Found this on mint:
Propagating Mint
The end of each runner grows just under the surface until it develops pale leaves that break up into the sun and begin to grow another plant. Nodules along each runner also form branches that break upward and form new plants. These new plants can be separated from the parent with a sharp spade, dug up and transplanted to a new location. Another method of propagating mint is to lay a plant down and bury it, forming a runner. New plants will emerge above the nodules on the main branch where the lateral branches begin. Once established, mint is quite vigorous and rather invasive, so it's wise to contain your mint patch by sinking barriers about a foot into the ground around the area where you want your mint to grow. Mint grows best in full sun, so it can be grown against a wall on the south or west side of a house or garage. Once established, mint will spread to fill the area allotted for it.