Thistles comprise weeds in many different genera, including Circium, Dipsacus, Carduus, Onopordum and Sonchus, which grow over wide swaths of the United States. These biennial plants first form rosettes, then grow into tall, pink- or purple-blooming plants in their second year. Because they are opportunists, they often appear in cultivated garden settings, where they prove a prickly nuisance to professional gardeners and busy moms alike. If you are interested in an easy, non-chemical approach to thistle treatment, don’t worry: vinegar has your back.
Vinegar is a broad-spectrum herbicide, meaning it will damage most plants with which it comes into contact, potentially killing them. Because it has such damaging results on plants of all kinds, not just thistles, you should be careful to apply it when there is no wind, restricting your applications to weeds only. The weed-killing ingredient in vinegar is ascetic acid, but in grocery store vinegars this concentration is only around 5 or 10 percent, not very strong. Go for a 20 percent solution, which is regulated, but you should be able to find at specialty garden centers.
If you do find vinegar with a 20 percent acid content, go ahead and use that straight on thistles. If you can’t find it, you can still make some fairly caustic mixtures at home. Combine 1 quart of white or apple cider vinegar with 4 ounces of concentrated lemon juice, then spray on plants. Or mix a tablespoon each of gin and cider vinegar and a teaspoon of dishwashing detergent in a gallon of water and go to town.
Because thistles are so tough, they may not respond as readily to treatment just on their leaves and stems. Instead, you might have better luck by first chopping down the plant and removing it, then applying the solution straight to the open wound. This will not only exhaust the plant by removing its source of food, leaves, but will get the herbicidal properties of the vinegar solution a lot close to the roots.
Apply Till They Die
The most important aspect of treating thistles is to remember that these solutions kill the leaves and stems and other fleshy parts of the plant, but not the roots. Even if you are chopping the plant down and applying directly to the roots, it may take a while to exhaust them. If you want to keep weeds down over the long haul, you will need to repeat applications each time the weed reemerges. Doing so during the heat of the day will have better results, and catching weeds before they set seed will help immeasurably in reducing the overall population on your property.
Keep it Safe
Solutions of acetic acid at 4 to 6 percent, as well as 20 percent, can cause skin, eye and internal irritation, so always keep them away from children; solutions with acid contents of 11 percent or more can cause burns both inside and out, though treatment is the same. If they come into contact with skin or eyes, flush with water for 15 minutes. If swallowed, give the victim a few cups of water or milk but do not try to make them vomit. If vapors are inhaled, get the person fresh air and perform artificial respiration if they stop breathing. If the victim does not respond readily to treatment, call Poison Control or 911.