Acetic Acid

• Commercial acetic acid sprays may be used by organic farmers to kill above-ground portions of plants and some roots. Spray or inject 20 percent acetic acid onto or into the weed. Household vinegar, at about 5 percent acetic acid, won't kill mature weeds, but will kill just-sprouted ones.
  
  

Boiling Water

• Hot water kills weeds, but it's not very easy to direct. Avoid using boiling water to kill flower or vegetable garden weeds, but feel free to apply it to sidewalk cracks and other areas where unwanted plants have invaded.
  
  

Biological Agents

• Insects, bacteria and other biological agents can help control weeds in gardens and public areas. Introducing specific bacteria, fungi and invertebrates can help keep weed problems to a minimum, but it's important to do so with care -- the wrong biological agent could hurt the local ecosystem. Avoid using broad spectrum pest-control methods that can also kill weed-eating insects.