Ditch stonecrop (Penthorum sedoides) of the ditch stonecrop (Penthoraceae) family is a short-lived, herbaceous perennial of wetlands. The genus name originates from the Greek words for “five” and “marker” in reference to the pentamerous (five-part) floral structure. The specific epithet translates to “resembling sedum,” also in reference to floral structure.* In the U.S., ditch stonecrop is the only species of Penthorum; a second species is known from Eurasia. Ditch stonecrop occurs throughout the eastern U.S. to eastern Texas, Nebraska, and eastern North Dakota. In Arkansas, it occurs statewide. Habitat preference is full to partially sunny sites with shallow, standing to slow-flowing water, where mucky soils remain wet in low-rainfall conditions, such as ditches, floodplain depressions, pond margins, and marshes.
Young plants, growing during the summer months, have a single aerial stem above water as well as leafy basal branches that arise from submerged rhizomes. Basal branches, up to six inches long, remain parallel to the soil surface with ascending tips. White, string-like secondary roots descend from the main root and the basal branches. With plant maturity, these become dark and matted.
Erect stems, 1 to 2+ feet tall at maturity, are terete, slightly ridged, and slightly zigzagged between the nodes (this more prominent on new growth). They are light green to pale red (in more sunny sites). Floral branches arise near the stem apex, each subtended by a leaf. Stem pubescence is sparse, coarse below and becoming finer distally. Inflorescence branches are glandular pubescent. An aerial stem apparently dies soon after it has produced fruit.
Plants that do not have submerged bases do not develop basal branches. If an upper portion of a stem becomes submerged, it develops basal-branch look-alikes.
Stem leaves are alternate and lanceolate-elliptic, to 6-7 inches long and 1½ inches wide, the blade tapering to a pointed tip and base. Largest leaves occur mid-stem. Blades are glabrous and sessile to short-petiolate, with fine, mucronate (tipped) serrations along their margins. Alternate, tightly clustered leaves of basal branches are about 1 inch long and ½ inch wide. Lower leaves of the aerial stems drop-off as tips of the basal branches continue to grow.
The inflorescence develops over the summer months. It consists of 2-4 flower-bearing “arms,” 1-sided racemes (technically cymes), extending from the tip of the main stem and, often, 1-2 axillary floral branches immediately below. Flowering begins at the basal flower and extends distally as the racemes uncoil from their initial fiddlehead shape. The eventually straight, ascending and arching floral branches bear several to 15 flowers on minute (< ⅛ inch) pedicels. Floral branches may be up to 2 inches long and have few to 20+ flowers. Floral branches below the terminal racemes may be 1¾ inches long and may bear one to several lanceolate leaves that are to 1¼ inches long and ½ inch wide. Along with the dense glandular hairs on floral branches, noted above, tiny deeply serrated bracts occur. The inflorescence may become reddish with fruiting.
Flowers have a bowl-shaped hypanthium or floral cup, which is to 3/16 inch across and ⅛ inch deep, bearing 5 triangular green sepals. The greenish-white flowers, typically lacking petals, have 5 (sometimes 6) pistils (ovary, style and stigma) united below and 10 stamens. The pistils are arranged in a central ring with the stamens both alternate and opposite to them. Each pistil comprises a single carpel. Each ovary has central placentation, a prominent post-like style (with a wide conical base), and a large convex stigma. At anthesis, styles are erect but flare outward in fruit, as the shiny greenish white stigmas become black.
Fruits retain the form of the flowers with the hypanthium, sepals, and 5 projecting styles persistent. The ⅛ inch long styles give the 5-carpellate fruits a prickly appearance. As fruits develop, the entire floral structure may become reddish, ultimately turning tan to dark brown at maturity. Seed dispersal occurs when the style with its conical base drops off a chamber to allow numerous tiny oblong (1/32 inch long) white seeds to drop away.
For a garden or natural area, ditch stonecrop needs a site that is continuously wet to occasionally flooded, such as a water garden, drainage, or pond margin. Plants would provide cover for water-based insects and amphibians. Ditch stonecrop is a good native plant for aquariums because of its leafy basal branches, which grow naturally submerged.
* Sedums are in the Stonecrop (Crassulaceae) family, in which ditch stonecrop was once included. There are four species of sedums known from the wild in Arkansas: the native Nuttall’s stonecrop (Sedum nuttallii), widow’s cross (Sedum pulchellum), and woodland stonecrop (Sedum ternatum), and the non-native yellow stonecrop (Sedum sarmentosum). All have succulent leaves.
Article and photographs by ANPS member Sid Vogelpohl