If it is the older, inside needles of your evergreen trees and shrubs that are yellowing and dropping, it is probably not a disease or an insect. It is normal fall needle drop, sometimes referred to as seasonal needle drop.
What is Fall Needle Drop?
Despite being called evergreens, the needles on evergreen trees do not stay green forever. The label "evergreen" refers to their habit of not dropping their leaves, or needles, before winter, the way deciduous trees do. However older needles are shed regularly, as newer needles fill in.
Fall needle drop is triggered by weather and other factors of the growing season, much like dormancy. So it doesn't happen like clockwork, but it is fairly regular.
Sometimes needle drop can also occur slowly, over several months, making it barely noticeable.
You may not even be aware of this regular shedding of needles, because the new needles fill in quickly.
Fall needle drop is most noticeable when several trees start to lose their needles at the same time, which is not uncommon since it is a seasonal process. It can be a startling sight, but it's a normal one for most evergreens. The inner most needles will turn yellow while outer needles stay bright green. The yellow needles eventually drop and carpet the ground around the tree. It may look alarming, but it is not just normal, it is also healthy. Older needles may also turn red or brown and go unnoticed before dropping.
Different types of evergreens will drop their needles at different rates. For instance, most pines will shed every 2--5 years, while spruce trees hang on to them for 5--7.
Eastern white pines can show their shedding dramatically. They tend to carry 3 years worth of needle growth during the growing season and drop the oldest year's needles just before winter; sometimes the 2 oldest years' needles. This can leave you with a sparse looking tree, with yellow needles throughout. It can take another season before the tree starts looking lush and green again.
Other pines, like the Austrian and Scotch pines, hang onto their needles for at least 3 years. This means there will be enough green needles on the trees to virtually hide the loss of the yellow needles.
When are Yellowing Needles a Sign of Trouble?
Yellow needles early in the season and the yellowing of newer growth are different stories. If that should happen, look for other causes, such as drought, insects, such as spider mites, or other symptoms on the needles, bark, and roots that could be causing desiccation.
If you see signs of yellowing in isolated portions of a tree, or if it starts in an isolated spot and starts to spread slowly, you should gather some needles and a couple of small branches and take them to your local cooperative extension or a good nursery and have them take a look for signs of disease or pests. Older healthy needles will have a few bumps and spots on them, but fall needle drop generally occurs throughout the tree.
Not All Conifers are Evergreen
It's true, not all cone bearing trees and shrubs are evergreen. Some, like the bald cypress, dawn redwood, larch, and tamarack, have needles that change color in fall and then drop from the branches. They are deciduous conifers, and behave just like leafy deciduous trees, such as maples and oaks. taken from "About Home". Hope you don't have this problem but if you do now you know something about it. Till next time, this is Becky Litterer from Becky's Greenhouse, Dougherty Iowa