Many of us look forward to the changes autumn brings to our broadleaved trees and shrubs, when leaves turn brilliant shades of red and gold before cascading to the ground. But did you know that autumn is also a time for conifers to refresh their needles, shedding the senescent inner foliage in preparation for new spring growth?
Unfortunately, most conifers do not put on a beautiful color show during fall needle drop. Our one local exception is western larch, which loses all of its needles in the fall after turning a stunning yellow.
Conifers will retain needles for three to seven years, depending on the tree species. As needles age, they become less efficient at photosynthesis. They build up a waxy coating to protect them from drought and wind, which eventually starts restricting carbon dioxide flow. Inner needles and needles lower on the tree also become shaded by new growth, eventually becoming a drain on photosynthesis.
In autumn, these less efficient needles will fade and drop. Sometimes this can be an alarming sight, particularly on ponderosa pines, but it is a natural and necessary process.
A very vigorous tree will retain more needles. If a tree is healthy enough to afford the cost of supporting marginally efficient foliage, it may hang on to six or seven years of needles. But if this same tree experiences stress from drought or damage, it may drop two or three years of old needles in the autumn, which can be shocking to a homeowner. Pines which have been severely stressed will sometimes drop all but the most recent year of needles, resulting in a “lion’s tail” appearance. If the tree is able to recover, it will eventually start retaining a normal needle cohort.
Many things may cause a conifer to lose its needles. Defoliating insects such as western spruce budworm, Douglas-fir tussock moth or pine butterfly will eat the needles, and sucking insects such as aphids or scales can cause needles to turn yellow. This damage is usually seen in the summer. Needle diseases can cause needles to turn brown and fall off in the spring, before the new needles emerge. But if an otherwise healthy tree has old needles turning brown and falling off in the autumn, normal fall needle drop is most likely the cause.
It is a good idea to clean up fallen needles. They may harbor pests or damaging fungi. Good sanitation makes the spread of insects or diseases less likely.
A WSU Master Gardeners of Chelan County column appears weekly in The Wenatchee World. Connie Mehmel is one of four columnists featured.