Spearmint (Mentha spicata) is a popular herb used globally for its sweet, mild flavor in culinary and medicinal traditions. The plant is a member of the mint family, Lamiaceae, known for its fragrant, serrated leaves and upright, square stems. Spearmint is a hardy perennial herb that reliably returns year after year, meaning gardeners do not need to replant it each spring.
The Perennial Nature of Spearmint
Spearmint is classified as an herbaceous perennial, meaning its above-ground growth dies back completely when temperatures drop, but its root system remains alive to regenerate in spring. This survival mechanism is centered on its extensive network of underground stems, known as rhizomes. These specialized, horizontal storage organs contain the energy reserves necessary for the plant to sprout new shoots.
The plant is remarkably cold-hardy, thriving in USDA Hardiness Zones 3 through 8 or 10. In these zones, the rhizomes can withstand significant winter freezing, allowing the plant to emerge with fresh foliage once the soil warms. The ability to spread via these rhizomes, rather than relying on seeds, makes spearmint vigorous in the garden.
Managing Aggressive Growth and Spread
While the perennial nature of spearmint ensures its return, the same rhizome system is responsible for its notorious, aggressive spread. If planted directly into a garden bed without restraint, the plant will quickly send out runners that can invade and overtake adjacent areas. This makes containment a necessary part of routine spearmint care for most gardeners.
The most effective strategy for managing this horizontal spread is to grow spearmint in a container. A pot or planter, preferably 8 to 12 inches deep, physically limits the reach of the rhizomes. This container can be placed on a hard surface, like a patio, or sunk into the ground.
When sinking a pot into the ground, it is wise to leave the rim of the container exposed by at least one to two inches to prevent rhizomes from escaping over the top edge and rooting in the surrounding soil. Another option is to use a root barrier, such as a plastic or metal sheet, buried 12 to 18 inches deep around the planting area to block the underground runners. Beyond root control, regular harvesting and pruning of the above-ground stems also helps manage the plant’s size and encourages bushier growth.
Preparing Spearmint for Winter Dormancy
As the growing season ends, usually with the first hard frost, the spearmint foliage will turn brown and die back, transitioning into winter dormancy. To ensure the survival of the rhizomes, cut back the dead stems to just a few inches above the soil line. This cleanup removes potential hiding spots for pests and diseases that could otherwise overwinter.
In colder USDA zones, applying a layer of protective mulch (straw or shredded leaves) over the soil surface helps insulate the root system. The mulch should be several inches thick to protect the shallow rhizomes from extreme temperature fluctuations. Potted spearmint requires greater protection because the exposed pot walls offer less insulation than the ground.
Potted spearmint can be moved to a sheltered, unheated location, like a garage or against a south-facing wall, to shield it from harsh winds and freezing temperatures. Alternatively, the entire pot can be buried in the ground for natural insulation. The dormant plant needs minimal care during winter, requiring only occasional, light watering to keep the soil from completely drying out.