Know Your Natives – Late Boneset

Late Boneset (Eupatorium serotinum) of the Aster, Sunflower, or Composite (Asteraceae) family is a tall herbaceous perennial with small, clustered, whitish flowerheads that lack ray florets. The genus name recognizes Mithridates Eupator who invented a “universal antidote” against poisoning and is said to have used a species of the genus in medicine. The specific epithet translates as “developing late” in reference to the time of flowering. The species is wide-ranging, from the Big Bend area of Texas to Illinois, east to the Atlantic and Gulf Coasts, and including most of Florida. In Arkansas, it occurs statewide. The common name “boneset” originates from past use of some species of the genus to relieve pain. Plants are common in a variety of open to partly shaded habitats––prairies, woodlands, rights-of-way, roadsides and fallow fields––on mesic to dry soils.

Young single-stem plants have a stubby central rootstock with long near-surface spreading fibrous roots. At the end of the growing year, stems die with new stems developing the following spring from bases of the old stems. Over time, this short-lived perennial develops a gnarly central rootstock supported by a mass of wide-spreading fibrous to ropy light-tan roots. As new stems develop, the older central portion of the rootstock decays.

Photo 1: New stems grow from base of dead stems as the central portion of the rootstock decays. Width of that portion of the root mat shown is 7 inches. Photo – August 14.

Plants, lacking basal leaves, have one to several stems that may be 2-6+ feet tall, those in mesic soils being taller. Early in the growing season, stems tend to be purple, but later become medium green with faint, reddish ridges. Main stems are straight and erect, with their upper half to two-thirds bearing opposite pairs of spreading to ascending, axillary (primary) branches that decrease distally in length. More robust plants branch more profusely. All branches typically terminate in an inflorescence. Near the branch tips, leaves and branches may become sub-opposite or even alternate. Stems and branches are uniformly puberulent.

Photo 2: As an herbaceous perennial, new spring growth originates directly from the rootstock. Plants do not have basal leaves. Photo – March 28.
Photo 3: Leaves are in matched decussate pairs, except that those near the tips of branches may be sub-opposite or even alternate. Photo – May 10.
Photo 4: Stems are erect throughout the growth-year. As shown, branches have begun to grow along the stem’s upper portions. Stems, early in the growth year, are a solid purple. Photo – June 3.

Leaves are decussate, petiolate, and broadly lanceolate to lanceolate, becoming narrower above. A typical larger lower stem leaf may be 9½ inches long (including a 2½-inch petiole) and 2½ inches wide. An upper leaf, at the base of the inflorescence, may be 1⅝ inches long (including a ¼-inch petiole) and ¼ inch wide, while the uppermost leaves (subtending the final floral branches) become tiny and almost linear. Margins of larger leaves are boldly serrate while those of smaller leaves become entire. Margins taper gently to acute apexes. Upper leaf surface is glabrous, lower surface and underside of petioles puberulent. Upper surface is a shiny green, lower surface a dull pale green with the main veins yellowish green. Venation, recessed above and expressed below, is longitudinal with 2-4 secondary veins parallel to the midrib. Petioles are swollen and clasping at the base. Lower stem leaves that do not subtend a branch, tend to subtend a rudimentary branch or tuft of leaves. With drying conditions, lower leaves drop off.

Photo 5: Paired decussate branches are subtended by paired decussate leaves. Note stem puberulence and somewhat clasping bases of petioles. Photo – July 12.
Photo 6: Lower stem leaves, mid-stem leaves, and upper-branch leaves, the upper surfaces shown on left, lower surfaces on right. Larger blades are widest just above base. Venation is longitudinal. Leaf at left is 7¼ inches long, including a 1⅞ inch petiole.

Plants flower from mid-August into October. Spreading clusters of small flowerheads arranged in corymbs at the branch tips produce a broad, flat-topped inflorescence. Corymbs, with 6-10 closely spaced flowerheads, bloom outward from the base. Each head comprises 8-15 disk florets; ray florets are absent. Inflorescence branches, peduncles and pedicels are clothed in a fine white puberulence.

Photo 7: Display of parts of a 5½ foot stem. The ¾-inch-diameter stem at left, lower leaves having dropped off, was at ground level. Paired branching (lower right) is consistently repeated into the inflorescence (upper center). Photo – August 8.
Photo 8: The overall inflorescence tends to be flat-topped. The inflorescence is an insect magnet and provides a hunting ground for this White Banded Crab Spider (Misumenoides formosipes). Photo – September 24.
Photo 9: Terminal clusters are composed of several corymbs, each having 6-10 flowerheads, each composed of 8-15 disk florets. All parts of the inflorescence below the corolla are densely puberulent. Photo – September 6.

Disk florets, with whitish, 5-lobed, tubular corollas, are set in a cylindrical involucre of up to a dozen imbricate, elliptical phyllaries, about ⅛ inch long, in 1-2 series. Purplish stamens remain hidden within the corolla throat. The white style pushes upward through the ring of fused anthers, at first displaying the pollen, and ultimately dividing and spreading to receive pollen from other florets.

Photo 10: Disk florets, with white, 5-lobed, tubular corollas, are packed in a cylindrical involucre. Stamens and bristly pappus are hidden at this stage of floral development. The divided style arms extend well beyond the corollas. Photo – September 6.

After fertilization, corolla, stamens and style are shed from the inferior ovary as the pappus of 20+ white hairs or bristles dries and radiates from its apex. Ovaries mature into dark brown, cylindrical, slightly ribbed achenes, less than 1/16 inch long, dispersed by wind.

Photo 11: As the stem dies, the pappus radiates from the top of the achenes, providing lift and wind dispersal. Photo – November 13.

In considering Late Boneset for a garden, there is a good chance that volunteer plants are already there. Multiple plants may create a weedy appearance, but individual plants can be attractive as specimens, especially where associated with other native plants of differing texture, structure and color. Bonesets, mostly avoided by deer, are attractive to many insects. Removal of the dying stems from this short-lived perennial before seed dispersal would help control self-seeding.

Photo 12: In this garden setting Late Boneset grows with False Aloe, Dittany, Rose Vervain, Hairy Blazing Star, and Hairy Skullcap. Photo – July 18.

Late Boneset is one of some 20 species and recognized hybrids in the genus Eupatorium that occur in Arkansas. Many of these other species have white flower heads that are similar to those of Late Boneset. The leaf characteristics of Late Boneset––the undivided blades, long petioles and marginal serrations––separate it from most of those other species.

Article and photographs by ANPS member Sid Vogelpohl

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