Slender Mountain Mint (Pycnanthemum tenuifolium) of the Mint (Lamiaceae/Labiatae) family is a rhizomatous plant with narrow, linear leaves that give the plant an attractive, airy appearance. The genus name is based on Greek words for “dense” and “flower.” The specific epithet is from the Latin for “slender leaved.” The species is widespread across the eastern U.S. from east Texas and southeastern Nebraska to Maine and northern Georgia, excluding most of southern Georgia and Florida. In Arkansas, occurrence is statewide except for limited areas near the Mississippi River. Habitats may be dry to wet and sunny to partially shaded, such as prairies, roadsides, and open woodlands. It is also known as Narrow-Leaf Mountain Mint.
This herbaceous perennial grows in clonal colonies from skinny, near-surface, branching rhizomes (underground stems). New rhizomes (1/16 inch in diameter) are white, smooth, and segmented (each segment ¼ to ½ inch long), with an opposite pair of tiny triangular brown bracts subtending the segments. Rhizomes develop long fibrous roots along their distal end and, in spring, the tips of these rhizomes become emergent as single stems. During summer months, multiple new rhizomes grow from near the end of the “old” rhizome. A dense root mat may develop, especially in loose mesic soils. Dead stems persist into the new year.
Erect, rigid, glabrous, typically reddish stems elongate to 2 to 3+ feet tall, with short branches in opposite decussate pairs along their upper half. Each branch is subtended by a leaf. A lateral branch does not rebranch unless the stem tip has been nipped. Branching continues to the stem apex which, for a flowering stem, terminates with a single flower head. The ascending branches (the lowermost to about 5 inches long) diverge from the stem at about 60⁰. Stems are slender––near the ground, ⅛ inch or less wide, becoming wiry and thread-like distally.
All leaves are simple and linear. Larger leaves may be 2 inches long and less than ¼ inch wide, widest at or just below mid-leaf. From mid-leaf, the long-tapering margins extend to a more or less sharp tip; a shorter taper extends to a blunt sessile base. Leaves have a slightly recessed upper midrib and an expressed lower midrib with an obscure pair of parallel secondary veins on each side. Leaves are glabrous. Margins are entire (smooth) and slightly revolute (downturned). They turn yellow in fall. Rubbed or even crushed leaves can be nearly scentless or with a mild minty scent caused by pulegone. Plants are usually much less aromatic than other members of the genus. Lower leaves are early deciduous.
Flowering occurs in June into July. Clusters of flower heads (¼ inch wide) at the tips of the main stem and uppermost branches form a broad inflorescence to 3 or more inches wide. Each head consists of 20 to 50 densely packed flowers. An opposite pair of small linear leaves (about 5/16 long and 1/16 wide) occurs immediately below the sessile head. Whitish linear bracts of unequal size, in 1 to 2 series, form an involucre subtending the heads. These elongate-triangular bracts, lighter green than the leaves, are loosely imbricate with long spiny-looking tips (but not prickly).
Flowers bloom centrifugally, from the center outward. The tubular calyx (to ¼ inch long and 1/32 inch wide) is divided above into 5 ascending, triangular lobes. Relatively large tubular corollas are white to pale lavender with an unlobed upper lip and a 3-lobed, lavender-spotted lower lip. The upper lip and lobes of the lower lip are apically rounded, with lips widely spreading to expose the throat. Attached to the corolla tube are 4 slender white filaments with 2-lobed lavender anthers exserted above the corolla. After pollen release, anthers and filaments become shriveled and brown. From the base of the 4-lobed ovary, the slender white style, with a divided apex bearing the stigmatic surfaces, extends well beyond the stamens. Exterior of the corolla as well as its throat within bear a long twisty pubescence.
With fertilization, each flower produces 4 tiny (1/16 inch long), black, smooth nutlets that mature in September, while stems and leaves are still green. Dispersal of the 1-seeded nutlets may be by flowing water and strong wind. Heads persists on dead leafless stems into the next growing season.
Slender Mountain Mints are a fine addition to a wild or native garden. They are interesting and decorative as well as ecologically beneficial for a wide variety of insects seeking nectar. The species is drought tolerant and deer resistant. Self-seeding seems to be minimal, however plants can be easily propagated by division. (In loosened garden soils, Slender Mountain Mint has the potential to form vigorous colonies that may be difficult to remove due to dense root-mats.) In its natural environment, colonies persist for many years.
Five other species of Mountain Mint occur in Arkansas: 1) White-Leaf Mountain Mint (Pycnanthemum albescens), 2) Short-Tooth or Clustered Mountain Mint (P. muticum), 3) Hairy Mountain Mint (P. pilosum), 4) Whorled Mountain Mint (P. verticillatum), and 5) Virginia Mountain Mint (P. viginianum). Of these, Whorled Mountain Mint and Virginia Mountain Mint (both state species of conservation concern) are most similar to Slender Mountain Mint due to their narrow leaves. Slender Mountain Mint can be distinguished by its linear leaves less than ¼ inch wide and by its glabrous leaves and stems. Natural hybridization has been documented among native Mountain Mints, though, so difficult-to-identify plants may occasionally be encountered.
Article and photographs by ANPS member Sid Vogelpohl