Know Your Natives – Toothed Spurge

Toothed spurge (Euphorbia dentata) of the Spurge (Euphorbiaceae) family is a drought-tolerant summer annual with milky sap and a complex, bizarre inflorescence called a cyathium. The genus name recognizes Euphorbus, a Greek physician.* The specific epithet is from the Latin for “toothed” in reference to the leaf margins. The common name “spurge” derives from the Latin for “purge,” from the use of Euphorbia species as a purgative. In the U.S., toothed spurge occurs primarily from northeastern Colorado to Pennsylvania, south to southern-most Texas and east to Alabama and the Carolinas, as well as in disjunct populations in Arizona and California. In Arkansas, the species occurs statewide. Habitat preference is dry to mesic sandy to clayey soils found in woodland borders and openings, prairies and disturbed habitats, such as fields and roadsides. It is also called summer poinsettia and green poinsettia (and was formerly named Poinsettia dentata).

Toothed spurge, germinating with warm temperatures, varies considerably in size and structure, as determined by both habitat and summer weather. In most habitats, stems and branches are a pale green, but they may be reddish in full sun, with plant height varying from several inches to 3 feet. Plants have a single stem with a branched tap root supported by descending fibrous roots. The terete stems are straight, stout and erect. Larger plants have lateral branches set-off the stem at about 45⁰. Plants typically do not have sub-branches of an appreciable size.

Leaves, in opposite decussate pairs lower on the stem, may be in whorls at mid stem and again opposite or alternate on the upper stem. Robust plants may bear multiple branches of varying size from enlarged nodes. Internodes vary from ½ to 3½ inches. At the time of flowering, with lower leaves dropping off early (caducous), leaves are concentrated at ends of branches where they are associated with the inflorescence (see below). Lowermost lateral stems are longest with higher lateral stems gradually decreasing in length. When damaged, stems and leaves exude white sticky acrid sap.

Photo 1: This young plant has characteristic simple toothed leaves. Lower leaves are in opposite decussate pairs. Photo – May 9.
Photo 2: Plants generally have an open structure as a result of long internodes and caducous lower leaves. As shown by inset, mid-stem nodes may be prominent and support “extra” branches. Photo – September 21.

Leaves are simple and petiolate with considerable variation in shape and size. Generally, lower stem-leaves are ovate-elliptic with a wedge-shaped (cuneate) base and acuminate to acute apex. Leaves on branches may become elliptic-lanceolate and those on plants in full sun lanceolate. A large elliptic leaf may be 4 inches long and 1¾ inches wide, including a 1-inch petiole. Leaf blades are generally medium green above and yellowish green below, although in sunny habitats leaves may be reddish above (or with a few randomly scattered purplish-red blotches) and greenish red below. Leaf margins, somewhat crinkly, vary from shallowly toothed on elliptic leaves to sub-entire on lanceolate leaves. Venation is offset-pinnate with secondary veins diffusing just before reaching the leaf margin. Veins are slightly depressed on the upper surface and prominently expressed below. All veins are lighter in color than the blade. Petioles are slightly winged below the blade, flattened above and rounded below.

Pubescence varies with habitat, but generally plants have erect filiform hairs overall, except for the inflorescence itself. Hairs of lower stems and branches may be soft or hispid, depending on habitat, while the upper stems and branches may be puberulent. Leaves are glabrous to somewhat pubescent above and densely pilose beneath. Petioles and floral bracts (see below) are pubescent beneath.

The inflorescence at the tips of most branches consists of flat-topped cymose clusters, to 1+ inches across, of tightly spaced one-sixteenth-inch-wide cyathia (described below). A well-developed cluster consists of 1-3 cymes, each comprising up to a dozen cyathia. Each cyme is set atop a straight floral stem and subtended by leaf-like lanceolate floral bracts (¼ to 2½ inches long) and larger floral leaves that are typically shaded white to reddish at the base of their upper surface.

Photo 3: This flat-topped cluster of tightly spaced cyathia consists of 3 cymes on separate floral stems with floral bracts and floral leaves. Note filiform pubescence. Photo – September 19.
Photo 4: Bracts and leaves immediately below the inflorescence have white to reddish shading at their bases, but are totally red in full-sun habitats.
Photo 5: Well-developed clusters, as shown, have 3 cymes. Each cyme is subtended by 2 floral bracts and 1 (sometimes 2) floral leaves. In display, floral leaves removed to show upper and lower surfaces. Leaves of full-sun plants (at right) tend to be lanceolate.

