First, focus on getting your lawn and garden as healthy as possible. The healthier your plants and the thicker your grass, the less room there is for weeds. Make sure to plant native species whenever possible. Native plants are better suited for persevering against the weeds of the local area. Then provide your plants with optimal soil conditions. Aerate soil before planting and fertilize with compost. Lay down newspaper to block weeds – it's organic, can be turned into the soil the following spring, and is less expensive than plastic. Strengthen your lawn by mulch-cutting and leaving the clippings (they're a great source of nitrogen) on the grass. And make sure to cut high – weeds get choked out and the lawn’s root system becomes hardier. For an all-natural and safe pre-emergent weed killer try corn gluten meal. It has been shown to inhibit the growth of dandelions, crabgrass, and several other annual weeds.
Even if you take every preventative measure, there's always at least a few weeds bound to show up. Start with the cheapest, most natural weed killer – your own hands. Pull weeds by hand or with a hoe or spade. There are also several different tools available to make removing weeds by their roots easier. Some other all-natural weed killers to consider:
Boiling water. Pour boiling water (you can re-purpose cooking water) on a weed and it will shrivel up within a couple days. This is a great way to clear a grouping of weeds popping out of or around driveways, walkways, and sidewalks. Take care though . . . boiling water will kill any plant it comes in contact with as well as the underground roots of nearby plants.
Salt. Salt (any type will do) is a serious plant killer. You can sprinkle just a small bit at the base of a weed to get rid of, but then the soil will become unsuitable for future plant growth. It's best to use salt on gravel walkways and driveways (just sprinkle it on) to keep anything from growing there for several months. Avoid using salt on concrete because salt erodes concrete surfaces.
Vinegar. Spraying vinegar directly on weeds will kill the leaves and cause them to shrivel up within a few days. Young weedlings will likely die all together, but more established weeds will need to be sprayed a few times before being completely eliminated. Avoid spraying vinegar directly on the soil since it also kills beneficial microorganisms which would ruin the soil for any future plant growth.
Rubbing Alcohol. Rubbing alcohol can be used the same way as vinegar. It too can ruin the soil, so make sure to only spray the leaves of the weed.
Soap. Add a few drops of liquid dish soap to any of the above ingredients to make it more effective. The soap breaks down the surface of the plant boosting the weed killer's absorption and thus its ability to do damage.
A Combination. Your best bet is a combination of ingredients. Mix a gallon of vinegar with one cup of salt and two tablespoons of dish soap. Spray this combination directly onto weeds or pour on entire areas where you want to eliminate plants completely.
Finally, you have two additional options for dealing with weeds. One: If you can't beat 'em, eat 'em. Common weeds such as dandelions, purlsane, chicory, and violet are all safe to eat. Two: If you can't beat ‘em, enjoy ‘em. Accept weeds as part of nature. As A.A. Milne put it, “Weeds are flowers too, once you get to know them.”