Q: I have poison sumac growing around my trees and up the trunks. What's the best way to eliminate it without harming the trees? — R.A. Scherer, St. Louis, MO
A: Poison sumac (Toxicodendron vernix) is a woody shrub or small tree that grows primarily in wet, boggy locations and sometimes in moist, acidic soils found in pinewood forests. It does not grow up the trunks of trees. However, poison ivy (Toxicodendron radicans), a close relative of poison sumac, does grow around and up the trunks of trees and is not restricted to moist soils. I suspect that's what you have.
Poison oak, another close relative, tends to stay fairly short rather than form vines that climb up trees. Poison ivy leaves are somewhat variable in their shape; people often think the more lobed versions are poison oak, but that's not necessarily the case.
There are several herbicides and woody brush killers labeled for poison ivy. Herbicides containing glyphosate (Roundup, for instance) are readily available and do a good job of eliminating poison ivy when sprayed or wiped onto the foliage. (Make sure you read and follow package directions.) Plants take in the chemical through healthy green leaves and transport it internally to the roots, without harming adjacent plants. It won't be absorbed by a woody tree trunk, so you can spray the poison ivy right on the tree as long as you're careful not to let any of the spray reach the tree's leaves. Also avoid spraying nearby leafy plants on the ground, unless you don't mind losing them.
If the poison ivy is old and well-established, repeat the herbicide application several times to completely eliminate it. Don't apply the herbicide too early in spring; wait until poison ivy leaves are fully expanded. It won't be effective toward the end of the season, either, once poison ivy leaves begin to exhibit fall color.
Be aware that all parts of the plants, living or dead, contain toxic oils that pose a threat to anyone who handles them. Use extreme caution in cutting down and removing poison ivy, poison oak, or poison sumac. Never burn them, as the fumes are also poisonous. Instead, bury the plants in an out-of-the-way place or bag them up to be hauled away. — Deb Brown, professor emeritus, University of Minnesota Department of Horticultural Science
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