The cyathium is the unique, primary inflorescence of the large, worldwide genus Euphorbia. The cyathium of toothed spurge is a bowl-like, glabrous, yellowish green to reddish involucre (the modified receptacle) of five united, pinkish fringed (fimbriate) bracteoles that enclose a central pistillate flower and a number of staminate flowers. A distinct funnel-shaped nectar gland, attached to the outside of the involucre, has a shiny green apex shaped like pursed lips. Each apparent stamen is morphologically an entire staminate flower reduced to a single stamen––a microscopic joint in its stalk marks the separation of the pedicel (below) from the filament (above). Each filament bears a single two-lobed pale yellow anther. Similarly, the apparent pistil is morphologically a reduced pistillate flower. After the pistillate flower “blooms” and the stigma is no longer receptive, the anthers release light yellow pollen––a behavior that reduces the chance of self-pollination. Cyathia develop from the base of the cymes outward.

Photo 6: Involucres of cyathia comprise fringed bracteoles (red arrow). Cyathia bear a female flower (white arrow), male flowers (yellow arrow), and a nectar gland (black arrow).

The pistil of the female flower has a glabrous, globose ovary with 3 spreading styles tipped by minute stigmas. At anthesis, when the pistillate flower is “in bloom,” the ovary is hidden within the involucre. The rounded 3-lobed ovary has three chambers, each containing an ovule. The ovary is attached to a gynophore (a stalk supporting the ovary) which, if the ovary is fertilized, lengthens and exserts the developing fruit from the involucre. The ovary then becomes significantly larger, initially dangling from its down-curved gynophore. As the fruit matures, the gynophore strengthens and becomes erect, holding the seed capsule upright above the cyme.

With maturity, the fruiting capsules become light tan before they dehisce explosively to disperse up to three seeds. Seeds are brown, ovoid, tuberculate, and 1/16 inch long, with a light colored hilum surrounded by a caruncle of the same color, and a dark furrow extending from the hilum to the seed apex. After the explosive dispersal of seeds, the gynophore, topped with remnants of the capsule and placenta, remains for a short time before dropping off.

Photo 7: In these 2 cymes (a third cyme represented by “stump” at center), female flowers are seen at 3 stages of development (red arrows – bottom to top). The “posts” (black arrow) are gynophores after capsules have explosively dehisced, with remnants of the capsule and placenta remaining at their apexes.
Photo 8: Seeds, shown with capsule sections, have uniformly spread tubercles and a furrow (see furrow on right seed). (Squares = ¼ inch)

Lacking a showy inflorescence, toothed spurge will not typically be considered an attractive candidate for a native plant garden. (On the other hand, the species is quite weedy and may arrive on its own as a volunteer.) In its favor, plants are morphologically intriguing as well as offering a good food source for birds, making them welcome additions to a natural area. Excessive self-seeding, can be controlled by full-plant removal early in its growth cycle. Sap can cause contact dermatitis and is harmful to eyes.

Photo 9: In this garden setting, this recumbent plant is attractive, but was removed at this stage to prevent self-seeding. All branches terminate with an inflorescence. Photo – August 8.

Toothed spurge is one of 27 species of Euphorbia that occur in Arkansas, including 9 non-native species and 5 species of conservation concern. Only 1 of the other 26 species may be confused with toothed spurge, namely, non-native David’s Spurge (aka toothed spurge), Euphorbia davidii. Both have opposite branching, toothed leaves, and one nectar gland. However, David’s Spurge has stiff strongly tapered hairs on lower leaf surfaces and the seed tubercules are not evenly distributed. David’s Spurge has been reported from Mississippi County only.

* Euphorbus, physician of King Juba II (50 BC-AD 24) discovered a medicinal plant in the Atlas Mountains of North Africa. King Juba named the plant “Euphorbia” in his honor. The name “Euphorbia” was officially assigned to the genus by Carl Linnaeus in 1753.

Article and photographs by ANPS member Sid Vogelpohl

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