Mint care: growing spearmint and peppermint in containers

Light

Mint is more flexible about light than most herbs. NC State lists spearmint for full sun or partial shade, and says peppermint actually prefers partial shade and needs a minimum of only three hours of sun a day. Missouri Botanical Garden rates peppermint for full sun to part shade. In a hot southern summer, an east-facing spot with morning sun and afternoon shade keeps the pot from drying out so quickly.

Indoors the problem is too little light, not too much. The University of Illinois Extension measures roughly 300 foot-candles right at an east or west window and 1,500 in front of a south one, so set an indoor pot as close to bright glass as you can. The RHS suggests a sunny windowsill for potted mint brought in for winter harvests. Stretched, pale stems mean it wants more.

Water

Unlike rosemary or lavender, mint likes its roots moist. The RHS asks for compost kept evenly moist, Missouri Botanical Garden rates peppermint for medium to wet soil, and NC State notes spearmint does not do well in dry soil. Water whenever the surface begins to feel dry, soaking the pot until water runs out of the bottom.

Containers are the catch. The RHS points out that pots dry out much faster than ground soil, especially in hot, dry weather, so check them regularly through the growing season. Moving a pot into partial shade during a heat wave cuts water use without hurting the plant. A mint that wilts badly usually perks up within hours of a good soak, but repeated drying stresses it and pushes it to flower.

Moist does not mean waterlogged. Use a pot with drainage holes and empty the saucer, because roots sitting in water for long periods still suffer, especially indoors in winter when growth is slow and the mix dries more gradually.

Soil and pH

NC State describes ideal mint soil as organically rich, well-drained and moist, and the RHS suggests multipurpose or soil-based compost for containers. A good bagged potting mix with a few handfuls of compost works well. NC State gives peppermint's best pH as 6.0 to 7.0, which most potting mixes already meet. Pick a wide pot rather than a deep one; the University of Maryland suggests 12 to 16 inches in diameter, which leaves the rhizomes room to run.

Humidity

Ordinary household humidity is fine. The more common issue is fungal disease when foliage stays wet in still air: NC State lists rust and leaf spot as the main problems on spearmint. Space pots so air moves between them and water at the base. Penn State Extension notes that forced-air heating makes winter rooms very dry, and grouping herb pots together helps if indoor mint dries at the leaf edges.

Temperature

Mint is tough. NC State rates spearmint hardy in USDA zones 4 through 9, and Illinois Extension calls mints extremely hardy perennials. Outdoors in winter the top growth dies back, and the RHS explains it regrows every spring, so a pot can stay outside in most of the country, ideally sheltered against a wall where the root ball will not sit frozen and waterlogged.

For a pot you bring indoors to keep harvesting, Penn State Extension recommends herb rooms of 65 to 70°F (18 to 21°C) by day and 55 to 60°F (13 to 16°C) at night, and notes that most herbs survive the mid to low 40s. Keep it away from heat vents, which dry it out fast.

Fertilizer

Mint needs little feeding. In a pot of rich mix it grows vigorously on its own, and extension advice for herbs in general is to feed lightly, because heavy fertilizer produces soft growth with less flavor. Penn State Extension suggests a low dose of water-soluble fertilizer every 2 weeks for herbs indoors. Outdoors, a pot repotted into fresh compost each spring often needs nothing more. Stop feeding when growth slows in late fall.

Work out the dose for your watering can with the fertilizer dilution calculator →

Repotting

This is the chore mint owners skip, and it is why many pots decline. The rhizomes fill the container within a few seasons, the center dies out and shoots get smaller. Illinois Extension recommends dividing mints every three to four years to keep them vigorous, and the RHS says the same: repot into a larger container, or divide the clump and replant one section back into the same pot.

Spring and fall are the best times. Slide the plant out, cut the root ball into pieces with a knife or spade, throw away the old woody middle and pot up the young outer sections in fresh mix.

• Rhizomes circle the inside of the pot and the center of the clump dies out
• Shoots get shorter and leaves smaller each year
• Runners escape over the rim or through the drainage holes

Propagation

Few plants root as easily. The RHS notes that stems placed in a jar of water soon produce roots, and softwood cuttings from new spring shoots take readily in compost. Division in spring or fall gives an instant full-sized plant. You can also pot short sections of rhizome at any time of year; the RHS suggests doing it indoors in November for fresh mint by Christmas.

Seed is the least reliable route. Many named mints, especially peppermint, rarely set seed, according to Missouri Botanical Garden, so buy plants or take cuttings to get the flavor you want.

Methods: Division of the clump, Stem cuttings rooted in water, Sections of rhizome, Seed (variable) · Best season: division in spring or fall; cuttings from spring shoots; rhizome sections any time of year.

Common problems

Rust is the disease to watch: the RHS describes dusty orange, yellow and black spots on the leaves, and NC State lists it alongside leaf spot as the common fungal problems on spearmint. Cut infected stems right down and dispose of them. Aphids sometimes cluster on soft tips, and regular harvesting removes most of them. Missouri Botanical Garden otherwise reports no serious pest or disease problems for peppermint, noting only that it is an aggressive spreader.

Not sure which one it is? Try the plant problem diagnosis tool →

Is the mint toxic to cats and dogs?

The ASPCA lists mint (Mentha species) as toxic to dogs, cats and horses. The toxic principle is the essential oils, and the clinical signs are vomiting and diarrhea with large ingestions. A pet that nibbles a leaf is unlikely to get sick, but keep pots out of reach of a dog or cat that chews plants. The RHS warns that pennyroyal (Mentha pulegium) is harmful if eaten, so buy culinary mints from a reputable source. Catnip and catmint (Nepeta) are not true mints; the ASPCA lists catnip separately as toxic to cats, with vomiting and diarrhea possible.

Check any other plant against the ASPCA lists →