Know Your Natives – Obedient Plant

Obedient plant (Physostegia virginiana*) of the Mint (Lamiaceae) family is an attractive plant with showy late summer to early fall flowers. The genus name is from Greek words for “bladder” and “cover” in reference to the inflated calyx that covers maturing fruit. The specific epithet refers to the type locality, Virginia, the provenance of the specimen originally described by Linnaeus. This species occurs from Texas to North Dakota, thence eastward to the Atlantic and Gulf coasts, along with scattered (and perhaps introduced?) occurrences in New Mexico, Utah and Montana. In Arkansas, obedient plant occurs throughout much of the Interior Highlands of the Ozarks, River Valley and Ouachitas, along with scattered occurrences on Crowley’s Ridge and several south-central Coastal Plain counties. The common name “obedient plant” refers to the plant’s flowers obediently remaining to one side when pushed. Another common name is false dragonhead, from similarity to species of Dracocephalum (not found in Arkansas). Obedient plant occurs in sunny prairies, glades and partially sunny woodland openings, in moist to wet, fairly well drained, fertile soils.

Obedient Plant - Physostegia virginiana
Photo 1: Established plants begin to produce basal leaves in mid-winter. Photo – February 20.

Obedient plant is a glabrous (hairless) herbaceous perennial with radiating, shallow, whitish roots and many segmented rhizomes (in some forms*). Rhizomes terminate in a bud or two that will produce clonal stems the following spring. Shiny, dark green, stout stems, to 3 or more feet tall, are four sided with prominently raised, rounded corners. Decussate (alternating at 90 degree angles) leaves and branches, originating between the corners, are typically in matched pairs. Between leaf and branch pairs, stems appear segmented. Segments may be up to 1½ inches long lower on the stem and to ½ inch long higher up. Each stem segment is straight and equidimensional except for a short, enlarged, lighter green portion immediately below the next higher leaf or branch pair. Plants in less sunny areas may have many branches while those in less sunny sites or less fertile sites may be unbranched. The stout stems are erect and tough, but the entire plant may recline under weight of the inflorescence. With drying soils, lower leaves may drop, while flowering continues.

Obedient Plant - Physostegia virginiana
Photo 2: This pair of stems was produced by one rhizome (extending off to the lower left). Current-year rhizomes and rhizome buds can be seen along with a vegetative bud (between the stems). Rhizome roots emerge at lines of segmentation (nodes). Stem bases are ¼ inch across. Photo Sept 29.

Leaves, dark green above and light green below, are lanceolate with very gradual tapers from mid-leaf to the acuminate apexes and sessile bases. Leaves of large plants tend to be broadly lanceolate (to oblanceolate) to 5 inches long and 1 inch wide, while leaves of smaller plants are narrower. Leaf margins along the distal half of leaf have sharp, well-spaced serrations with tiny prickle-like tips, while serrations lower on leaf gradually reduce in size toward the stem. Leaf axils that are not subtending a branch or floral spike have rudimentary buds that can break dormancy if a distal portion of the plant is broken off.

Mature stem leaves are pinnately veined, with secondary veins unnoticeable on the upper side and slightly noticeable below. The light green upper midvein is channeled. The lower midvein is whitish green and strongly expressed. Secondary veins arch off the midvein and become parallel to the margins before fading away.

Obedient Plant - Physostegia virginiana
Photo 3: In this sunny site, the plant has many branches, each of which will produce one or more terminal floral spikes. Photo Jun 16.

Floral spikes with sessile flowers terminate the main and secondary branches and also grow directly from leaf axils. Blooming occurs for a month or more from late summer into early fall. Terminal spikes tend to be straight while lateral spikes curve upward. Lanceolate, decussate bracts, with entire margins, are widely spaced along the spike below and become tighter above. Lowermost bracts may subtend rudimentary buds while bracts along most of the spike subtend individual flowers. Bracts, at anthesis, are about half as long as calyxes (see below). Flowers are sessile (no pedicels). Terminal spikes may be 8 inches long with as many as 100 flowers, while lateral spikes are shorter with correspondingly fewer flowers. Flowers on the main stem and lateral branches reach anthesis at the same time, with flowering proceeding along the spike from base to apex. Individual corollas are open for several days and then drop from the receptacle, leaving behind the calyx and developing fruit.

Elongate, round-tipped buds open into puffy, light pink to lavender (sometime white) corollas that are up to ½ inch wide and 1¼ inches long. Corollas have a constricted, laterally flattened base set within a relatively small (to ¼ inch long and ⅛ inch wide), conical, 5-lobed calyx. From their constricted base, corollas abruptly flare into a broad, puffy tube that narrows slightly at the throat’s broad opening. Corollas have an arching “top-rib” along the proximal two-thirds, which strengthens the rather flimsy structure and provides for attachment of stamens (see below). The throat opening has a 3/8-inch-long projecting, slightly hooded rounded upper lip and a half-as-wide recurved lower lip with two small lateral lobes and a broader central lobe. Flower color is more intense in the areas of the lobes and along the top-rib while fading elsewhere, including the throat which is marked with a splatter of dark purplish spots. At anthesis, the length of the calyx is longer than the subtending bracts.

Obedient Plant - Physostegia virginiana
Photo 4: Anthesis proceeds from spike base to spike apex. Note the relative length of bracts and calyxes along the spike. (A green lynx spider [Peucetia sp.] can be seen at the base of the spike. It is a hunting spider, often preying on flower-visiting insects.)

Flowers have four straight, slender stamens and a straight,  slender style. Stamens are adnate (attached) to the underside of the corolla’s top-rib (see above) with a side-by-side pair positioned to either side of a central style that is held against the top of the throat by the stamens. Anthers dangle just below the outer edge of the upper lip with a divided (bifurcated) stigma in between and slightly farther out. The two-lobed anthers, when releasing pollen, have a circular appearance (similar to a horse’s hoof). Filaments have soft shaggy hairs (villous) on their upper portion. With fertilization, a flower produces four dull brown, smooth 1-seeded nutlets with rounded “sides” and two angled sides. Nutlets, less than ⅛-inch long, are dispersed by gravity.

Obedient Plant - Physostegia virginiana
Photo 5: Flowers have a bifurcated stigma with a longer upper portion that angles up while the lower portion angles sharply down. Note the two-lobed anthers and villous filaments.
Obedient Plant - Physostegia virginiana
Photo 6: Lower flowers have dropped off as upper flowers bloom. A bumblebee fits snugly into the throat, so that there is an excellent chance that pollen will be deposited on its back. The style connects to a tiny 4-lobed ovary deep within the calyx.

Obedient plant is an attractive wildflower, ideal for a sunny to partially shady garden or natural area with moist to mesic soil. However, in a preferred habitat, this plant can spread aggressively from rhizomes. Plants may need staking when in bloom to remain erect and seeded spikes may need to be removed to better control plant spread. The very attractive flowers persist for a month or more in late summer into early fall. It is favored by hummingbirds, bumble bees and carpenter bees. (Carpenter bees are not pollinators because they “steal” nectar by cutting through the base of the flower.)

Other species of the same genus in Arkansas are: narrowleaf obedient plant (Physostegia augustifolia), foxglove obedient plant (Physostegia digitalis), and slender obedient plant (Physostegia intermedia). Physostegia virginiana can be distinguished by the combination of its relatively late flowering time and wider leaves with sharply toothed margins. The most similar species is Physostegia angustifolia, but it has narrowly lanceolate leaves (usually less than ½ inch wide) and primarily blooms in late spring to early summer.  Physostegia digitalis also has wider leaves, but they are less toothed and their bases distinctly clasp the stem.

  • Physostegia virginiana has multiple forms of which two have been classified as subspecies, namely, Physostegia virginiana subsp. praemorsa and Physostegia virginiana subsp. virginiana. Subspecies praemorsa is said to have larger flowers, to form clumps, and to favor upland prairies and glades in comparison to subspecies virginiana which is said to have smaller flowers, to form colonies via rhizomes, and to favor wetter bottomlands and streambanks. Wild Arkansas plants have all been classified as subspecies praemorsa by most authorities, but there may be several different forms in the state, and it remains unclear as to what names best apply (perhaps both subspecies listed above, perhaps varieties or species synonymized under those two, or perhaps as yet undescribed entities).  To confound the issue, the species is commonly cultivated and cultivars may escape into the wild.  The species is in need of further field and taxonomic study.

Article and photographs by ANPS member Sid Vogelpohl

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