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Why are my evergreens dropping their needles?
Contrary to popular belief, evergreens do not keep their needles forever. Older, inner needles discard and drop off after one or more years depending on the evergreen. Sometimes the drop occurs slowly. On other occasions, many needles may turn yellow all at once in late summer or fall. Because weather triggers the condition, many evergreens are likely to show symptoms in the fall. If you are not familiar with this natural process, it could cause you a great deal of concern.
Each species of evergreen usually keeps its needles for an definite length of time. White pines provide a good example. They usually keep three years of needles in summer and two in winter. The three-year-old white pine needles turn yellow throughout the tree in fall. The tree will appear particularly unhealthy when these yellow needles outnumber the green ones. Austrian and scotch pine also keep their needles for three years and Red pine and Norway pine keep theirs for four years. Needles on arborvitae also called "white cedar" usually turn brown rather than yellow when they age. They often remain attached much longer than mature pine needles. Japanese yew needles turn yellow and drop in late spring or early summer of their third year.
When your evergreens yellow, look closely. If the yellowed needles are old needles, the yellowing is most likely natural. If it is new or young needles that are yellowing and dropping, you may have a problem. If this is the case, take a sample to a professional for a diagnosis.