With all of the news about mountain pine beetles, it is easy to mistakenly believe that a natural fall phenomenon - needle drop on pine trees - means that pine beetles have attacked a tree. If pine needles turn brown from the interior of the tree, and the outer needles stay green, there is no cause for alarm.
Everyone knows that deciduous trees lose their leaves in the fall, but fewer people learn that evergreen trees also lose their old needles sometime in the late summer or fall. Needle age at which drop occurs varies among evergreens and within species. Eastern white pine loses two year old needles. On the other extreme, bristlecone pine sheds needles that are 14-17 years old. Colorado spruce needles last 8-10 years, Ponderosa pine needles for 3-4 years and Austrian pine for 4 years.
There may be a problem if there is yellowing or dieback on the tips of branches. Consider drought, salts, root damage, spray damage, soil compaction, conifer aphids, mountain pine beetle and other factors. Occasionally "deciduous conifers" such as baldcypress, larch and dawn redwood are found in Colorado landscapes. These conifers lose all their needles every autumn, to be replaced the following spring.
For more information, see the following Colorado State Extension fact sheet(s).
For more information, see the following Planttalk Colorado™ script(s). http://www.ext.colostate.edu/ptlk/2123.html
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