The solution? Contain it.
Dig a hole big enough for a large flower pot. Because of my cold climate, I recycled an old plastic clay colored pot because the actual clay pots will crack with the weather extremes.
Next, sink the pot in the ground and fill it with potting soil or good quality soil and amendments. Mint grows well in a semi-shaded area. I'm adding this pot to my medicinal/kitchen herb garden just outside my kitchen.
Plant your mint directly in the pot. This will allow your mint to grow where you want it in the flower bed, but without the roots spreading beyond the borders of the actual pot. Note that I kept the top of the pot well above ground level so that the runners won't "jump the fence". Soon this will fill in beautifully and should last for years while staying tame.
To start more mint plants, just cut a stem of mint, place it in water for a few days and it will begin to send out small roots. When you have quite a few new roots, transfer it to potting soil and you have a new plant. (Sorry, I took a picture of this, but can't find it!).
There are so many wonderful varieties of mint; peppermint, spearmint, chocolate mint, pineapple mint, apple mint, ...
Mint tea... I can hardly wait!
