Experts and BHG readers answer.
trees with thorns
Thanks for writing. I highly advice AGAINST using motor oil to control these trees. It's not an effective treatment against them and it causes environmental damage. In fact, it's illegal in some areas to do this.
You'd be much better off using an herbicide such as Roundup. Roundup is specially formulated to kill plants and have minimal damage to the environment. You can either spray it on the tree leaves or cut the trees off at the ground and apply it directly to the stumps. (This latter method is more effective.)
The Roundup will be absorbed into the root system and start killing the roots so the trees don't resprout any more.
---Justin, Senior Garden Editor, BHG.com