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Vinegar is a penny-wise and environmentally-safe means for killing weeds. It is easy to use and will not harm the environment. Simply place some undiluted household vinegar in a mister/sprayer bottle and spritz weeds to saturate their leaves. Usually within 6 to 48 hours the plant growth will wither and die. Young seedlings less than two weeks old will be killed entirely, older weeds will have their top-growth killed but may still resprout from the roots.

What is vinegar and how it it made? The word vinegar comes from two Latin words: *vinum* meaning wine and *acer* sour. Vinegar can be naturally made by the decaying of plant parts in anaerobic situations. Acetic acid (often called ethanoic acid) is created by the fermentation of alcohol. Common household vinegar has about a 5% acetic acid concentration and is prepared from grapes (wine), apples (cider), or grains (malt). Fermentation creates vinegar when sugars are converted into alcohol and carbon-dioxide....the alcohol reacts with air and specific bacteria to make vinegar. Concentrated acetic acid created by synthetic processes is not considered organic and is not used by the organic agricultural community. Vinegar (made from plant sources) with high acetic acid concentration created by distillation or freeze-evaporation process is deemed acceptable for organic garden use.

Will the acetic acid in vinegar harm my soil? It can temporarily reduce the pH of soil but this effect typically lasts only a couple of days. Acetic acid rapidly breaks down in water and so for the best results do not spray weeds just before a storm. The rains will dilute the vinegar and greatly reduce its effectiveness.

Is there some research data available on this? The United State Department of Agriculture is currently researching the use of vinegar as an organic herbicide at their research facility in Beltsville, Maryland.
Results so far are very promising and have shown that using vinegars with 10, 15, or 20% acetic acid concentrations have had an 80 to 100% kill-rate with many common weeds. Common household vinegar (with a 5% acetic acid concentration) has successfully killed the top-growth of Canada Thistle, one of North America's most noxious and widely spread weeds. However, regrowth from the roots did occur. To achieve the best results in eliminating Canada thistle spray very young plants every couple of weeks with household vinegar (5%).

Where can I buy high concentration acetic acid vinegar? Look for it at Farm Supply Stores. Vinegar sold specifically for Home-Canning purposes also has an acetic acid concentration higher than five percent.

Do I need to use high acetic acid concentration vinegar for weed control on my home property? I don't want to use anything that is caustic. Young weeds will be killed by spraying with plain household vinegar (5%), older weeds may require some additional spraying for effective control.
Wear protective clothing and eye shields while using high concentration vinegars. Skin irritation from contact, lung irritation from fumes, and very severe and permanently damaging eye irritation from splashes are all possible.

This might also be helpful:

Will boiling water kill weeds? Newspaper as weed control?
Organic Gardening Forum