Answer: Round-Up should work, but it will work better if you can get it down into the root system. If it is too large to cut down, you can use a wooden or metal stake held at an angle and pounded down into the root system, then removed. Make several holes at the base of the plant and into the soil several inches, then pour Round Up into the holes. Or, cover the entire plant with black plastic or a dark colored tarp. Without sunshine, the plant will die. When you've gotten rid of the prickly top, be sure to dig out the roots or they will resprout.
Good luck with your project!
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