Know Your Natives – Arkansas Beardtongue

Arkansas Beardtongue (Penstemon arkansanus) of the Plantain (Plantaginaceae) family, formerly of the Figwort (Scrophulariaceae) family, has showy white tubular flowers with a fifth stamen that is sterile, typically bearded (pubescent), and tonguelike in form, a hallmark of this large genus. The name Penstemon derives from the Greek words “penta” (five) and “stemon”(stamen). The specific epithet notes the species’ principal range in the Interior Highlands of Arkansas. It is also found in eastern Oklahoma, far-south Missouri, and far-south Indiana. Habitats include well-drained soils on sandstones and shales in open woodlands.

This herbaceous perennial has several to many stems in a tight clump growing from a tough, coarse rootstock. Springtime stems emerge from beneath previous years’ dead stems as well as from new basal plants that arise both from the center and the periphery of the rootstock. Stem bases that contact soil take root. Stems are unbranched except within the inflorescence. They have a grayish sheen due to a minute pubescence.

Photo 1: These three root/plant segments are from a plant that had a dozen such segments in a tight clump. New stems growing from base of dead stems can be seen on right.

Arkansas Beardtongue plants have slender stems to 24+ inches tall and ascending lanceolate leaves to 3+ inches long and ⅝+ inch wide. Leaves are dark green above and yellowish green below, and occur in opposite, decussate pairs (rotated 90⁰ degrees). Leaves are glabrous to minutely pubescent (particularly on the veins), slightly coriaceous (leathery), broadest at mid-leaf with a gentle taper to a rounded clasping base and a blunt-tipped apex. Distal margins have short, well-spaced, prickly teeth. Upper midribs are narrowly channeled while lower yellowish midribs are prominent. Leaves of new offset plants are elongate-spatulate, to 5½ inches long and 1 inch wide, with a broadly lanceolate blade tapering to a 2½ inch, partially winged petiole.

Photo 2: Stems of this spring-time plant, in a sunny site, are purple with a sheen of microscopic white pubescence. Clasping cauline leaves are in opposite pairs. Photo – April 5.
Photo 3: The new basal plant, growing from center of the rootstock, has spatulate leaves with partially winged petioles. The leaves are slightly leathery. Photo – May 18.
Photo 4: This two foot plant has multiple stems that branch only in the inflorescence. Photo – May 10.

Arkansas Beardtongue bears flowers for about a month in late April to early June––among the first of several species of Arkansas’ native penstemons to bloom. The inflorescence, a terminal, open panicle, consists of a stem bearing 6-14 cymes with 3-16 flowers per cyme. Cymes are 1¼-3½ inches long, the panicles to 6+ inches long. The panicle consists of several to a half dozen opposite pairs of ascending floral branches, subtended by small to tiny leaflike bracts. Flowers occur mostly in pairs between the bases of paired branches. Branches within a mature inflorescence tend to become entangled. Pubescence of the stems extends onto the inflorescence branches.

Photo 5: Display shows: 1) leaves of a basal plant with upper surface shown on left and lower surface on right, 2) upper surface of a stem leaf pair, 3) lower surface of a stem leaf pair, 4) upper surface of leaf pair at base of inflorescence (in inverted position), and 5) a segment of a floral branch with bracts.
Photo 6: The open panicles of these two stems comprise multiple cymes. Flowers between the bases of axillary branches have produced fruits while flowers on lateral branches are in bloom. Photo – May 15.

The white tubular flowers, with purple nectar guides, are to ¾ inch long with a 2-lobed upper lip and a 3-lobed lower lip. All lobes are rounded, the upper lobes smaller. The upper lip is recurved; the lower lip projects forward, a landing platform for insects. The 5 lobes terminate a gaping inflated throat that abruptly decreases in diameter to the tube. The nectar guides, extending from the center of lobes down the throat, are also evident on the exterior of the flowers. A pale green cup-shaped calyx, with a length and width of ⅛ inch (at flowering), bears 5 spreading triangular lobes.

