Back in November, after some of the other herbs had died for the winter, the mint began to take over the space where the basil once was. The mint also spreaded around the herb bed in general, with stems curling around any open space along the ground. We had cut some of the other herbs’ stems down to the base of the soil, including thyme, sage, and chives, because their stems had died from the winter temperatures. The mint continued to grow in the cold weather, and take over any space the dying plants had left open, until below freezing temperatures in January caused the mint to die for the winter as well. However, all the perennials will grow back from their roots once it is spring again. Meanwhile, annuals such as basil will have to be replanted.
The expansion of the mint plant was not unusual behavior. Mint plants start growing as a small, round bush shape. Then, the mint will send out new stems called runners horizontally across any open space, following the top of the soil. New sets of roots and leaves will begin to grow vertically at different spots along the runners, and these sets of roots and leaves are called rhizomes. While mint can take over a garden if allowed to grow many runners and rhizomes, there are ways to keep mint plants small and controlled. You can cut back the runners and pull the rhizomes out of the soil regularly. Leaves on the rhizomes can be harvested and eaten just like the leaves on the main plant. You can also place mint in a separate container or place a small wall around it so the runners do not wrap themselves around other plants in your garden.