Answer: Ortho Season Long Weed Killer around your trees will not damage the trees. However, it may not be strong enough to kill poison ivy and oak. For this you need something like Round Up, which will kill the weeds but should not be sprayed on the trunk of your tree. So, here's what I would do: cut the Poison Ivy/Oak down at ground level and immediately paint Round Up on the cut stubs. Then pull the poison ivy/oak out of the trees. Repeat the applications on any new sprouts. Round up will be translocated to the roots of the weeds but will not travel in the soil and so will be safe to use around your trees. Don't spray the stuff or damage can occur. Just paint it on the weedy growth you want to control. Good luck with your project.
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