Mulch your mint patch, if desired. Mentha grows too vigorously for weeds to bother it much, but if your garden space lacks adequate water or dappled shade, mulch helps keep the herbs cool and moist.
Don't worry about fertilizing your mints. The herbs adapt just fine to almost any kind of soil, so save your compost and fish emulsion for pickier plants.
Unless your mints live in containers or grow in the Dust Bowl, watering is not a concern. Mints can take care of themselves! But do check the soil of your potted-up herbs, including mints, as container soil dries out much more quickly than garden soil.
Keep pruning, or pinching back, the tips of the plants to encourage healthier, bushy growth and plenty of leaves. The plants, especially peppermint, quickly get a leggy, scraggly look when left unpruned. If possible, however, let some of the plants flower. The purple blossoms look lovely in any garden, and make a unique addition to salads, herbal vinegars, jellies and jams.
Even if you do nothing, the plants will simply die back in the cold months and emerge hale and hearty the following spring. For neatness' sake, though, you may want to trim the plants close to ground level in the autumn.
For the colder months, consider potting up a bit of mint for an indoor container. Keep the mint watered but not overly moist, and place it where it will receive some shade. As you might have guessed, however, indoor mint plants are not much fussier than their outdoor cousins.