[charli (OP)]

Listener Denise wonders:

Gamba grass (Andropogon gayanus) is a tall (5m+) clumping weed that has devastated well over a million hectares of northern Australia, outcompeting all other plants and leaving only vast monocultures of grass. It also fuels dangerous, fast-moving wildfire. The only solutions offered so far have been to dig the weed out, difficult as it is a perennial with a robust root system, or by poisoning it with glyphosate.
However, several years ago I discovered a way of killing Gamba. I push the plants over, stamp on the base to crush the stems, and then apply vinegar (4% acetic acid) to the base. The biggest weeds are dead within two days. Can someone tell me why vinegar is such an efficient weedkiller?

[Colin2B]

Vinegar absorbs moisture from the leaves causing them to shrivel and die. It is more effective on annual weeds as perennials will often regrow unless the roots are killed. Domestic vinegar is 5% acetic acid but a 20% is used in agriculture.
It is often mixed with salt and soap to make it more effective. One disadvantage is that you are changing the pH of the soil if you use it a lot, but maybe it’s a case of which is worse.

[Bored chemist]

One disadvantage is that you are changing the pH of the soil if you use it a lot,

Not as much as you might think.
The soil bacteria will fairly rapidly oxidise the acetic acid to CO2 and water. The pH will return pretty much to where it was.

[Colin2B]

One disadvantage is that you are changing the pH of the soil if you use it a lot,

Not as much as you might think.
The soil bacteria will fairly rapidly oxidise the acetic acid to CO2 and water. The pH will return pretty much to where it was.

Thanks, that’s useful to know.

[Bored chemist]

Also, the real reason vinegar kills plants is that life is complicated and dunking it in an acid bath screws things up.

Given that the vinegar works and that some people will be much happier using that rather than glyphosate etc, it's a useful bit of information.