For organic gardening, vinegar can function as a natural weed killer (depending on your definition of "organic"; see below). It is the acetic acid in vinegar that 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 (5%) in acetic acid, so, if you are serious, you will purchase the stronger stuff (see below).
Vinegar As Natural Weed Killer: Limitations
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. Why? Because 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.
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. Do not spray on a windy day, as the wind could carry your vinegar spray where you do not want it to go -- namely, onto your grass or onto plants you paid good money for at the garden center.
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).
Note: if, on the other hand, you are thinking of reducing lawn size by killing a big patch of grass on purpose, there are better (easier) ways to get rid of grass across such large expanses.
There is another limitation in using vinegar as a natural weed killer, but this limitation extends to chemical weed killers, as well: namely, that you will probably have to re-apply the vinegar to get the job done, as weeds often refuse to go quietly.
This is especially true of established perennial weeds, toughened by years of coping with environmental challenges; 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. If you are of a mind to get rid of weeds organically, one assumes that you will be motivated to make repeated applications, as necessary.
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 the 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 in 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 -- via exhaustion.
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:
- You need to saturate the weeds' leaves with the vinegar, and rain would wash too much of the vinegar off the foliage.
- The real damage to the sprayed weeds begins the next couple of days after the application when the sun hits the leaves.
Some people who use vinegar as a natural weed killer like to boil the vinegar, prior to application. Through such boiling, you may actually be able to gain a concentrate higher in acetic acid, although I have not yet experimented with this option in any scientific way. As an aside, we could speculate that it would not hurt, at least, to boil the vinegar; in fact, many folks report success killing weeds by simply pouring scalding water on weeds. So I suppose, theoretically, the use of boiled vinegar could allow you to attack weeds with an additional weapon (namely, heat).
Note: One way to improve the effectiveness of an 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 (the type you scrub plates within the sink, for example, Dawn or Palmolive) 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 (20%) than that found in ordinary vinegar. Such products can be purchased at farmer's stores or from restaurant supply businesses, but perhaps the easiest way to get your hands on such a product is to buy it on Amazon.com. Note, however, that the potency of these high-percentage acetic acid products renders them unsafe. And while on the subject of safety, let me observe that the dyes, antibacterial agents, and other ingredients in the typical dish-soap surfactant will also leave the organic gardener less than sanguine. These facts can cause us to question the whole concept of using vinegar as a "natural" weed killer.
If you want to get technical about it, the most natural (as in "safest") way to get rid of weeds is generally via hand-removal. But removing weeds by hand is not always feasible: the weed in question may be too persistent, the area covered by weeds may be too extensive, or your health may prevent you from being up to the task. Thus the need for herbicides, and thus the interest in organic herbicides, which, while not always harmless, will perhaps do less harm than other herbicides.
Other Uses for Vinegar
Those of you who have held an ongoing grudge against vinegar for its ability to make your mouth pucker may finally be able to grant vinegar forgiveness. For that same sourness can make vinegar lethal to weeds. Vinegar can also be used for cleaning purposes around the home, as an alternative to chemical cleaners.
Now that I have you thinking about the controversial issue of herbicide use, read up on two other unsavory tasks: