Why are the pine trees dropping needles?

      Fall Needle Drop - Why Evergreen Needles Turn Yellow in the Fall
 
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. While evergreens are never totally without needles, older needles are shed regularly, as newer needles fill in.
 
We are used to seeing the leaves of deciduous trees change color and drop in the fall.
 
But when you see your evergreens start to turn yellow, it can cause you concern. There are diseases and pests that can harm evergreen needles, but 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?
 
Fall needle drop refers to the tendency of evergreens to shed some of their older, inner needles at the end of the summer. It 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 not only is it normal, it is also healthy. Older needles may also turn red or brown and go unnoticed before dropping.
 
Do All Evergreens Experience Needle Drop?
 
Different types of evergreens will drop their needles at different rates. For instance, most pine trees 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 pine 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.
 
However, evergreens are continuously shedding needles as new needles fill in, much like the hair on your head. You will know it is fall needle drop if it is occurring 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. Don't panic if you have one of these trees and you start seeing large amounts of needles dropping in fall. They will leaf out again in the spring.
Taken from https://www.thespruce.com/fall-needle-drop-
Till next time this is Becky Litterer, Becky's Greenhouse, Dougherty  Iowa