Despite being called evergreen, the needles on evergreen trees don't stay green forever. The label "evergreen" refers to the trees' habit of not dropping their leaves, or needles, before winter the way deciduous trees do. But while evergreens are never totally without needles, they do regularly shed older needles as newer needles fill in. Some diseases and pests can harm evergreen needles, causing them to yellow and drop. But if it's the older inside needles that are yellowing and dropping, it's probably the normal fall needle drop, sometimes referred to as seasonal needle drop.
The Fall Needle Drop
Fall needle drop refers to the tendency of evergreens to shed some of their older inner needles usually at the end of the summer and into fall. It is triggered by weather and other factors of the growing season, much like plant dormancy when plants go into a sort of hibernation to wait out the cold winter months. Sometimes the needle drop occurs slowly over several months, making it barely noticeable. You also might not be aware of the shedding because the new needles fill in quickly.
The needle drop is most noticeable when several trees start to lose their needles at the same time, which is not uncommon as it is a seasonal process. It can be a startling sight, but it's a normal one for most evergreens. The innermost needles will turn yellow while outer needles stay bright green. The yellow needles eventually drop and carpet the ground around the tree. Older needles also might turn red or brown before dropping.
Evergreen Trees That Experience Needle Drop
Different types of evergreens will drop their needles at different rates. For instance, most pine trees shed their needles every two to five years while spruce trees hang on to them for five to seven years.
Eastern white pines can show their shedding dramatically. They tend to carry three years' worth of needle growth during the growing season and drop the oldest year's (or two years') needles just before winter. This can leave you with a sparse-looking tree with yellow needles throughout. And it can take another season before the tree starts looking lush and green again.
Other pines, such as the Austrian pine and Scotch pine, hang on to their needles for at least three years. This means there will be enough green needles on the trees to virtually hide the loss of the yellow needles.
Furthermore, it's important to note that not all cone-bearing trees and shrubs are evergreen. Some, such as the bald cypress, dawn redwood, larch, and tamarack, have needles that change color in the fall and then drop from the branches. They are deciduous conifers and behave just like leafy deciduous trees, such as maples and oaks. So don't panic if you have one of these trees and start seeing large amounts of needles dropping in the fall. They will leaf out again in the spring.
When Yellowing Needles Are a Sign of Trouble
Yellow needles early in the growing season and the yellowing of newer growth are different stories. If that should happen, look for other causes besides normal needle drop that could be causing the desiccation, such as drought or insect infestation.
If you see signs of yellowing in isolated portions of a tree, or if yellowing starts in an isolated spot and slowly spreads, gather some needles and a couple of small branches to take to your local cooperative extension or a good nursery. Have an expert look for signs of diseases or pests.