Know Your Natives – Downy Phlox

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.

Photo 1: This mature plant has several gnarled taproots and smaller ropy roots. A stem-remnant from the previous growth-year can be seen (red arrow). New stems may grow directly from the rootstock or from base of old stems (yellow arrows). Photo – August 19.
Photo 2: Basal portion of stems survive through winter. Short leafy branches may develop which survive through the winter, dying with spring-time growth. Photo – January 9.

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.

Photo 3: Initially, opposite leaf pairs are closely spaced. Leaves and stems are initially purplish, becoming medium green with maturity. Photo – March 3.
Photo 4: New winter-time growth of plants with many stems is ground-hugging. Early growth of branches can be seen at bottom of photo. Photo – March 20.
Photo 5: The simple linear leaves of this non-flowering stem have revolute margins, prominent midveins and obscure secondary veins. Middle leaf on left is 4½ inches long and ½ inch wide. Photo – June 9.

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.

Photo 6: Lanceolate floral bracts become increasingly smaller toward apexes of stems and branches. Elongate flower buds are tightly rolled. Photo – April 30.
Photo 7: Flowering stems often have several branches; all in bloom at about the same time. Flower shape and color vary. Blue flowers in background are Crested Iris. Photo – April 18.
Photo 8: Apexes of corolla lobes are generally rounded to varying degrees. Corolla lobes of the white-flowered plant here have minutely pointed apexes. Anthers remain within the throat. Photo – April 30.
Photo 9: This many-stemmed plant has white flowers with prominent purplish nectar guides. Photo – April 30.

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.

Photo 10: Stem, pedicels, floral bracts and calyx of this plant have dense glandular pubescence. Calyx lobes are erect in their lower portion and spreading in their upper portion. Photo – June 9.
Photo 11: With filaments fused to the tube at varied points, anthers are staggered. Flowers have three-part fused pistils with spreading stigmas. This flower is ¾ inch wide and 9/16 inch long. Pistil is ⅛ inch long.

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.

Photo 12: Flowers, on straight pedicels, grow directly from axils of floral bracts. Flowers are arranged in panicles. Bracts and calyx lobes have a similar appearance. Photo – May 27.
Photo 13: Dry calyx lobes recurve sharply to fully expose seed capsules. The spherical, light tan capsules contain several brown to dark brown seeds. Seed surfaces are covered by papillae. Photo – May 27.

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.

Photo 14: Downy Phlox with other spring-time plants, including: Wild Geranium, Celandine Poppy and Wild Comfrey. Photo – April 30.
Photo 15: Downy Phlox is a great nectar sources for butterflies and moths, including the showy Eastern Tiger Swallowtails (Papilio glaucus). Photo April 10.

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

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