Most evergreens require full sunlight to thrive, so low light can cause a slow decline and foliage drop beginning in the center of the tree. This condition is most likely to occur if the tree was planted in a poor, shaded location or is adjacent to trees that have grown large enough to cast a shadow on the evergreen.
- Utah State University Cooperative Extension; Needle Drop of Evergreens; Sherman V. Thomson and Scott C. Ockey; January 2000
- Ohio State University Extension; Yellowing, Dieback and Death of Narrow-Leafed Evergreens; Stephen Nameth, et al; June 1996
- University of Saskatchewan Extension Division; Browning of Evergreens Can Be Controlled; Grant Wood
- Photo Credit Jupiterimages/Comstock/Getty Images
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