I weed out one area and then it grows in another area. I know it spreads by underground roots but how am I to stop this invasive herb from taking over my garden?
Yes…it does that. The only way to control mint is to plant it in a large pot buried in the dirt, but with the rim just above the surface. This stops it sending out runners.
The only way to get rid of it is careful hand weeding. Wet the soil thoroughly and leave it for several hours to soak in, then start teasing out the plants. Make sure you get it ALL. You may have to do this several times, as it is easy to miss bits, or for pieces to break off. Keep a few pieces for your new confined pot, and put the rest in the trash. Do not try to compost it, as you may not kill it all in the compost heap.