Why is our cottonwood tree shedding yellow leaves in summer? - Ask Extension
All summer, 2015, our huge cottonwood has been shedding yellow leaves! We've been raking like it's fall! The fallen leaves are yellow with brown spots...
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Why is our cottonwood tree shedding yellow leaves in summer? #270776
Asked August 17, 2015, 12:23 PM EDT
All summer, 2015, our huge cottonwood has been shedding yellow leaves! We've been raking like it's fall! The fallen leaves are yellow with brown spots. The tree has green leaves on it, too. Huge tree, has not done this in 26 years we've been with it. What to do?
Arapahoe County Colorado
Expert Response
Amy,
there's a few possible reasons or causes for this; or a combination of these causes:
First, cottonwoods are "drought-deciduous". This means that they often produce many leaves during the cooler and moist periods of spring, and then have to shed some leaves as June heat and dryness arrive...as sort of a water conservation effort. This spring was very rainy and humid - good for leaves!
Next, the sudden and severe 'polar vortex' freeze of Nov 11-12, 2014 may have killed some vascular (water-conductive) tissue of many types of trees. That would mean they would leaf out normally in spring, but "crash" with the onset of hot dry weather in late June, dropping leaves as a result.
Finally, all the rainy humid weather in spring promoted the growth of leaf spot diseases caused by various fungi (Marsonnina, Septoria). These leaf spot diseases often cause premature leaf drop. Details: http://www.ext.colostate.edu/pubs/garden/02920.html
What to do:
*check a few twigs to make sure the tree has formed buds that will produce next year's new growth and leaves
*water the tree's rooting area seasonally and during dry warm snowless periods in winter months. The rooting area of a 26 year old cottonwood will be very large, perhaps the entire back yard.
*continue to rake up and dispose of leaves; this will reduce the fungal spores which could cause more leafspot disease next year, if weather conditions are conducive.
*Have a tree service prune out/remove any deadwood (twigs/branches that fail to leaf out next spring).
there's a few possible reasons or causes for this; or a combination of these causes:
First, cottonwoods are "drought-deciduous". This means that they often produce many leaves during the cooler and moist periods of spring, and then have to shed some leaves as June heat and dryness arrive...as sort of a water conservation effort. This spring was very rainy and humid - good for leaves!
Next, the sudden and severe 'polar vortex' freeze of Nov 11-12, 2014 may have killed some vascular (water-conductive) tissue of many types of trees. That would mean they would leaf out normally in spring, but "crash" with the onset of hot dry weather in late June, dropping leaves as a result.
Finally, all the rainy humid weather in spring promoted the growth of leaf spot diseases caused by various fungi (Marsonnina, Septoria). These leaf spot diseases often cause premature leaf drop. Details: http://www.ext.colostate.edu/pubs/garden/02920.html
What to do:
*check a few twigs to make sure the tree has formed buds that will produce next year's new growth and leaves
*water the tree's rooting area seasonally and during dry warm snowless periods in winter months. The rooting area of a 26 year old cottonwood will be very large, perhaps the entire back yard.
*continue to rake up and dispose of leaves; this will reduce the fungal spores which could cause more leafspot disease next year, if weather conditions are conducive.
*Have a tree service prune out/remove any deadwood (twigs/branches that fail to leaf out next spring).
