Slender Mountain Mint
Pycnanthemum tenuifolium
Mint family (Lamiaceae)

Description: This native perennial plant is 1-3' tall, branching frequently to create a bushy effect. The slender stems are hairless. The slender opposite leaves are up to 3" long and ¼" across. Each leaf is sessile, linear, and hairless, with a prominent central vein and smooth margins.

The upper stems terminate in small flat heads of flowers. The short tubular flowers are white, often with scattered purple dots, and individually about ¼" long. The corolla is divided into an upper lip and a lower lip with three lobes. The reproductive structures of each flower are white, except that the anthers are purple. The calyx is divided into several slender green lobes. The blooming period is early to mid-summer, and lasts about 1–1½ months. There is no floral scent, although the foliage has a mild mint scent and somewhat stronger minty taste. The small dark seeds are without tufts of hairs, but are small enough to be dispersed by gusts of wind. The root system consists of a taproot and rhizomes. Slender Mountain Mint can spread vegetatively, forming colonies of closely bunched plants.

Cultivation: The preference is full sun and moist to slightly dry conditions. This plant often grows in rich loam, as well as soil containing rocky or gravelly material. Foliar disease is less troublesome for this mint species than many others. The leaves may assume a yellowish appearance during a major drought. This is an easy plant to grow.

Range & Habitat: Slender Mountain Mint occurs occasionally in every county of southern and central Illinois, but is less common and more sporadic in northern Illinois (see Distribution Map). Habitats include moist to slightly dry black soil prairies, moist meadows and gravelly areas along rivers, openings in woodlands, moist thickets, acid gravel seeps, limestone glades, and abandoned fields.

Faunal Associations: The flowers are very attractive to many kinds of insects, including long-tongued bees, short-tongued bees, wasps, flies, butterflies, skippers, beetles, and plant bugs. These insects usually seek nectar. Among the wasps, are such visitors as Thread-Waisted wasps, Bee Wolves (Philanthus spp.), Scoliid wasps, Tiphiid wasps, Sand wasps, Spider wasps, and Eumenine wasps. Flies visitors include Soldier flies, Syrphid flies, Mydas flies, bee flies, Thick-Headed flies, and Tachinid flies. The seeds are too small to be of much interest to birds. Mammalian herbivores usually don't browse on this plant because of the minty taste; the foliage may contain anti-bacterial substances that disrupt the digestive process of herbivores.

Photographic Location: The photographs were taken at Meadowbrook Park in Urbana, Illinois.

Comments: This plant has a delicate, somewhat airy appearance. Slender Mountain Mint closely resembles Pycnanthemum virginianum (Common Mountain Mint), except that the former has hairless stems and leaves that never exceed ¼" across. Common Mountain Mint, on the other hand, has lines of white hairs on its stems, and some of the larger leaves will exceed ¼" across. This latter plant tends to be taller, stouter, and less branched in appearance; it also blooms a little later in the year. The photographed flowering plant is still in the bud stage, while the photographed flowerheads are beginning to bloom.

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