Article by Sam Waggener, City of Lone Tree Forester
What is Needle Drop?
No, we’re not talking old vinyl records here. We’re talking about a process all plants go through called “programmed cell death,” in which plants respond to environmental factors like diseases, nutrition, time of year, moisture, herbicide application, root damage, insects, sunlight, and so on.
If you have an evergreen tree, it can be somewhat alarming to see the needles start to fall off starting in the summer months and leading into fall. Yes, evergreen trees and shrubs do have green needles all year round, but this doesn’t mean that the needles live forever. Just as deciduous trees lose their leaves, so do conifers. Some species even do this every year! Some deciduous conifers include Bald Cypress, Dawn Redwood, and American Larch, all of which can grow in zone 5, the hardiness zone for most of the front range. Conifer needles have varying life spans and can last anywhere from 2-8 years. Species that shed their needles every two years are White Pines and Scots Pine. Species that shed needles every three years are Lacebark Pine and Ponderosa Pine. Austrian Pines take four years to shed needles and Mugo Pines take five years to shed.
Noticing Needle Drop
Sometimes this development goes unnoticed, this is because new growth has filled in the voids and the older needles have yellowed, browned, and fallen to the base. Trees and shrubs naturally shed foliage, and this is intended to give the plant a free mulching. This mulch creates a better environment for the plant, increasing the number of microorganisms, aids in temperature regulation for the roots, increasing moisture retention and soil aeration, not to mention is free fertilizer.
There are also species that we more often forget, and these are the broadleaf evergreen shrubs, some examples are Boxwoods, Manzanita, Yew, and Holly. These broadleaf shrubs drop leaves in very small increments and are often unnoticed as well. Don’t be alarmed this is the tree going through the regular progression of programed cell death.
Older shrubs and trees drop foliage that is closer to the trunk and base of the plant, this is mainly due to the lack of sunlight the leaves or needles are receiving. Which is why when pruning evergreens, it is important to prune properly. If pruning, only the outer portion of the branches this can stunt growth, lead to diseases and insects, not to mention will disrupt wind-loading during storm events and will eventually kill off that branch. This will kill off the branch because conifers only produce needles on the outer most portion of the branch. If you notice needles turning yellow in other areas of the tree or shrub besides the inner portion, then it could be something else.
Check to see if the current year of growth is discolored or wilted, if tips of branches are showing signs of browning or yellowing this might be a more serious disease or insect problem and should be diagnosed.
As always, if you have any tree or plant questions, feel free to contact me at [email protected]. Until next time, happy growing, Lone Tree!