I was going to discuss watering issues this week, but a house call about saving aspen trees has gotten me sidetracked. The new owners of a recently purchased house in Cedaredge wanted to know if the aspens in their yard could be saved. My answer was a qualified yes.
Some of these aspens were 20 years old while others were root sprouts that had developed into small trees thus far. The previous owner had let the aspens fend for themselves and cut them down as they died from a multitude of causes.
The older aspens were planted on the east side of the house (good) but never were watered except by rainfall (bad). Their leaves were the size of nickels, showing extreme stress. There were also many dead branches and the presence of cytospora canker, an incurable branch and trunk disease.
Here are my recommendations for these older trees: Do everything to remove all the stress these trees are now under. I would begin watering every 21 days until the first freeze and then every two months. I would return to the 21-day frequency in the spring. The key is to water deeply but infrequently. Overwatering can suffocate tree roots, leading to yellow leaves and eventual tree death. I would also put two inches of bark mulch in the aspen planting area for water conservation and cooling of tree roots.
I would prune out all the deadwood. I would also apply imidacloprid insecticide as a soil drench in the very early spring to help control the poplar twiggall fly and perhaps for flatheaded borers (metallic wood borers).
In the spring, when the new leaf buds begin to open, I would spray the developing foliage three times at seven- to 10-day intervals to minimize marssonina blight on leaves. I would also apply a nitrogen fertilizer, ammonium sulphate (21-0-0-24S) in the spring. No fertilizer should be applied to trees and shrubs after July 4 to avoid winter kill problems.
The smaller, younger cottonwoods are being damaged by deer and need a small diameter wire cage placed around them to protect them from buck antler rubbing. These same trees show evidence of flatheaded borer activity and should receive the imidacloprid treatment as well. I would also spray these trees and the older trees between June and August for egg-laying adult flatheaded borers with a permethrin insecticide product. These applications would be every two weeks and would not be repeated in subsequent years unless the borers returned. Spray the entire tree, with a focus on their trunks.
Most borers attack only trees that are stressed, as these trees are. So promoting vigorous growth and health is our goal here. Since aspen trees are only marginally adapted to our lower elevations and clay, alkaline soils, we must do everything we can to make them comfortable. Planting on the east and north sides of our houses is best. West and southern exposures can scorch their leaves. Also, rock mulches will reflect unwanted heat into the trees, making them uncomfortable.
Aspens, while drop dead gorgeous with their attractive bark, rustling leaves and golden fall color, can be a pain to maintain in our lower elevation landscapes. You might consider alternatives if you are not prepared to do those things that increase their success. While my suggestions above were addressing trees with problems, keeping aspens stress-free using my management tips will go a long way in helping them live long and prosper.
Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology.