One way mint plants expand is by sending out stems called runners horizontally across the soil. Sets of stems and leaves called rhizomes will start to grow vertically at points along the runners. Last year, our mint plant only began to get a noticeable amount of runners in late fall once the annual herbs had died and left more open space. However, this season the runners are coming much earlier.
In August, we began to see runners taking over the herb bed. One runner had grown along the soil under the lemon thyme, and had developed a rhizome on the other side of the lemon thyme. This shows that the runners do not need open space as we had previously thought. Instead, the runners can follow the soil under other plants, making the runners harder to cut back. After we cut back these runners, new ones had grown in by the following week!
While runners and rhizomes can allow mint to overgrow the plants around it, this is preventable. You can either cut runners with scissors close to the main plant to remove them or place a barrier between the mint and other plants. Give the mint its own planter or place some wood around it. The runners are not able to get passed the wooden sides of our raised beds as quickly as they spread across the soil. Remember to watch for and keep runners under control, and your mint and surrounding plants should be fine.