Downy Phlox (Phlox pilosa*) of the Phlox (Polemoniaceae) family is a semi-evergreen perennial forb with showy flowers of variable color and shape. The genus, Phlox, is derived from the Greek word meaning flame – a reference to flower color of some species. The specific epithet, pilosa, is from Latin meaning “hairy” – a reference to the plant’s pubescence. In the U.S., it occurs in an area bounded from south-central Texas and eastern Oklahoma, north along the eastern boundaries of states from Kansas to North Dakota, then eastward to New Jersey, south along the Atlantic coast to central Florida and then back to Texas; principally excluding the Appalachian Mountains and portions of the lower Mississippi River floodplain. In Arkansas, occurrence is basically statewide. Habitats vary from mostly sunny mesic to dry sandy to rocky sites in open woodlands, prairies and meadows. It is also known as Prairie Phlox.
Plants develop stubby, branched taproots supported by smaller ropy roots and secondary ropy roots which may grow from basal stem-nodes with ground contact. The terete stems, few to many, grow to 18+ inches long. New stems originate overwinter from axillary buds located at nodes at the base of old stems or directly from the root crown. New stems may be erect or ground-hugging and spreading before becoming erect. They tend to be purplish before becoming medium green at flowering. In similar sunny sites, at the time of bloom, plants with few stems have an open structure while those with many stems have a round-mounded structure. The slender stems are stiff, rather tough and densely short-pubescent with hairs that may or may not be gland tipped. The main stem and axillary branches along the upper portions of the stem bear flowers. Late in the growing season, after seed dispersal and dying of flowering stem/branches, short, leafy, secondary branches may develop along the remainder of the stem. These secondary branches tend to survive over winter but die with new spring-time growth.
Leaves occur in opposite pairs – – adjacent pairs rotated 90⁰ (decussate pairs). Initially, nodes of leaf pairs are practically touching along the stem, but nodes become widely spaced with stem growth. Leaves vary from linear to lanceolate and oblanceolate to ovate with rounded to truncate bases and acuminate, sharp apexes. The simple leaves, which grow to 4½ inches long and ¾ inch wide, lack petioles (sessile leaves) and have smooth (entire) margins. New leaves have deeply furrowed midribs above and prominent revolute (curled under) margins while mature leaves become more planar. The initial purplish shading of new leaves becomes medium green above and a pale green below. Other than the midrib, venation is obscure above and below. Leaves are moderately to densely pubescent with short white hairs on upper and lower surfaces. Hairs on the margins of leaves (ciliate hairs) and midrib hairs tend to be longer. Leaf pairs, extending into the inflorescence as floral bracts, become increasingly small. With drying conditions, basal leaves drop off.
Downy Phlox flowers from April into June. Flowers on the stem and branches are on short, straight, stem-like, pubescent pedicels growing directly from axils of floral bracts. Early in the bloom-period, the inflorescence appears to be of terminal dome-shaped clusters. Clusters, composed of up to 50 flowers, can be up to 3 inches across. As the stem elongates, the panicle-nature of the inflorescence becomes apparent. (See Photo 12.)
The flat-faced flowers grow to ½ inch wide and ¼+ inch long and have five petal-like lobes united at the base into a long slender tube. In bud, lobes are in a tight elongate roll atop the straight tube. The pink to purplish or white flowers typically have a patch of contrasting colors at lobe-bases enhanced by an outline of a lighter color (nectar guides), along with a darkened throat. Nectar guides on white flowers may not be noticeable. Corolla lobes, with unnotched apexes, may be obovate to oblanceolate – lobes of some plants have sharp points. Exterior of flower may be glabrous or the tube may have scattered pubescence.
Flowers, set in pubescent calyxes, have five stamens (filament + anthers) and a three-part fused pistil (ovary + style + stigma) with a nectary disk at the pistil’s base. The white filaments are fused to the tube at varied points so that the yellow, elongate anthers are staggered. Anthers, remaining within the tube, produce yellow pollen. The white style, set atop a smooth pale green ovary, terminates with a three-part spreading stigma positioned well below the anthers. Calyxes, to ½ inch long, have a short cupped tubular base with five very long, lanceolate lobes. Calyx lobes are erect in their lower portion and widely spread in their upper portion. Calyx lobes, which tend to have a purplish tinge, have dense glandular or non-glandular pubescence. Calyx lobes and floral bracts are similar in appearance.
Following fertilization, ovaries develop into green spherical fruits in late spring. With drying, capsules become light-tan and calyx lobes recurve sharply. The three-chambered, smooth capsules contain several brown to dark brown seeds each with a rounded side and two flat sides. Seed surfaces have irregular bumps (papillae). Seed capsules that drop from plants while intact are buoyant and surface water flow may aid with seed dispersal.
In regard to gardening, Downy Phlox can be a specimen plant or share space with other spring-blooming plants in rock gardens, cottage gardens, or prairie settings. Plants grow well in various dry to mesic, well-drained soil in full sun to partial shade. Its early growth and variable flower shape and color and seed capsules add interest. Plants spread by seed but not annoyingly so. It is a great nectar source for moths, butterflies, and hummingbirds as well as a pollen source for small bees.
Other species and subspecies of the genus reported in Arkansas are: Broad Leaf Phlox (Phlox amplifolia), Sand Phlox (Phlox bifida subsp. bifida), Starry Sand Phlox (Phlox bifida subsp. stellaris), Wild Blue Phlox (Phlox divaricata subsp. laphamii), Annual Phlox (Phlox drummondii) [native to Texas but considered introduced in Arkansas], Smooth Phlox (Phlox glaberrima), Perennial Phlox / Garden Phlox (Phlox paniculata), and Moss Phlox (Phlox subulata) [introduced in Arkansas but native to portions of the Northeast U.S.]. Of these species, Downy Phlox is most similar to the comparably sized, pubescent Wild Blue Phlox. Downy Phlox can be distinguished by 1) its later bloom period, 2) linear to lanceolate leaves, 3) flowers being variably colored, and 4) lack of clonal sterile stems. Wild Blue Phlox flowers earlier in the spring, has broad leaves, has consistently blue or bluish-purple flowers, has sterile clonal stems, and lacks a tap root.
*Numerous subspecies of Downy Phlox have been described and named by various authorities over the years, and the USDA Plants Database currently recognizes nine based on type, length and location of hairs and leaf shape. In Arkansas, two subspecies are generally recognized: 1) Downy Phlox (Phlox pilosa subsp. pilosa) and 2) Ozark Downy Phlox (Phlox pilosa subsp. ozarkana) based on leaf shape and presence or absence of glandular hairs. Some cryptic other or undescribed subspecies may also occur.
Article and photographs by ANPS member Sid Vogelpohl