When you invite mint plants (Mentha spp.) into your garden or backyard, they turn into the quintessential houseguests that just won't leave. Most types of mint spread by underground shoots called rhizomes. This includes both peppermint (Mentha x piperita) and spearmint (Mentha spicata), two of the most popular garden mints in the U.S. To rid your yard of mint, you'll have to dig out those rhizomes or else prevent the plants from growing by smothering them.
Removing Rhizomes and Roots
With care and patience, you can dig up and remove all of your mint plants, including their rhizomes. Mint rhizomes are white when they grow underground and can be reddish, green or purplish when on the soil surface. About as thick as cooked spaghetti, a mint rhizome sprouts tufts of thin white roots about every 1 inch along its length. Dig up the rhizomes and roots on a dry day when the soil is moist but not wet; the task can be done any time of year. Push a garden fork into the ground about 1 foot from the base of the mint plant. Lever the fork upward, loosening the soil clump, and gently pull the rhizomes and roots out of the soil. The removed plant parts should be set aside for trash disposal. Repeat the procedure across the mint-infested area, working backward so that you don't tread on dug-up soil. Remove every rhizome and root piece you see, no matter how small it is.
Smothering the Plants
A layer of light-excluding material can be used to starve mint plants and kill them. In early spring, when the mint plants are 3 to 4 inches tall, cut them to ground level with a weed trimmer or hand shears. Spread lightproof material, such as newspaper four sheets thick or landscape fabric, over the mint-infested area. If you use newspaper, overlap the sheets by at least 4 inches. Spread a 3-inch-thick layer of wood chips, shredded bark or other organic mulch on the newspaper or fabric to hold it in place. In late fall or the following spring, remove the covering or incorporate the remains of the newspaper and mulch into the soil. By that time, most of the newspaper will have decomposed.
Preventing the Plants From Returning
Whichever elimination method you use, mint plants sometimes reappear. After you get rid of mint plants, wait one month before before replanting in the affected area. Closely check the soil for new mint shoots every three to four weeks during the growing season, and pull up all shoots that appear, removing as much of the root system as possible. Mulching bare soil in the affected area with a 3-inch-thick layer of organic mulch deters weak mint shoots and prevents mint seeds from sprouting. Garden compost, leaf mold and shredded bark are suitable organic mulches to use.