White-Leaf Mountain Mint (Pycnanthemum albescens) of the Mint (Lamiaceae) family is a tall, upright perennial forb with a strong minty scent and has an open, gangly structure. The genus name is based on Greek words for dense (pyknos) and flower (anthos) in reference to its flower clusters. The specific epithet is from a Latin word (albescentem) for “becoming white” in reference to the uppermost leaves and bracts. Its primary area of occurrence extends from the Florida panhandle, across southwestern Alabama, Mississippi (excluding the lowlands), Louisiana (excluding the lowlands), east Texas, east Oklahoma, south Missouri and Arkansas. In Arkansas, it occurs statewide except for some portions of the Mississippi Alluvial Plain. Habitats are variable from partial shade in open woods, to full sun in prairies, from pinelands to oak woodlands and from mesic to dry soils that may be sandy to rocky to clayey.
During summer months, new rhizomes grow from the sides of rhizomes that developed the previous growth year, near the current year’s stem. These smooth, white rhizomes have decussate nodes bearing rudimentary buds that do not develop unless the rhizome is damaged. In early winter, the growing tip appears as a new stem, old stems having died and began decaying. With growth of a new stem, the “umbilical” rhizome dies and a separate clonal plant with a retained portion of rhizome which becomes and develops wiry, fibrous roots. Colonies tend to be somewhat open and isolated one from another. The new year’s stem growth has purplish stems and ovate, petiolate, early-deciduous, purplish-green leaves; basal leaves are absent.
Mature stems are square in cross-section with distinct corners and flat sides. The erect stems have widely spaced, opposite leaves with internode lengths to 4 inches that are fairly equal, decreasing nearing stem apex. Stem leaves subtend ascending to wide-spreading branches that may be from near-zero-inch long (only a pair of leaves is seen) to 8 inches long; lengths quickly shortening near stem apex. A 4-foot stem, with a base-width of ⅛+ inch, may have up to 8 axillary branch pairs (primary branches) along its upper portion. Primary branches, square along their proximal portions, have base-widths less than half that of stems. Primary branches have one to several opposite leaf pairs with axillary buds which may develop into very short secondary branches bearing a single leaf pair. Leaves along primary branches orient toward sunlight and become aligned along sides of the branches. Stems and branches are covered by soft, dense, spreading pubescence which decreases with age. Lower primary branches typically terminate with a leaf-pair while upper primary branches often terminate with flower clusters. Leaves, branches, and flower clusters at the same node tend to be about the same size. At the end of the growing season, the hollow stems die back to the rhizome and readily break off and mostly decay by late spring.
The simple leaves of a mature stem are lanceolate to broadly lanceolate and to 3¾ inches long (including a ½ inch petiole) and ⅞ inch wide. Leaves of the branches are usually half that size. Leaf blades may taper onto petioles. The smallest leaves have entire margins while larger leaves’ smooth margins are interrupted on their upper half to two-thirds by 4-8 shallow teeth as blade width decreases to acuminate apices. Pinnate venation, recessed above and expressed below, is well spaced. Secondary veins, mostly opposite but may be alternate, extend from the mid-rib at about 45⁰ and become parallel to leaf margins as they fade. The lower end of secondary veins may trend along midrib, toward petiole. Leaves near the inflorescences (floral leaves, herein) are strongly whitened above and below while other leaves (green leaves, herein) are olive-green above and whitish green below, with midribs of floral and green leaves a lighter shade. Floral leaves have dense, white, appressed, matted puberulent pubescence on both surfaces. Green leaves have short hirsute pubescence above and minute, short-matted pubescence below along with relatively long marginal (ciliate) pubescence. Leaf pubescence extends onto the petioles where it transitions to the longer, spreading pubescence of the stem or branch.
From July into September, the inflorescence consists of terminal and lateral clusters on stems and branches. Single clusters, to ⅞ by ⅝ inch, are axillary to pairs of opposite leaves so that an opposite pair of single clusters, often, visually merge into a double cluster. Each single cluster is on a stalk that is ⅛ to ¼ inch long. Clusters (single and double) are broadly rounded when viewed from above and disk-like with a flat bottom and slightly convex top when viewed from the side. A single cluster is composed of 2-3 cymes consisting of one to several pedicellate flowers positioned between a lateral pair of opposing “arms” bearing multiple pedunculate flowers along a curved stalk. A final opposite whitish leaf pair subtends a double cluster with further divisions of arms and cymes being subtended by increasingly small leaf-like, spade-shaped, whitish bracts followed by linear to filiform whitish bracts which occur along and at the terminus of the cymes. Thus, double clusters are ringed by whitish leaves, leaf-like bracts, and bracts and clusters have a spiky appearance. Blooming flowers are seemingly randomly scattered across a cluster because pedicellate flowers bloom first, followed sequentially by the pedunculate flowers.
Flowers have a white, wide-flared corolla consisting of a lower lip with a broad central lobe and 2 elongate lateral lobes and an upper lip with a single lobe. Purple spots are scattered across the lower lip with the greatest concentration being across the cupped central lobe. From the front, corollas are about ¼ inch tall and ⅛ inch wide across the lower lobe while the upper lip is about ⅛ inch long and 1/32 inch wide. Flowers are glabrous on the outside while throat is pubescent. Upper lip is positioned along a straight-line with the axis of the calyx while lower lip lobes curve downward sharply. Lips unite to form a tube that is inserted into a pale green, terete calyx with a widened top and pinched base. Calyces are 3/16 inch long and 1/16 inch wide. Calyces (see Photo 12) have 2 lips; an upper lip that is 2-lobed and a lower lip that is 3-lobed; sometimes all lips and/or lobes cannot be seen. Lobes are stubby-triangular. Calyces (as are pedicels, peduncles and adjoining stalks) are densely covered with white, appressed, puberulent pubescence.
Flowers have 4 stamens (filament + anther) and a pistil (ovary + style + stigma). The white filaments (adnate to the sides of the throat) and style are fine-thread-like. The elongate 2-lobed anthers, balanced at the filaments’ tips, are exerted from the corolla. The strongly exserted anthers change from white, to orange (with pollen release) to brown. The style, as anthers dry, extends slightly beyond the anthers and unequally divides slightly at its tip to expose 2 unequal stigmatic surfaces. The tiny green ovaries (too small to photograph) have 4 ovules which mature to 4 wheat-shaped, brown nutlets in the fall.
In regards to gardening, considering its gangly appearance during most of the growing season, White-Leaf Mountain Mint may be best suited for a naturalistic garden. The near-inch-wide flower clusters have numerous tiny flowers which appear delicate when viewed up-close. White leaves and bracts associated with the inflorescence causes clusters to be showy. Plants do not seem to be aggressive spreaders, either by root or seed, but is easily propagated by rooted rhizomes. It has a fine minty scent when touched and provides nectar and pollen to insects for a month or more. White-Leaf Mountain Mint is not a preferred food by deer.
Other species of Pycnanthemum that occur in Arkansas are: Short-Tooth or Clustered Mountain Mint (P. muticum), Hairy Mountain Mint (P. pilosum), Slender or Narrow-Leaf Mountain Mint (P. tenuifolium) and Virginia Mountain Mint (P. viginianum). White-Leaf Mountain Mint is most similar to Clustered Mountain Mint, but White-Leaf Mountain Mint can be distinguished by its lanceolate to broadly lanceolate leaves and flower clusters that are more loosely arranged with noticeable, interspersed bracts. Additionally, bloom period of Clustered Mountain Mint is about a month earlier.
Article and photographs by ANPS member Sid Vogelpohl
