Know Your Natives – Carolina Horse Nettle

Carolina Horse Nettle (Solanum carolinense) of the Nightshade (Solanaceae) family is a toxic* perennial with attractive flowers and fruit and piercing prickles. The genus name is Latin for “quieting” in reference to the narcotic properties of some species. The specific epithet suggests that Linnaeus, who named the species, examined a specimen from the Carolinas, where the plant does indeed occur. Carolina Horse Nettle is found from east Texas north to Iowa and east to the Atlantic Coast from southern Maine to Florida. Additionally, there are scattered occurrences in some western states, likely as a result of introductions. In Arkansas, it grows statewide. Bull Nettle is another common name. Habitats include a wide range of soils in a wide variety of sites, from prairies to woodland margins, but this aggressive weedy native grows most vigorously in disturbed sandy to gravelly soils, including cultivated fields.

Carolina Horse Nettle, with taproots 5 feet or longer (!), and near-surface lateral roots, is an herbaceous perennial typically with tough, over-wintering dead stalks. The main stalk, 1-3 feet tall with alternate wide-spreading branches, bears scattered prickles (attached to the epidermis) as well as a more or less dense covering of stellate (star-shaped) hairs. The straight, rigid, yellowish prickles along stalk and branches, angling slightly downward, have widened bases and pin-like (sharp!) tips. Prickles are to ⅛+ inch long.

Photo 1:  Carolina Horse Nettle is a deciduous perennial with a very deep tap root and near-surface lateral roots. Slender taproots grow to 5+ feet deep while long skinny lateral roots are near the surface. Red * marks a cut-off taproot from which a single lateral root extends for 4½ feet (wound together for photo) to a second cut-off tap root marked with a yellow *.

The alternate coarse leaves are oval to elliptic with a wedge-shaped (cuneate) base. They measure 2-4+ inches long, including a ½ inch petiole, and 1-3 inches wide. Smaller leaves tend to be unlobed; the larger blades have 2-4 prominent rounded-triangular lobes of varying size (largest at midleaf) and an elongate lobe-like apex. Margins are wavy and lipped (somewhat swollen). Surfaces are armed with scattered prickles on veins (mostly the midrib) and petiole. Sessile stellate pubescence, moderate above, denser beneath, does not obscure the green tissue. Venation is pinnate with the secondary veins terminating near lobe tips.

Photo 2: This plant has a main stalk and several branches. Larger leaves have two to four lateral lobes and an elongate lobe-like apex. Terete stems and branches are protected by rigid prickles.

Flowers emerge from May to October in axillary and terminal clusters, with several to 20 flowers, the inflorescence elongating to a raceme as fruits develop. Rachises are free of prickles while peduncles may have scattered small prickles. Flowers bloom sequentially from the base of racemes as the rachis continues to lengthen and straighten. In bud, the inflorescence bears down-bent pedicels with dangling oblong buds. With anthesis, pedicels become ascending so that flowers face outward to upward. Lower portion of the rachis has sparse short hairs while pubescence of pedicels and calyx is more dense.

Photo 3: This raceme has single flowers along the rachis, but plants may have compound racemes (see upper budded cluster in Photo 2). Stub at left is a pedicel.

The perianth is pentamerous. The calyx tube bears 5 narrowly triangular lobes (¼ inch long), with a few small prickles. The white to lavender corolla, ¾-1 inch wide, bears 5 broad, delicate, pointed, widely spreading lobes about the same length as the tube. Five large anthers and a rather stout style project forward from the spreading corolla. The distal portion of the style protrudes just beyond the anther tips. The prominent, bright yellow, tubular anthers, on short white filaments, have a terminal pore for pollen dispersal (poricidal dehiscence). Flowers do not produce nectar. With the widespread corolla, bumblebees are able to easily settle across the anthers and stigma. When a bee vibrates its thoracic muscles (sonication), pollen loosens within the anther and exits via the apical pores. Airborne pollen or pollen attached to the ventral side of a bee adheres to the large sticky stigma.

Photo 4: Dense stellate pubescence occurs on abaxial side of upper leaves and on upper rachis, pedicels and sepals. At anthesis, corollas are widely spread so that the calyx has a star shape.
Photo 5: Display shows several flower buds (lower left), a flower just opening (lower right), a flower at anthesis (upper right), and a flower at anthesis with its stamens removed (upper left). Note style and superior ovary of flower at upper left.

Fertilized ovaries develop into smooth spherical fruits (berries), ½+ inch in diameter, without prickles or pubescence. Fruits, initially a shiny pale green, with slashes of dark green extending to mid-fruit from the pedicel, become a deep yellow at maturity. Over winter, leaves having fallen, the dangling yellow fruits and their calyxes persist on the dead stems. During the winter months, a fruit’s epidermis becomes tough and dimpled. A fruit may bear 80± yellowish slightly flattened oval seeds. Damaged fruits have a memorable, pungent scent.

Photo 6: The immature ½+ inch spherical fruits are a pale green with green slashes extending from the pedicel. mimicking the sepals. Fruits are glabrous.
Photo 7: Tough, yellow, oval fruits persist on dead stalks. The 80± seeds per fruit, embedded in a gelatinous substance, are slightly flattened.

For a garden, Carolina Horse Nettle would provide bold texture, and distinctive flowers and fruits. However, considering its piercing prickles and very deep taproots, a gardener would probably not want to introduce it. Once entrenched, it is difficult to control––cut roots propagate new plants. It has been listed as a noxious weed over most of its range due to its aggressiveness in disturbed sites. Plants are suitable for a natural area (woods or prairie) where shading and competing plants can restrict its spread. Being a native plant, it does provide sustenance to native insects, including bumblebees and tobacco hornworms (Manduca sexta), birds and small mammals.

Photo 8: Carolina Horse Nettle, and other members of the Solanum genus, is a host plant for the Carolina Sphinx Moth (Manduca sexta) larvae (often called tobacco hornworms).

Other plants in the genus Solanum which occur in Arkansas are Western Horse Nettle (Solanum dimidiatum), Silver Leaf Nightshade or White Horse Nettle (Solanum elaeagnifolium), Garden Tomato (Solanum lycopersicum), Black Nightshade (Solanum ptychanthum), Buffalo Bur (Solanum rostratum), and Hairy Nightshade (Solanum sarrachoides). Solanum dimidiatum is a more robust plant with larger leaves with 5 to 7 lobes and stalked stellate hairs, larger blue flowers (up to 2 inches wide) and larger fruit. Solanum elaeagnifolium is covered by densely matted short silvery pubescence and has entire or slightly wavy-margined leaves. Solanum lycopersicum is the non-native garden tomato, with the dissected leaves, glandular pubescence, and larger fruits (usually red-orange at maturity). Solanum ptychanthum is an annual species, without prickles, with no or few hairs (non-stellate), and with small flowers and black fruit. Solanum rostratum has deeply lobed leaves, yellow flowers, and prickly calyxes. Solanum sarrachoides, another non-native, is densely hairy with simple (non-stellate) glandular hairs and has small black fruit.

* Plants in the Solanum genus (including tomatoes, potatoes and eggplants) contain solanine, a natural defense toxic compound. Concentration of solanine varies by species and by plant part. For Carolina Horse Nettle, the greatest concentration is in the green fruit.

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