Question
IF I pour boiling water on a tree will it kill it?

Answer
Depends on the size of the tree and its roots. A small less than 3 foot tree would more than likely be killed by the boiling water depending on the amount of water that is used. But in general I doubt that boiling water poured on the roots of a tree will kill it unless you used a great deal of water. If you want to kill the weeds near a tree try vinegar or a herbicide called Roundup. Vinegar alone - Full strength, undiluted 18 percent acidity vinegar can be an effective weed killer.