Photo 7: A cyme with the main stem terminating with a flower pair (past anthesis), as growth continues via lateral branching. Pubescence becomes more noticeable toward and onto the flowers.

Flowers have 1 pistil (ovary + style + stigma), 4 fertile stamens (filaments + anthers), and 1 staminode, the “beardtongue.” The glabrous ovary, 1/16-inch-long and half as wide, has a raindrop shape. The straight thread-like 1/2-inch style, white with purple shading, bears a flat round stigma. The style is mostly hidden by the stamens. The white staminode (⅝ inch long), curving down from the upper side of the throat and positioned on the lower lip, faces upwards with spiky light-yellow hairs along the upper surface of its wider and flatter distal half. Style and filaments are hairless. Dense short glandular pubescence covers the exterior of corollas, calyxes, and pedicels.

The four fertile stamens (twice as thick as the style) occur as a longer pair (½ inch) and a shorter pair (⅜ inch). The staminal filaments are fused below to the floral tube. Their somewhat contorted positioning works to facilitate contact with pollinating insects that enter the flower. The white filaments of each stamen-pair curve away from each other in the throat and then recurve so that anthers face each other at the mouth of the tube, with the shorter pair positioned just below the longer pair, and all remaining beneath the upper lip. The dark purple anthers each consists of a pair of elongate lobes connected to the tip of the filament. At anthesis, anther lobes spread wide to expose a white surface of pollen.

Photo 8: Pedicels, calyxes, and exterior of flowers have glandular pubescence. Straight light-yellow hairs are on the distal half of the staminode. Anthers of the longer pair of stamens can be seen. Photo – May 28.
Photo 9: With corolla lips pulled back, the staminode projects beyond the anthers. Filaments attach to the purple side of the 2-lobed anthers. Lower anthers have yellowed. Purplish style is centered among the anthers.

Ovaries of fertilized flowers develop into teardrop-shaped hardened capsules. Capsules may be pale green to purple before drying to a bronzy brown in mid-summer. Sepals and style are persistent––the pointed tip of the glabrous capsule is a remnant of the style. Dry capsules, about ¼ inch long, split into two chambers from tip to base to release numerous tiny black seeds. The black seeds are irregularly shaped and about 1/16 inch wide.

Photo 10: This leaning stem, with entangled cymes, bears green to purple developing fruits. Inset photo shows dry infructescence on a grid of ¼-inch squares. Photo dates: main – May 15, inset – July 24.

For a garden, Arkansas Beardtongue is not as showy as other native beardtongues. Mature plants produce hundreds of flowers with nectar and pollen over a month-long period for small bees and other insects. Compact clumps expand slowly from year to year. If new seeded plants are not wanted, drying panicles are fairly easy to remove but may become lost in other vegetation. For a more tidy springtime appearance, removal of the upper portion of old stems may be needed (retain old bases for new stem growth).

In addition to Arkansas Beardtongue, seven other beardtongues occur in Arkansas, of which four have white flowers: Foxglove Beardtongue (P. digitalis), Nodding Beardtongue (P. laxiflorus), Pale Beardtongue (P. pallidus), and White Wand Beardtongue (P. tubiflorus). In Photo 11, P. digitalis and P. tubiflorus are shown with P. arkansanus, where differences in flower and sepal size and shape can be seen. Of the two remaining white-flowered species, P. laxiflorus can be distinguished by its persistent basal leaves, hanging inch-long pale lavender flowers with throats 2 times as long as the tubes, and orange staminode hairs on staminodes that are more exserted from the throat. Penstemon laxiflorus occurs primarily in the West Gulf Coastal Plain. The other species, P. pallidus, can be distinguished by its less complex branching habit with clustered flowers, dark-yellow staminode hairs, and densely pubescent stems and leaves with slightly longer pilose hairs. Penstemon pallidus occurs primarily in the Ozark Highlands and northern Crowley’s Ridge.

Photo 11: Display showing cymes of P. tubiflorus (left), P. digitalis (center), and P. arkansanus (right). As well as differences among flowers and calyxes, hairs of the staminodes are also distinctive.

Article and photographs by ANPS member Sid Vogelpohl

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