Thistle is ... the mosquito of the plant kingdom for me. I'm sure somewhere in nature's circle thistle, like the mosquito, serves a purpose, yet what I do not know. Thistle is prickly, painful, invasive, and tough as nails to "weed out." This summer we joined a Community Garden and when we first saw our plot we it was jam packed with thistle. I knew I needed a weed killer, a strong and effective one. I also knew I wanted it to be completely safe as I garden with our toddler and puppy. We use vinegar for several other homemade solutions, so I decided to start research there, maybe there was a simple fix, turns out the required ingredients were already staples in our kitchen.
Vinegar is an all natural broad-spectrum herbicide, meaning it will kill most plants with which it comes into contact with. Because it doesn't discriminate between weeds and other plants, please 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 most grocery store vinegars this concentration is only around 5 or 10 percent, which isn't quite strong enough. If you can find a solid 20 percent solution, that as is, in a spray bottle will work.
If you can't find one, you can create a more potent vinegar based weed killer with the below options -
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.
Since weeds by nature are so tough, and this mixture is all natural it does not kill on spot like RoundUp or other cancer causing killing options. That's ok by me for the greater health of all, myself included. Instead of applying just on their leaves and stems, try first chopping down the plant and then applying the solution straight to the open wound at the base by the root. I just carry scissors with me and snip it down, spray, done. This will not only exhaust the plant by removing its source of food, the stems and leaves, but will get the herbicidal properties of the vinegar a lot closer to the roots. You will likely need to repeat each time the weed reemerges thought for about 2-3 additional applications. Doing so during the heat of the day will yield the best results.
Although this is vastly safer than the chemical counterparts, solutions of acetic acid can cause irritation. In other words, if my son touches a bit of this or I do, or our pets, we'll be ok, we've gardened a plenty all together and been right as rain. That being said I don't leave our child or pets alone with this either, and certainly you aren't going to drink or splash this about. If this comes in contact with your skin or eyes and any burning sensation begins, flush with cold water for 15 minutes. Vinegar of 20% on your eyes will be painful, it's similar to having lemon juice in your eyes, only think with a concentration of 10-15 lemons. If swallowed, drink a few cups of water or milk to help dilute and flush the solution from your system.