By Adam Austing and Katie Drewitz, University of Minnesota Extension
Despite their name, evergreen needles do not stay green forever. Older, inner needles discolor and drop off after one or more years, depending on the species. Sometimes the drop occurs slowly. Other times, large numbers of needles yellow simultaneously in late summer or early fall, making for a striking spectacle. Due to the condition being triggered by the weather and the season, many evergreens are likely to show symptoms at the same time. This phenomenon could easily be mistaken for heavy disease or insect pressure. Each species of evergreen tends to keep its needles for a defined length of time. Thus, cycles or patterns of needle drop develop for each species, although variation exists from tree to tree and from year to year.
White pines are the most dramatically affected trees. This species typically bears three years’ needles in summer and two years’ needles in winter. However, vigorous shoots may have only one year’s needles still attached by November of any given year. These needles may not extend the length of the branch. Third-year white pine needles turn yellow throughout the tree. The tree will appear particularly unhealthy if the yellowed needles outnumber the green ones of the current season. This is natural.
Austrian and Scots pine usually retain needles for three years. Red pine ordinarily drops its needles in the fourth year. Thus, three or four years’ green growth will outnumber yellowed needles, even at peak periods, and none of these species will appear as unhealthy as the white pine periodically does. Arborvitae (white cedar) needles usually turn brown rather than yellow when they age, and often remain attached much longer than matured pine needles. Yew needles turn yellow and drop in late spring or early summer of their third year. Spruce and fir needles also yellow and drop with age, but since these trees retain their needles for several years, needle drop is often not visible unless you search for it on inner branches.
The yellowing and drop of evergreen needles described above occurs naturally. It is important that you do not confuse this natural seasonal needle drop with various disease and insect problems that can seriously reduce the vitality and esthetic value of the tree. The facts that it is a seasonal occurrence and that the symptoms are distributed throughout the interior portion of the tree are helpful clues to proper diagnosis. Seasonal needle drop control is not possible, nor would it be beneficial.
If you have questions about this or any other horticulture-related topic, please reach out to your local Extension Educator. Residents in Wright County can email firstname.lastname@example.org or call (320) 249-5929.