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Over the Garden Fence

Where gardeners come to find out what's happening out in the yard.

Yellowing Evergreen Needles

Posted by Richard Hentschel -

Down the Garden Path

Richard Hentschel, Extension Educator

It has not been a kind couple of years for many of our needle evergreens in the landscape. The 2012 season of high temperatures and lack of rainfall really set up the needle evergreens for needle casts and blights, all causing needle loss.

One of the typical evergreen cycles is that of annual needle loss as a result of growing a new set of needles each year and one that should not concern us. Under normal growing conditions, evergreens shed their needles on a regular schedule, losing the oldest that naturally creates that bed of needles at the base of the tree. Needles are kept two to three years before sloughing off as new ones grow at the tips of each branch. Arborvitae and white pine drop needles when those needles reach two years old for example.

While this can occur any time from mid-summer on, Master Gardeners are getting a lot of calls, especially on arborvitae right now. An evergreen tree losing interior needles is normal and expected each year. What can make that more obvious in some years are the annual rates of growth. If the evergreen trees grew well in 2011 for example, the needle loss may be as much as 50% of the tree needles if the growth in 2012 and 2013 was far less due to the stresses of 2012 and now still recovering in 2013. One indicator that this is normal is that the needle loss is uniform throughout the evergreen tree rather that impacting a singular branch or branches within the canopy. A second visual clue is looking at the buds for next year. If you see viable buds and the ends of the branches are flexible, all is in place of new needles again in 2014.

There are other conditions that favor needle loss such as being planted too close together so the needles end up being shaded out. This is common in an evergreen windbreak or in the landscape where there was only really room for one tree and more than that were planted to quickly fill in the area. This kind of needle loss is really caused by a lack of sunlight to sustain the needles rather than a needle disease.

While most often evergreen trees recover from environmental stresses, there are conditions where it makes it harder. If the evergreen trees are not widely adapted to our climate and soils, they will struggle to grow well. If the site where they are planted is not providing the conditions for favorable growth, they will not grow well. Spruce and pine are susceptible to attack by disease organisms during stressful growing seasons. For spruce the main disease can be Cytospora canker and next in line are the needle casts and blights. If a treatment is warranted, sprays will need to be applied while the new growth in 2014 is expanding, especially for casts and blights. Each product is slightly different and following the label instructions is critical. Large evergreen trees may require the professional arborist to visit a couple of times for 2 or 3 years to get the foliar diseases back under control. The whole time treatments are being made, annual fertilization and seasonal watering will aid in the recover.

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