Vinegar and orange oil kill by chemically BURNING the foliage off the plants. Doing this when the sun is shining directly on the plants compounds the effect. Higher temperatures add to the speed. By defoliating the plants, you often kill them. Sometimes it take a second application. But ultimately the roots exhaust themselves and the plant dies.
Maybe you were expecting the vinegar to work the same way as Roundup...which is a poison. You have gotten something else in the bargain along with your dead weeds. Sorry you decided to opt out and use Roundup. My second option has always been fierce mowing or a weedeater, followed by the vinegar & orange oil. That combination does the trick every time. The good news is that the vinegar and orange oil you already had in the soil will help to deplete the secondary, toxic effects of the Roundup more quickly.
Kathe
