Synthetic weed killers are good at killing undesirable plants, but they are also toxic and can be fairly expensive. Fortunately, there are a number of alternatives to store-bought herbicides. Whether you want to kill a whole swath of crabgrass or eliminate one or two stubborn thistles, there are techniques that will let you safely kill the plants you don't want while saving the ones you do.
The easiest way to kill a wide patch of weeds is to smother them. Lay newspaper on top of the weeds to create a barrier at least four papers deep. Hold the paper down with rocks if you want to plant a garden after your weeds die. If you have no plans to grow plants in that area soon, cover the newspaper with a layer of mulch. The paper will kill the weeds and decompose under the mulch. If you want to grow plants in that spot in the future, you will find nothing but soil under your garden mulch.
Many weeds spread quickly, popping up new plants almost before you can dig up the old ones. Spread a layer of cornmeal over the ground to control the weeds in your garden. According to This Garden is Illegal, "corn Gluten is a pre-emergent," a substance that stops seeds from growing into plants. If you have perennial plants plagued by annual weeds, you won't even need to weed after spreading the cornmeal; the weeds will die off at the end of the season, while the plant will continue to grow the next season.
Being doused in enough boiling water will kill nearly anything, weeds included. Cut weeds to near the ground and mark the spot where they are with a stick or flag. Fill a pot halfway with water and bring it to a boil (you can fill the pot more, but it will be easier to spill and burn yourself). Carefully carry the pot out to your weeds and pour the water on the weeds. The boiling water will scald the base of the plants, killing them quickly. Be careful, since boiling water will also kill desirable plants.
For a safer alternative to boiling water, try rubbing alcohol. Fill a spray bottle up with rubbing alcohol and spray it liberally on weeds. As the alcohol evaporates, it will draw water out of the plants, dehydrating them and killing them. Spray the weeds every couple of days if they do not seem to be wilting quickly. If your weeds are near plants you don't want to hurt, apply the rubbing alcohol to the weeds with a cotton swab to avoid injuring desirable plants.