For organic gardening, vinegar can function as a natural weed killer. The acetic acid in vinegar gives it the power to kill weeds; the higher the percentage in the vinegar, the deadlier it will be. The type used for culinary purposes is relatively low (five percent) in acetic acid, so, if you are serious, you will need to purchase the stronger stuff.
If you are battling lawn weeds, take care to apply the vinegar directly onto the weeds themselves, not letting it come into contact with your grass.
The fact that vinegar is a "natural" product does not mean it can't be harmful if misused. Vinegar is non-selective, and it can harm your grass as well as the weeds.
To avoid damage to grass, consider "painting" the vinegar directly onto weeds with a brush. If you do spray with vinegar, do not pull the trigger until you are right up close to the targeted weed.
Because of this limitation, I do not think of vinegar as being an especially effective method for killing weeds in lawn areas. It makes more sense to use vinegar in areas where lawn grass and other landscaping plants will not be in the way, such as on patios or walkways (where you have weeds pushing up through cracks). If on the other hand, you are thinking of reducing lawn size by killing a big patch of grass on purpose, there are easier ways to get rid of grass across such large expanses.
You will probably have to re-apply the vinegar to get the job done.
This is especially true of established perennial weeds; vinegar will be more effective on younger weeds and weeds with an annual life cycle. An example of a perennial weed is dandelions (see below), while crabgrass, for instance, is an annual. But considering the limitations of herbicidal products as a whole (and particularly of organic ones), this objection is hardly a telling argument against the use of vinegar as a natural weed killer.
When to Apply Vinegar (and How Often)
When practicing natural weed control, take to heart the dictum, Know your enemy! Study up on the weeds you are battling before you use vinegar on them. Target annual weeds before they set seed, to prevent them from spawning a new generation to give you headaches next year. Depending on the weed, this can be in spring or summer, and one spraying may be enough.
Perennial weeds are not as easy to defeat. Take dandelions, as an example. It is a good idea to snap the flowers off dandelions whenever you see them so that they do not propagate themselves via seeding. But these perennials, although their leaves die back in winter, do live on through their roots. So preventing them from going to seed is not enough.
Therefore, the idea behind spraying dandelions with vinegar is basically to engage in a war of attrition throughout the growing season. Each time you apply the herbicide, the plant will die a little bit more. With repeated sprayings, a final death will arrive.
How to Apply
Listen to your local forecast, and find out when your region will be experiencing a few continuous days of sunshine. At the beginning of this period, spray or paint the vinegar onto the weeds you wish to kill.
Why is a sunny period required? Two reasons, first you need to saturate the weeds' leaves with the vinegar, and rain would wash too much of the vinegar off the foliage. The second is that the real damage to the sprayed weeds begins the days after the application when the sun hits the leaves.
One way to improve the effectiveness of herbicide is by mixing a surfactant into it. The surfactant does not kill the weed; rather, it helps the herbicide do a better job of killing the weed. For an herbicide to have the intended impact, it must come into contact with the vegetation and stay in contact long enough to penetrate. This is easier said than done because a weed's leaf is protected by a waxy coating that can repulse attempts at penetration. The role of a surfactant is to counteract this defense, thereby allowing penetration to take place.
In homemade recipes for herbicides, dish soap is often used as a surfactant. One ounce of soap per gallon is the recommended mixture.
The Stronger Stuff
It is possible to buy products with a higher acetic acid content than that found in ordinary vinegar. Such products can be purchased at farmer's stores or from restaurant supply businesses. Note, however, that the potency of these high-percentage acetic acid products renders them unsafe.