Whether you’re planting it in the ground or a pot, peppermint loves rich soil with a pH of between 6.5 and 7.0. Mulch helps it stay moist and enriches the soil. For indoor peppermint plants, add some compost to the top of the soil every few months. The good thing about peppermint is that it's very cold-hardy. As long as it has good drainage and is maintained properly, it will do well.

While they are adaptable and somewhat forgiving, peppermint can't stand being overwatered or extremely dry. The best course of care is to do a soil finger test: press the tip of your finger into the soil. If it feels dry down to the first knuckle, your plant needs water. To provide that humid touch, mist peppermint plants in between waterings, especially if you live in a drier climate or are growing indoors.

Peppermint is a great companion plant for brassica vegetables, such as cabbage and cauliflower. It’s also great to plant with onions, carrots, tomatoes, and roses, just to name a few. Peppermint naturally deters insects, such as flea beetles, that chew holes in leafy green vegetables. Its smell confuses carrot root flies and onion flies, and it keeps aphids away from tomatoes and roses.

The constant use of fertilizer on peppermint plants is common in commercial cultivation. Due to the constant cutting of the plant, they need a good balance of nutrients, such as phosphorous and nitrogen, to help the leaves grow quickly. A small, potted peppermint plant benefits from a little support in spring when it pushes out new growth. During that season, use some liquid fertilizer once a month or every six weeks.

If you already have a healthy mint that you want to propagate, it’s a relatively simple process. Cut about 3.5 inches of the young stem, right above the next set of leaves. Place that stem in a glass of water and leave it for a couple of weeks in an open location with plenty of air and light. Once an abundant root system has formed, it’s ready to be potted. If you have more than one seedling and are planting in a garden, make to plant them at least 1.5 feet apart.

If you want to germinate peppermint from seeds, sow them indoors in the winter so your seedlings will be ready to plant in the spring. You can so sow the seeds directly in the garden; wait until spring and make sure the bed is lined with wood to prevent the plants from running wild and taking over. It may take about a year for the plant to settle before you can harvest.

Once your peppermint stems are 7 or 8 inches tall, you can snip a few sprigs and leaves. Mint loves to be pruned regularly, but if you haven't touched it in a while, the best way to know that it needs a good shearing is if the leaves look shorter than normal. That's when you can cut it back by about a third and give it a chance to focus its energy on developing new growth.

Dusty brown spots on peppermint leaves are a sign of mint rust, a fungal infection caused by Puccinia menthae, which causes stunting or death. If this rust turns black, the fungus is ready to produce spores that will infect the rhizomes — the horizontal underground stems — and the surrounding soil. The best way to deal with this disease is to discard the infected mint and plant a new patch. Surviving rhizomes have to be washed in water no hotter than 111 degrees Fahrenheit for 10 minutes, then cooled in cold water and replanted.