At this time of year, it is common for people to worry about their white pine and arborvitae trees. After a summer of appearing green and healthy, the inner needles of these evergreens are suddenly turning yellow or brown. Many people assume that the trees are sick or dying. In fact, though, these plants are simply losing some of their older needles in a natural--and completely healthy--part of their life cycle.
Despite being called "evergreens", conifers do not keep all their needles indefinitely. Older needled turn yellow or brown, die, and are shed regularly. For white pines and arborvitae, all of last year's needles are lost simultaneously in the autumn, causing the trees to appear more brown than usual at this time of year. Scots and Austrian pines retain their needles for three years, so their seasonal needle loss is less noticeable. Other conifers, such as firs and spruces, retain their needles even longer, so even though they lose their oldest needles, the effect is virtually unnoticeable.
The degree of needle loss seen can vary from tree to tree and year to year. Needle drop is often especially noticeable after stressful summers or falls.
Although it may appear alarming at first, seasonal needle loss is a natural part of the life cycle of the tree. However, browning of the needles at other times of the year may be a sign of disease. If you suspect disease, you may submit a sample for confirmation to the Plant Disease Clinic (323 Bessey Hall, Ames, IA 50011) or your local Extension office.
This article originally appeared in the 10/8/2004 issue.