The Penstemons are the 3rd most prolific genus in North America. Also known as beardtongues, they have 3/4 to 2 inch long trumpet shaped 5-parted flowers with flaring lobes, five stamens and specifically the flower throat is open with a flat base. The stem will have opposite, stalkless leaves.
The Foxglove or White Beardtongue is a perennial, probably introduced to the state long ago, whose stems are erect, smooth and purplish color with a height of one to four feet.
Leaves are quite variable, basically spatulate to lance shape, broad at the base where they touch the stem (no stalks) or may be slightly clasping. They have a shiny appearance and usually have fine teeth on the edges. The central leaf vein is prominent. The plant has a basal rosette of leaves that are stalked - these remain after the stem leaves have dropped off and the seeds have matured.
The inflorescence is considered to be an interrupted thyrse, that is, a branched inflorescence in which the main axis is raceme like but with lateral branches like cymes, a structure where each branch ends in a flower. The cymes are 5 to 12 flowered, with ascending branches, placed 2 per stem node and arise from opposite bracts on the stem, looking somewhat like a whorl, but actually a structure known as a 'verticillaster' and most prevalent in the Mint family. The entire inflorescence forming a cone shape atop the stem. All branches of the inflorescence are sparsely to densely glandular-hairy in the upper parts only.
The flowers have a white corolla, sometimes with purple veining for nectar guides. The corolla tube of five connected lobes abruptly inflated forming an upper lip with two rounded lobes and the lower lip has three. The flower tube is longer than the flaring lobes but short enough to let the stamens be accessible within the lobes. There are 4 true stamens in pairs of different lengths, with whitish smooth filaments and a single smooth style which hug the upper part of the corolla tube. A fifth, a larger staminode (infertile or false stamen) lies at the bottom of the tube with a slightly curved tip hairy with whitish hairs while the filament has more yellowish hair. The green calyx has 5 long-pointed lobes (4-8 x 2-3 mm) that are reflexing. The outer surface of the corolla and the calyx have glandular hair.
Seed: Flowers mature to brown clusters of ovoid seed capsules up to 14 mm long which then split open to release numerous dry brown flattened angled seeds. These are shaken out of the capsule by the wind. Seeds need 30 days of cold stratification to break dormancy. If stratified, plant when the ground is cool - early spring after the last hard frost or in the fall and let winter take care of the dormancy period. Seeds are very small - 130,000 to the ounce.
Habitat: The plant grows best in full sun, but will tolerate some shade (if you don't mind stems leaning toward the sun) with adequate moisture in loamy type prairie soils. It grows from a fibrous root system and forms a woody caudex. It will self-seed.
Names: Penstemon is from the Greek pente, or "five," and stemon, refers to "stamen." Digitalis refers to the finger shape of the flower. Penstemons used to be in the Figwort (Scrophulariaceae) family but contemporary Botanists have now placed the Penstemons into the family Plantaginaceae. The name 'Beardtongue' refers to the hairy tip of the sterile stamen that is visible in the corolla. The author names for the plant classification are as follows: 'Nutt.' refer to Thomas Nuttall (1786-1859) English botanist who lived and worked in America from 1808 to 1841. On his many expeditions he collected many species that had been originally collected by Lewis and Clark but lost by them. His work on this species was incomplete for publication and was updated and republished by 'Sims' which refers to John Sims (1749-1831), English botanist, first editor of Curtis’s Botanical Magazine, founding member of the Linnean Society, plant collector and author. In older literature this species may be listed as P. laevigatus Ait. var. digitalis.
Comparison: The closest relative is P. calycosus where the calyx lobes are slightly longer, but much narrower (5-9 x 0.8-1.9 mm), the corolla pale lavender or violet, and the pollen sacs are usually smooth. This species is not found in Minnesota but overlaps in many states east of the Mississippi River. Another Beardtongue which has pinkish to lavender blue flowers is the Showy Beardtongue, Penstemon grandiflorus Nutt., but there the flowers are clustered in 2s or 4s along the upper stem leaf axils; and Pale Beardtongue where the stems and flowers are hairy, and the flowers are white and smaller with the entire plant much shorter. There is a nursery cultivar with deep purple stems, leaves and light pinkish-purple flowers called "Husker Red".
Above: The inflorescence is a conical thyrse with verticillasters atop the stem.
Below: Note the delicate veins of the nectar guides in the flower tube and the fine glandular hair on the outside of the corolla and on the flower stalks. There is fine white long hair on the tip of the sterile stamen - those and the upper branches of the thyrse are the only parts of the plant with hair.
Below: 2nd photo - The fibrous root system of Foxglove Beardtongue.
Below: 1st photo - The stem leaves are sessile (no stalks), shiny green and have small teeth on the edge. 2nd photo - The rosette of basal leaves, which are stalked and remain as the plant forms seeds and the stem leaves have dropped away.
Below: The seed capsules of the Beardtongues are ovoid in shape and contain numerous flattened angled seeds.
Below: An extensive grouping on the Goldenrod Trail at the far east end of the Upland Garden.
Notes: Foxglove Beardtongue is indigenous or naturalized to the area near the Garden. Eloise Butler introduced Foxglove Beardtongue to the Garden on April 30, 1915 with 6 plants sourced from Horsford's Nursery in Charlotte Vt. Then on June 21 and June 29, same year, she sourced more from right within Glenwood Park (which surrounded part of the Garden and may have come from the Park Board Nursery located there) and more in 1917, same source. In 1919 another dozen came from Horsford's. Martha Crone noted planting this species several times, totaling 80 plants, in the fall of 1937. Like Eloise, she used the prior scientific name of P. laevigatus Ait. var. digitalis, which is now treated as a synonym for P. digitalis. She did not list her source for the plants. However, by the time of her 1951 inventory of plants in the Garden, she did not list it, but she did list four other Penstemons, (two of which are considered native in Minnesota): Large-flowered Beardtongue (P. grandiflorus) (native), Slender Beardtongue, (P. gracilis) (native), Western White Penstemon (P. confertus), and Blue Western Penstemon (P. caelestinus). The name Penstemon caelestinus is no longer accepted, instead use Penstemon albertinus Green, the Alberta Penstemon.
In North America Foxglove Beardtongue is considered native to the eastern half of the U.S. except Florida, but considered introduced in the lower Canadian Provinces where it is found.
Native or introduced? The Minnesota DNR lists P. digitalis as found in five counties of Minnesota, including Hennepin where the Garden is. Only Houston County has an adjoining population across the Mississippi in Wisconsin. It is currently on the DNR "watch list". The DNR nor U of M Herbarium authorities note it as introduced, thus leaning toward native or long-since naturalized.
The four Penstemons definitely considered native to Minnesota are: P. grandiflorus, Large-flowered (Showy) Beardtongue; P. albidus, White Penstemon; P. gracilis, Lilac Penstemon; P. pallidus, Pale Beardtongue.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"