as long as you don't just stand there and hold it pointed right at the roots for a long time it shouldn't be a problem. use common sense. it's not gonna kill the tree like it would a weed. obviously you don't wanna burn a big black spot on the root of the tree but even doing that it would be fine. a tree has many roots not just one. trees are almost impossible to kill. if i could kill them with a propane torch i'd be doing that on every yard i mow. trees are terrible for grass and mowing in general.