Foxglove beardtongue (Penstemon digitalis) of the Plantain (Plantaginaceae) family, formerly of the Figwort (Scrophulariaceae) family, is the largest of five white-flowered beardtongues in Arkansas. It is found throughout much of the eastern U.S. The genus name is from Greek words translating to “five stamens.” The specific epithet refers to the foxglove-like flowers of the genus Digitalis. Another common name is smooth white penstemon. The common name of the genus, “beardtongue,” describes the sterile, typically hairy (bearded) fifth stamen (a staminode) that is characteristic of all Penstemon species. Preferred habitats are well-drained, mesic, loamy soils in sunny prairies, sunny road drainages and partially sunny woodlands.
Plants have a thick clump of light colored fibrous roots and a woody caudex that may produce adjacent off-set plants. Ground-hugging basal leaves may survive winter and be present when spring growth appears. Mature caudices have multiple growth points that produce multiple stems 1 to 3 inches long bearing basal leaves only as well as one to several slender flowering stems from 3 to 4 feet tall. The unbranched flowering stems, becoming erect with maturity, are sturdy and very smooth (glabrous). The only pubescence on the plant occurs in the inflorescence.
Photo 1: In mid-March, along with over-wintering basal leaves, several rosettes of new leaves have appeared. New leaves are on stems that will either remain short with leaves only while other new leaves are of stems that will become tall and produce flowers.
Photo 2: In this mid-April photo, rapidly growing flowering stems are not erect but will become erect with further growth.
Basal leaves occur as closely spaced, decussate (rotated 90 degrees) and opposite pairs on the short stems. Leaves, with smooth margins, are a shiny medium green on the upper surfaces and a lighter green below. Basal leaves, with a total length to 11 inches, have a lanceolate blade to 5 inches long and a tapering winged petiole to 6 inches long. Young basal leaves and over-wintering leaves may have purplish shading, especially along petioles and lower blade surfaces. Leaf midribs are sunken above and sharply raised below. Widely spaced secondary pinnate veins curve gently toward leaf apex, but fade away without reaching leaf margin. Tertiary veins are obscure. Veins are the same color as leaf blade except lower-surface veins are a light yellowish green. Basal leaves persist after the flowering/fruiting stems have dried; a few that are ground-hugging persist into spring.
Leaves on flowering stems (cauline leaves) are in widely spaced (to 6 inches apart) opposite decussate pairs. These leaves, sessile to clasping, are lanceolate to oblanceolate, becoming small and elongate-triangular below the inflorescence. Generally, leaves have a rounded base and a long-tapering acute apex. Lowermost leaves may have wings that widen toward the base. Leaf length ranges from 8 inches, along lower portion of stem, to 2 inches and less, just below the inflorescence. Margins tend to be finely dentate, with teeth of lower leaves widely spaced and those of upper leaves more closely spaced. Leaf coloration is about the same as that of basal leaves. Venation is also about the same, but secondary veins are more closely spaced.
Photo 3: Two basal leaves (11 inches long) are displayed at bottom of photo with other leaves being cauline leaves. Upper leaf surfaces shown on left and lower surfaces shown on right.
In mid- to late-spring, the terminal inflorescence occurs as opposite pairs of branched ascending clusters (cymes), beginning about 6 inches above last leaf pair. Three to five opposite pairs of primary cymes tend to compose an inflorescence with spacing between pairs decreasing upwards to a cluster of flowers. Cymes are generally branched into secondary cymes. Primary and secondary cymes are subtended by decreasingly small linear bracts that have broadened, clasping bases. Each division of a cyme tends to have one to three flowers per branch. A terminal inflorescence has a length to 8+ inches and a width of up to 3 inches.
Photo 4: The terminal inflorescence is an elongate cluster of cymes. Photo taken in mid-May.
Foxglove beardtongue’s cream colored flower buds have a bulbous appearance before opening as white (sometimes with purplish shades) swollen corollas to 1½ inches long. Corolla tube abruptly enlarges to become strongly two-lipped (bilabiate), the lower lip with three larger lobes, the upper with two slightly smaller lobes. All five lobes, broadly rounded at their distal ends, have a similar appearance and similar length. Corollas, more broad than high, with a nearly flat lower inner surface, may have a few longitudinal purple veins (insect guides) along lower portion of the tube. Flowers are perfect (both male and female parts) with four fertile stamens and a pistil along with a prominent staminode. Two pairs of stamens, attached to the corolla tube, arise from the flower’s center so that their anthers are positioned at the top of the enlarged portion of tube. Anthers are held in see-saw fashion at tips of filaments. Style, straight with the small stigma, is centered between and in close proximity to the two anther pairs. The staminode, centrally positioned at the bottom of tube, has scattered long spiky hairs near its distal end. Spiky hairs are also scattered along lower surface of throat. Corollas are set in a small bell-shaped, medium-green calyx edged with five narrowly-triangular ascending to flaring lobes. Exterior of corolla, calyx and pedicels are densely covered with short, sticky, glandular hairs. Filaments, staminode and style are white, as are the glandular hairs. Anthers produce white pollen.
Photo 5: Large corollas are set in relatively short calyxes rimmed with five narrowly-triangular lobes. Glandular hairs cover exterior of corolla and calyx. A staminode can be seen in flower near photo-center.
Photo 6: Display shows a bud, an open complete flower and a flower with most of corolla removed. Note shape of flower, small calyx, glandular hairs, two-part anthers and straight hairs at distal end of staminode.
With fertilization, ovaries enlarge to form raindrop-shaped capsules that taper to the apex tipped by the drying style. Capsules enlarge to about three times longer than calyx, turning light brown at maturity and splitting from tip to base to release numerous tiny, angular, irregularly ridged, brown seeds.
In a garden or natural area, foxglove beardtongue would stand out due to its height, large widely spaced stem leaves and its showy foxglove-type flowers. Flowers may be present for a month in mid-spring. Timely removal of seed capsules can prevent any undesired self-seeding. Foxglove beardtongue has been listed by the Missouri Prairie Foundation as a “Grow Native!” plant (recommended for home landscapes).
Four other white-blooming beadtongues occur in Arkansas: Arkansas beardtongue (Penstemon arkansanus), nodding beardtongue (Penstemon laxiflorus), pale beardtongue (Penstemon pallidus), and white wand beardtongue (Penstemon tubaeflorus). Foxglove beardtongue can be distinguished by its 1) large size (plant and flowers), 2) glabrous nature, except for its glandular hairs on and near flowers, 3) foxglove-like corollas with lobes that are similarly flared and with similar length, and 4) a staminode that has scattered long straight hairs near its distal end.
Article and photographs by ANPS member Sid Vogelpohl