Answer: Bishop's weed or Bishop's Goutweed is extremely difficult to eradicate because just a small sliver of a root can grow into a new plant. I have had some success with herbicides containing glysophate (such as Round-Up). Thoroughly spraying each and every plant that sprouts, allowing it to die out completely, and then covering the area with a thick layer of mulch eventually gets rid of it. Be careful when using Round-Up - its a non-selective contact herbicide and will kill anything it touches.
Best wishes with your project!
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