Friday, July 25, 2014

It’s July! Why Is My Tree Dropping Its Leaves? by Patti O’Neal

Photo Patti O'Neal
Trees on the Front Range are under a lot of stress these days.  Right now we are seeing two things:  leaves turning yellow and dropping or leaves just dropping.  This is a common reaction of trees at this time of year, especially given the spring weather we had. 
This spring, we had cool, damp weather which encouraged trees to put on a great deal of leafage.  They have been green and full and lovely until now.  Now the hot, dry, low moisture conditions have persisted for several weeks, accompanied by hot, dry winds.  The trees cannot sustain the abundant growth they put on earlier in the year and are basically cutting their losses and letting go of growth they are now unable to sustain.   This is a natural response of trees to low moisture situations.  This response is often preceded by the yellowing of the leaves, another response to the lack of moisture or just the inability to take up enough water from the root system to match the transpiration of moisture from the top of the tree from the hot, dry conditions.   This can also be caused by rapid temperature fluctuations which we have experienced lately as well.

Photo Patti O'Neal
Be sure and check the areas around the tree to make sure your irrigation heads are putting out the amount of water they should and are hitting the areas they are meant to to be sure your tree is being adequately watered.  
As long as there are no symptoms of insect infestation or disease and the tree is being supplied with adequate water, this condition is not serious.  Watch your tree carefully for the next few weeks as water stress will make a tree more susceptible to many pests.  
Having said all this be cautious that your issue is not being caused by overwatering your trees.  Similar symptoms can result from this as well.  Overwatering is a common problem in Colorado, particularly in irrigated turfgrass areas.  This can be very difficult to diagnose as trees do not show symptoms of damaged roots from water logged soils until sometime after the overwatering occurs. 
The most common symptom of overwatering in broad leaved trees is leaf drop with some interveinal discoloration of the leaves.  The symptoms of leaf discoloration due to excess moisture in the soil usually begin in the bottom leaves and inside the tree and work up, rather than from the top down as with drought stressed trees.  Ash, aspen, honeylocust, birch and maples are the most susceptible to drowning.
Trees do best when there is time for the soil to drain and dry slightly between irrigations.  They also do better if watered more deeply, less frequently – just like your lawn does.  
So, leaf drop or leaf drop preceded by yellowing of the leaves is due to the dry conditions we are having after our cool, wet spring.  The tree is relieving itself of excess greenery it is unable to sustain.  The situation will stop as the weather settles out.  Leaf drop accompanied by leaves that are yellow with intervienal greening is likely due to overwatering.  Check the soil around the tree to a depth of 6-12 inches for excess moisture an dial back the water.  Remember less frequently – more deeply.  
For more information on care of your trees, check out CSU Extension Fact sheet 2.932 Environmental Disorders of Woody Plants.