For a basel bark application, I think you need an ester based herbicide with an oil based surfactant to penetrate the bark. You can use glyphosate to spray the leaves, but it would not be all that effective to kill the roots. In addition, spraying the leaves could result in herbicide drift. I think Triclopyr is probably more effective on woody plants.
I also might try cutting the sapling down and applying a herbicide to the stump. I think when you do that you apply the herbicide at full strength.
