Answer: You are right, comfrey is an invasive plant and difficult to eradicate once it takes hold. You will need to try a combination of methods, a few times, to get it under control. If at all possible, dig up the roots so you can get all of the root system, as opposed to breaking it up with a tiller. You may need to dig to a depth of 12 inches or so. If spading is difficult for you physically, perhaps you could hire a high-schooler to help? As soon as you see any of the stuff sprouting, dig it out. Round-Up's main ingredient is called glyphosate. Glyphosate is a systemic. This means when it is sprayed on a plant, the plant absorbs and distributes it throughout its system. Eventually, it kills the entire plant, including the roots. However, it needs to be applied when the plant is actively growing, and for tenacious weeds, it may need to be applied again. Follow product instructions exactly with any herbicide. The spray can drift to other plants, particularly if there is any breeze. Spray in the morning when air is usually calmest and cover any nearby plants. Another solution is to dab the hard-to-control weeds with glyphosate in a wick applicator (VERY low chemical use that way), which is preferable in a food-producing garden.
Soil solarization is another possibility, but I don't know if it will work with comfrey roots. Basically, you are heating up the soil to "cook" pathogens and weed seeds. You need to solarize during the hottest part of the summer, for up to 3 months. To solarize, smooth out the area (removing sod, plant debris and rocks), moisten the soil, lay a 4 mm to 6 mm thick sheet of clear plastic over bare soil, and seal the edges of the plastic with rocks or soil. This will naturally heat up to over 140 degrees F, which kills most pathogenic organisms and weed seeds in the top 4-8" of the soil, depending on soil type and temperatures. I'm sorry there isn't any easy "cure." As a tip for the future, if you want to grow something that can be invasive (mint, for example), sink a 5-gallon container into the ground, which will contain the root system.
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