Know Your Natives – Ditch Stonecrop

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.

Photo 1: In this wetland habitat, ditch stonecrop grows with arrowhead (Sagittaria platyphylla).

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.

Photo 2: Display of young plants showing a single emergent erect stem with well-spread leaves and several submerged leafy basal branches. Rhizomes have a pointed lower tip, as identified by red arrow above center plant. Photo – November 8.
Photo 3: By raising this clump of muck, water drained off to expose the leaf-shrouded basal stems on these three plants, each with a single aerial stem. Photo – November 8.

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.

Photo 4: These plants (foreground) have not developed the leafy basal branches because their bases are not submerged. Plants in mid-background are cardinal flowers (Lobelia cardinalis).

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.

Photo 5: The two outside leaves are aerial stem leaves with upper surface shown on left and lower surface on right. The basal branch, at center, has a basal-stem-leaf at its left (upper surface) and its right (lower surface). Large leaf on left is 6 inches long and 1½ inches wide.

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.

Photo 6: This group of plants is at the blooming stage. A plant may have several floral branches below the terminal inflorescence. The plant with the red asterisk appears to have three especially long floral branches. Photo – August 17.

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.

Photo 7: In addition to the single flower that terminates the stem at the base of the inflorescence (partially hidden in photo), flowers extend in secund (1-sided) fashion along coiled arms. Convex stigmas terminate post-like styles. Note sepals, dense glandular pubescence, and tiny leaf-like bracts along the inflorescence axes. Photo – July 27.

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.

Photo 8: Fruiting capsules, which may become reddish, have a prickly appearance due to persistent styles and stigmas (black in photo). Flowers have a central “empty” disk that may aid with insect pollination. Photo – November 8.
Photo 9: This inflorescence arm (tip at lower left) has dried and the conical styles are starting to drop off (see capsule at base and 4th capsule from lower left). Several cap-like dropped styles are displayed above the arm, along with seeds. Squares = ¼ inch. Photo – October 16.

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

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