Vinegar as herbicide

Introduction
USDA research indicates that vinegar can be used as an herbicide. Acetic acid is not absorbed into root systems, so vinegar will kill top growth, but perennial plants will reshoot. Commercial vinegar does not exceed 5% and solutions above 10% need careful handling. Stronger solutions (i.e., greater than 5%) may be needed for effective herbicide effect. In some studies, the herbicide effect only worked when the acetic acid was applied to young weed seedlings (less than two weeks after germination).

Use in organic agriculture

 * "organic roundup" - acetic acid
 * briefly makes the soil more acidic, but this effect goes away after a few days, as the vinegar is broken down by microbes in soil
 * can be produced on the farm from wine, cider, potato mash, or other sources of ethanol;
 * can easily be produced from pyrolysis ("wood vinegar")
 * some sources claim that it works as an insecticide when sprayed on leaves [needs verification / source]
 * 20% vinegar concentration to kill small weeds. Adult weeds may be killed by even higher concentrations - this then becomes dangerous to humans (acetic acid is corrosive - handle with care !)

Links

 * Wikipedia: acetic acid, vinegar and acetic acid bacteria
 * Gardenweb: Organic Weed Controls ~ Vinegar as an Herbicide
 * Article: "Vinegar as an Organic Weed Killer"