For years I have used vinegar (4% acetic acid) to kill gamba grass (Andropogon gayanus). This monstrous, fire-prone weed grow to four metres in heigh and in clumps over a metre in diameter. Gamba infests over 1.5 million hectares of the Top End. It carries destructive fire that within a decade can transform a forest into a grassy monoculture. I stamp on the base of big plants to push them over, stamp on the lower stems to keep them flat and then apply vinegar to the base. With smaller plants there is no need to push them over. Again I just pour vinegar on the base. The biggest weeds are dead within two days. No need for protective clothing, very uncomfortable here in the Top End. Flattening the grass gets it out of the tree canopy thus reducing the potential for canopy fire. This is an ever-present risk during the Dry. Two years ago there were twelve fires along our kilometre-long road, one of which nearly killed our fire chief. Gamba grass water uptake probably exceeds 2 megalitres a growing season. Another reason for getting rid of this threat.
Certified GKH Gardening Expert
While vinegar will kill the foliage above ground, it doesn't have any effect on the roots. It may work well for annual weeds. Here is more: