It’s that time of the year when a tapestry of fall foliage color begins to evolve across the rural landscape. While we may not have the brilliant foliage that exists in the hardwood forests of the New England states , we do have the vibrant reds of vine maples and golden glow of large leaf maples and Oregon ash. When showcased among the emerald green colors of our native conifers, our fall foliage display can be quite spectacular.
While most folks are comfortable with deciduous trees losing their leaves in the fall, there is concern among gardeners when conifer needles begin yellowing, browning and dropping from the tree. Although evergreens stay green all year long, it doesn’t mean that individual needles live forever. In fact, it’s normal for some of the needles on evergreens to turn yellow or brown and fall from the tree this time of year. The seasonal needle loss is a natural occurrence.
Seasonal needle loss is perhaps most conspicuous on our native Western red cedar when the older foliage turns brownish red in color. The phenomenon, commonly referred to as cedar flagging, is most frequently the result of moisture stress. Summers of prolonged, dry weather will cause more flagging than moist, overcast summers. Previous weather stresses, from cold winters, or dry springs, summers or falls, may have already damaged the tree, and could result in flagging during a summer or fall which is not particularly dry. Unless weather or cultural stress are severe, trees with cedar flagging should recover. Fall and winter winds blow most of the dead foliage out of the trees before Spring.
Interestingly, trees growing near lakes or other bodies of fresh water may also develop flagging during dry summer weather, even though they are near a source of water. Such trees have shallow root systems, dependent on high water tables in these locations. When droughts occur, the water table drops, leaving the root system without adequate moisture. Unless weather or cultural stress are severe, trees with cedar flagging should recover
Evergreen needles have varying life spans, depending on the species. Arborvitae and pine needles live for 2 years, while spruce needles live for 3 years or longer. Some species of evergreens have a more noticeable leaf drop than others. In the fall, arborvitae and white pine often drop their 2 year old needles all at once which can be quite alarming if you don’t realize it is perfectly normal.
Environmental stress such as drought and hot temperatures may cause greater than normal loss of needles. The normal pattern of seasonal needle loss is a gradual discoloration and eventual loss of inner needles from the top to the bottom of the trees. In contrast, fungal diseases often cause browning of the newest (outermost) needles, death of the entire branches, or thinning of needles on just the lower branches.
On other species, needle drop occurs gradually, with a small number of needles falling at one time. Foliage developed during the current year (at the branch tips) remains green. The older needles of yew for example, will turn yellow and drop in late spring or early summer. Broad leaved evergreens, such as rhododendrons, drop their 2-3 year old leaves in late summer and early fall.
Inner and lower needles that are hidden from light are usually the first to drop. Pruning excess growth and dead limbs can help open the plant to more light.
By Don Tapio (Master Gardener)