Not the OP, but I'd just point out that all you should need to do is get the stem above the boiling point of water, which is well below the combustion point. W

Not the OP, but I'd just point out that all you should need to do is get the stem above the boiling point of water, which is well below the combustion point. What wavelength to use for that could be an interesting discussion. Makes me wonder whether you could use masers (microwave lasers) rather than optical lasers for a more efficient "heating the weed from the inside" effect.