In 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 acetic acid percentage, the deadlier it will be. The type of vinegar used for culinary purposes is relatively low in acetic acid (around 5%). So if you are serious about weed control, you will need to buy products with a higher acetic acid content which are available at garden supply stores or from restaurant supply businesses.
The potency of high-percentage acetic acid products renders them unsafe, so use care when handling. Do not let them come in contact with your skin, and avoid touching your face.
When to Apply Vinegar
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.
For instance, learn when annual weeds set seed, so you can target them before this to prevent them from spawning a new generation. Depending on the weed, this can be in the spring or summer. If you catch them in time, one application of vinegar often will be enough to solve your weed problem.
Perennial weeds are not as easy to defeat. Take dandelions as an example. It's a good idea to snap off dandelion flowers whenever you see them, so they don't propagate themselves via seeding. However, while their leaves die back in the winter, these perennials typically live on through their roots. So preventing them from going to seed is not enough. That's where applications of vinegar throughout the growing season come into play. Each time you apply the herbicide, the plant will weaken. With repeated sprayings, a final death should occur.
How to Apply Vinegar
If you are battling lawn weeds, you must take care to apply the vinegar directly onto the weeds themselves, and don't let it come in contact with your grass or other plants. The fact that vinegar is a natural product doesn't mean it can't be harmful if misused. Vinegar is nonselective, meaning it can kill more plants than just weeds.
To avoid damaging grass and other wanted plants, consider painting the vinegar directly onto the weeds with a brush. If you choose to apply it with a sprayer, do not pull the trigger until you are right up close to the targeted weed.
Wait for a forecast of at least a few continuous days of sunshine before you apply vinegar. At the beginning of this period, spray or paint the vinegar onto the weeds you wish to kill.
There are two reasons why a sunny period is important. First, you need to saturate the weeds with vinegar for it to be effective, and rain would wash off too much of the vinegar from the foliage. And second, the real damage to the weeds begins in 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 itself does not kill the weed. Instead, it helps the herbicide work. For herbicide to have the intended impact, it must stay in contact with the vegetation long enough to penetrate. This is easier said than done because a weed's foliage is often protected by a waxy coating that can repel attempts at penetration. The role of the 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. An ounce of soap per gallon is the recommended mixture.
The Limitations of Vinegar
Because it's nonselective, vinegar is not 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 aren't in the way, such as on patios or walkways where weeds are pushing up through the cracks.
Furthermore, you will probably have to reapply 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 a dandelion while one type of annual weed is crabgrass. However, considering the limitations of herbicidal products as a whole—and particularly of the organic ones—having to reapply is hardly a deal-breaker in using vinegar as a natural weed killer.