On smaller stems (1"?) the oil will eventually wick around and cover all sides. On stems up to ~3", if the stem is sloped and not straight up and down, and you apply herbicide above knee height, the oil will usually wrap around as it runs down the stem. This assumes you're applying it heavy enough on the one side. I apply a thin stream of 20% triclopyr 4, and use just enough to wrap around the stem in a band about 3x the stem diameter. No need to fully cover the bottom 18" of the stem like some old sources say. On larger stems I do hit both sides. Disclaimer - this is on autumn olive, bush honeysuckle and privet. I don't have buckthorn here.