Description: This is a native perennial plant up to 3' tall and branching frequently, often with a bushy appearance. The green or reddish stems are strongly four-angled and have scattered white hairs along the ridges. The opposite leaves are up to 2½" long and narrowly lanceolate or linear. They are sessile, and have smooth margins. The largest leaves are ¼ - ½" across. When damaged, the foliage releases a strong mint scent.
Numerous flattened heads of small white flowers (often with purple dots) occur at the ends of the upper stems. Each head is up to ¾" across and can contain up to 50 flowers. However, only a few of these are in bloom at the same time, beginning with the outer circle of flowers and ending towards the center. Each tubular flower is about 1/8" long and 2-lipped. The blooming period occurs during the middle of summer and lasts about a month. Each small flower produces 4 tiny, finely pitted, dull black seeds. These seeds are distributed to some extent by the wind. The root system produces rhizomes, which spread a short distance from the mother plant. Soon, a small colony of plants are formed vegetatively.
Cultivation: The preference is full or partial sun, and moist to average conditions. The soil can contain loam, sand, clay, or gravel this plant is not fussy about soil texture. During drought, the lower leaves will turn yellow and fall off. This plant is easy to grow, and less subject to foliar disease than some other mints, such as Monarda spp. However, stressed out plants sometimes succumb to rust.
Range & Habitat: Common Mountain Mint is widely distributed in Illinois, but is uncommon or absent from southern Illinois and a few western counties. It is occasional to locally common in moist to mesic black soil prairies. Other habitats include moist sand prairies, moist meadows in woodland areas, thickets, fens, swamps, and rocky bluffs. This is probably the most common Mountain Mint in Illinois.
Faunal Associations: Many insects are strongly attracted to the flowers, including various bees, wasps, flies, small butterflies, and beetles. Typical visitors from these groups include honeybees, Cuckoo bees, Halictid bees, Sphecid wasps, Eumenine wasps, bee flies, Tachinid flies, Wedge-shaped beetles, and Pearl Cresecent butterflies. Most of these insects seek nectar. Mammalian herbivores and many leaf-chewing insects apparently find the mint fragrance of the leaves and stems repugnant, and rarely bother this plant.
Comments: The name 'Mountain Mint' is something of a misnomer, because this plant and the majority of other members in this genus do not usually occur in mountainous habitats. Common Mountain Mint is similar in appearance to Pycnanthemum tenuifolium (Slender Mountain Mint). It can be distinguished from the latter species by the white hairs along the ridges of its stems, and the occurrence of leaves greater than ¼" across. Another difference is the presence of a strong mint fragrance in the crushed leaves of Common Mountain Mint, while the leaves of Slender Mountain Mint usually have a milder scent.