In tests, though, this mixture was enough to kill Canadian thistle, giant foxtail, smooth pigweed, and two other weeds up to 2 weeks old. The vinegar caused the young weeds' leaves to shrivel up, turn brown, and die. But older plants needed stronger vinegar solutions--and sometimes more vinegar--to finish them off. For example, against older Canadian thistles, the scientists found they had to drench the pesky plants' roots to totally kill them. Just spraying the leaves didn't quite do the trick.
The scientists also did a little testing on cornfields. They found they could kill most--or all--of the weeds there without harming the corn. Their tests also showed that vinegar only makes the soil slightly more acidic, and only for a few days.
The weed scientists are glad the farmers and gardeners discovered vinegar as an organic weed killer. And the farmers and gardeners are happy to receive scientific information about how to use it best.
"Usually scientists are the first to discover things like chemicals that kill weeds," said Teasdale. "But this time, it was the opposite of the way it usually works. The farmers and gardeners discovered this natural weed killer. We scientists came in toward the end, with proof about how effective and safe it is."
-- By Don Comis, Agricultural Research Service, Information Staff