Question: I planted several aspen trees around my home because they were recommended as a reliable native plant. Now I am experiencing one problem after another: yellow leaves, leaf spots, leaf miners, and sprouts everywhere.
Now two trees are dying and I have not been able to find out what is wrong. Why is this tree planted so much if it has so many problems?
Aspen: Native seems to be a magic word nowadays. Aspens are indeed very beautiful and well adapted in our surrounding mountains. Unfortunately, many native plants have many “native pests” as well. When we are admiring plants in the mountains, we often are not as critical and do not observe as closely as we would in our own landscape.
Aspens are beautiful in an informal or natural landscape setting where you want to imitate a natural “woodsy” setting. Their informal, irregular branching habit, white bark, and quaking or rustling leaves can be very attractive in irregular clusters of several trees. In a natural setting, plants are not perfect. Branches die, leaves get spots or turn yellow or have holes eaten in them and we hardly notice. But if you want aspen trees to be perfect without a blemished leaf or leafless branch, plan on spending a lot of time to keep them that way.
Unwanted sprouts can be pulled out like any other weed. Keep a shovel or pruners handy so when a long horizontal runner comes up with a sprout you can cut it off. Small sprouts in the lawn can be mowed off with the grass. When they get too large or numerous, you can either dig them up or spray with lawn weed killer. Lawn weed killer will kill the sprouts without doing harm to the mother tree.
Insects can be sprayed if they become too numerous.
Spinosad will control most insects including leaf miners, but not borers. Borers bore holes into the bark of trunks and main branches. You will often see sap running out of borer wounds or find little piles of sawdust on the ground underneath. The best time to spray for borers is early in the spring as a preventative treatment before they get inside the bark. Systemic pesticides are available which can reach borers already under the bark. Full service nurseries or commercial landscape and pest control service companies can make recommendations as to which pesticide is most appropriate for your particular situation.
The black spots on leaves are caused by a fungus know as aspen leaf spot. Black spots appear on the leaves in the spring. If spots are numerous enough or grow large enough, leaves turn yellow and fall off. Trees usually develop new leaves to replace the ones which have dropped.
Aspen leaf spot can be controlled by spraying with a copper fungicide early in the spring just as leaves are opening. If you wait until the tree is in full leaf before spraying, you will probably be too late.