25% Triclopyr (Garlon 4) in diesel works great for basal application on poison oak. It takes about a month but kills it dead dead dead. Crispy dead. I only have to lightly spray about 18" of the stem. So useful for plants whose leaves are 30' up in a tree and unreachable by foliar spray. I also use it on cut stumps of brush (mostly coyote brush) to kill the plant. Otherwise coyote brush resprouts within days. You have to apply it to the cut right after cutting, like within a half hour. Direct undiluted application of homeowner grade glyphosphate with a paintbrush works too.