Tuesday, October 30, 2007

Landscape - Needle Drop in Evergreens

Normal needle or leaf drop in evergreen species is often confused with disease. The following is an article on the subject.

A common concern by laypersons during autumn concerns the color change and shedding of evergreen needles or leaves. Quite often it is difficult to convince the plant owner that yes evergreens do shed old foliage like the deciduous trees or shrubs during autumn. Sometimes the changes are so dramatic or occur so quickly that one will mistakenly blame a pest or other malfunction for normal foliar senescence and shedding.

Each year evergreen trees and shrubs will produce new foliage and they will shed some of their old foliage. Since the foliage of an evergreen can live from one to several years, the amount of shedding will vary with the individual evergreen species. In most cases the foliage will first turn yellow, then straw colored and eventually brown, at which time it drops to the ground. When viewed from a distance it appears as though the interior sections of foliage are yellow or brown and the only green foliage remaining is closer to the terminal end of the branch.

Pine (Pinus) trees produce needles in sets of 2 to 5 per fascicle (bunch). All of the needles contained in an individual fascicle will turn color and be shed off at roughly the same time. Pines shed their oldest needles in autumn. Most pines retain needles for 3 to 5 years. An exception is the white pine (Pinus strobus), which retains its needles for only 1 year. Hemlock (Tsuga) and yew (Taxus) produce individual leaves (needles) which are attached to a twig. These leaves will drop individually sometimes over an extended period of time. Hemlock and yew can retain needles for 3 to 5 years. Spruce (Picea) and fir (Abies) also produce single needles on twigs. Many of these species will retain needles for up to 5 or 6 years. On spruce and fir the shedding of foliage is not always restricted to the oldest needles but it does concentrate in that area. Arborvitae which bears its foliage as scale-like leaves that cover a tiny twig retains its leaves for only 1 year.

What about the broadleaf evergreens such as rhododendron and azalea (Rhododendron), holly (Ilex) and mountain laurel (Kalmia)? These plants produce leaves individually on their stems. Holly and laurel will retain their leaves for only one year. The Rhododendrons and azaleas will sometimes retain their leaves for 1 or 2 years. Holly, unlike most other evergreens shed their one-year-old leaves in the spring as soon as the new leaves start growing. Many species of rhododendron shed leaves in autumn, but it is not unusual to find some species shedding leaves at other times during the season. Leaf shedding on these plants is often influenced by environmental factors such as drought stress and severe winter weather.

Often leaf or needle shedding goes unnoticed in some seasons. This can occur when new leaves or needles conceal old foliage which is shedding on interior sections of the plant. But in some years it may be very noticeable, especially on white pine and arborvitae and this is often due to varying rates of growth from season to season. The length of stem and needle growth may be reduced during years when drought conditions prevail in comparison to a season where maximum stem and needle length occur. When this scenario occurs it is not uncommon to find that more than 50% of the needles/foliage will be shed in autumn on white pine and arborvitae. If the same length of growth occurs each year than 50% of the needles/foliage would shed in autumn. Some other factors which can affect the length stems, needles, and leaves in any one season are recent transplanting, root damage from construction or trenching, soil compaction, and poor soil drainage as well as disease and insect pests.

Old yellow and brown foliage/needles that shed and appear on interior sections of evergreens during late summer and autumn seldom indicate a serious problem. Occasionally, pests are involved. But more often than not we will diagnose this yellowing to be a normal phenomenon. Remember that normal leaf and needle drop occurs every year on every evergreen.

Article by Thomas Kowalsick, Extension Educator, Cornell Cooperative Extension - Suffolk County, NY. For the full publication with pictures go to http://counties.cce.cornell.edu/suffolk/HortFactSheets/factsheets/Normal%20Needle%20and%20Foliage%20Shedding%20on%20Evergreens.pdf

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