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Canada thistle grows naturally throughout most of Canada and much of the northern and western United States. This perennial plant has a large root system that spreads out widely and digs down deep into the soil. The extensive root system makes it a difficult weed to control, but there are effective ways to kill it off. According to the USDA, vinegar is one of the most effective ways to control Canada thistle. If you add salt to the equation, Canada thistle can die off in a matter of a few hours.
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Fill an empty spray bottle with vinegar. Do not dilute the vinegar with water as this makes it less efficient at killing weeds.
Cut the stem of each thistle plant with a knife. A small slice is all that is necessary, so do not cut the stem in half.
Spray one to two sprays of vinegar directly on the cut of each thistle plant. The cut allows the vinegar to spread to the roots more quickly, which in turn kills the plant more quickly. Do not spray the soil with vinegar.
Sprinkle a pinch of salt at the base of each plant. Salt dehydrates the plants which makes them die even faster. Too much salt will lower the quality of your soil, so use no more than a pinch of salt for each plant.