We all know that it is normal for deciduous tree leaves to change color and drop at this time of year. What doesn’t seem to be understood by many people is that “evergreens” drop their needles at this time as well. Not as colorfully, but they are dropped none-the-less.
Here is a photo, courtesy of the US Forest Service – showing what is considered a “normal” needle drop at this time of year on a pine.
Note that the yellowing of the needles is confined to the older material; the current season’s growth is normal and healthy appearing. Consequently, evergreens have a “needle life-span” that is roughly unique to each species. Leaf life-span is the age when leaves are shed from a plant. In conifers this includes leaves with acicular (needle-like), awl-like, and scale-like forms. Two extreme examples of leaf life-span differences are larches and bristle cone pine. Larches are deciduous conifers that retain needles for 5 to 6 months (a growing season) and shed them annually. In contrast, needles exceeding 40-years old have been documented on bristlecone pine. Typically, bristle cone pine trees retain needles for approximately 15 years, rather than the maximum 40 year plus extreme. Other examples between these extremes include spruce (5-7 years), Douglas-fi r (4-8 years), and white pine (2-3 years). Leaf lifespan varies between conifer genera (i.e., spruces, pines, firs, larches, etc.) and species within a genera.
Needle drop will vary from year to year with the same evergreen: extremely dry weather will cause the drop to occur a little ahead of schedule and in a grand fashion (all at once, or so it appears) while in normal growing seasons, the needle drop may be very little or held off until the following spring.
I encourage homeowners to make a ‘sport’ of this; start with this year recording the date that needle drop is observed and whether or not it is heavy, normal, or minimal, and see how this data varies from year to year.
– Ron Smith, NDSU Horticulturist and Turfgrass Specialist, ISA Certified Arborist