Horsenettle or Carolina horsenettle [Solanum carolinense L.][SOLCA][CDFA list: B] Photographs Map of Distribution

Robust horsenettle or Torrey's nightshade [Solanum dimidiatum Raf.][SOLDM][CDFA list: A] Photographs Map of Distribution

Silverleaf nightshade or White horsenettle [Solanum eleagnifolium Cav.][SOLEL][CDFA list: B] Photographs Map of Distribution

Lanceleaf nightshade [Solanum lanceolatum Cav.][Bayer code: none][CDFA list: B] Photographs Map of Distribution

White-margined nightshade [Solanum marginatum L.][Bayer code: none][CDFA list: B] Photographs Map of Distribution

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GENERAL DESCRIPTION: Noxious perennial herbs to shrubs, usually with creeping roots and prickles on the stems (may be sparse or lacking on robust horsenettle, silverleaf nightshade, and lanceleaf nightshade). Foliage is covered with star-shaped hairs. Foliage, flowers, and fruits are variable and often similar among species. Star-shaped hair characteristics (visible with 20x magnification) can help with species identification. Foliage and berries of this nightshade group contain variable amounts of several glycoalkaloids and can be toxic when ingested by livestock or people. Dried plant material does not lose its toxicity. In horsenettle, toxic compounds are most concentrated in fall. Acute toxicity symptoms include gastrointestinal irritation and nervous effects such as apathy, drowsiness, salivation, trembling, breathing difficulties, progressive weakness, paralysis, and unconsciousness. Symptoms of chronic poisoning in cattle by horsenettle include appetite loss, emaciation, rough coat, constipation, and a form of dropsy (ascites). Poisoning may or may not result in death. Sheep and goats are more resistant to poisoning than cattle or horses. Ironically, berries are consumed by many species of birds and small mammals. Fruits of silverleaf nightshade and white-margined nightshade also contain solasodine, a steroid compound used commercially to synthesize steroid hormones. Horsenettle, silverleaf nightshade, and robust horsenettle can be troublesome in agricultural fields and pastures, especially those receiving summer irrigation. Large infestations can reduce harvest yields of crops and the carrying capacities of pastures by competing with desirable plants for nutrients and soil moisture.

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SEEDLINGS: New shoots from roots resemble seedlings, but lack cotyledons. Descriptions not available for robust horsenettle, lanceleaf nightshade, and white-margined nightshade.

MATURE PLANT: Stems usually prickly, openly branched. Leaves alternate, simple, +/- dull green, ovate to lanceolate, to ~ 15 cm long, usually with wavy to coarse-lobed margins. Foliage is covered with minute star-shaped hairs (requires magnification), typically yellowish to straw-colored except where noted. New foliage is more densely covered with hairs. Refer to the table Comparison of nightshades (Solanum spp.): with spiny prickles and star-shaped hairs for specific descriptions of star-shaped hairs and leaf shapes, lengths, and margins.

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FLOWERS: May-September. Flower clusters are modified cymes (oldest flower at tip of main axis). Often lower flowers are bisexual while upper flowers have reduced female parts and are functionally male (horsenettle, white-margined nightshade). Corolla star-shaped, 5-lobed. Sepals lack prickles (except white-margined nightshade). Anthers erect, longer than filaments, spreading or loose around style. Refer to the table Comparison of nightshades (Solanum spp.): with spiny prickles and star-shaped hairs for specific descriptions of flower color and diameter. Insect-pollinated.

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FRUITS and SEEDS: Berries round, not enclosed halfway to completely in a spiny calyx. Refer to the table Comparison of nightshades (Solanum spp.): with spiny prickles and star-shaped hairs for specific descriptions of mature berry color and size range. Immature berries green. Seeds numerous, +/- ovate, flattened.

POSTSENESCENCE CHARACTERISTICS: Foliage of horsenettle, robust horsenettle, and silverleaf nightshade dies back after the first fall frost, and dead stems may persist for several months. Dead stems of silver leaf nightshade typically lose prickles and have a few wrinkled yellowish fruits.

HABITAT: Plants usually grow in places disturbed by people or livestock, especially those with summer moisture or irrigation. Plants tolerate considerable drought because of deep root systems.

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PROPAGATION/PHENOLOGY: All reproduce by seed and vegetatively from creeping roots (except possibly white-margined nightshade). Fruits and seeds disperse with agricultural activities, water, mud and soil movement, and animals. Root fragments disperse primarily with cultivation or other human activities. In winter, roots of horsenettle, robust horsenettle, and silverleaf nightshade go dormant and foliage dies back. Roots generate new shoots in spring. Seeds germinate spring through summer.

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MANAGEMENT FAVORING/DISCOURAGING SURVIVAL: Colonies of nightshades with creeping roots are difficult to control or eliminate by mechanical methods. Shallow and deep cultivation does not disturb enough of the root system to eliminate infestations and can increase the problem by dispersing root fragments. Deep ripping under dry conditions may reduce but typically does not eliminate infestations of silverleaf nightshade. Weekly mowing prevents most seed production and can help weaken roots by reducing carbohydrate reserves, but does not eliminate infestations. Confine livestock from infested pastures for 6-7 days before moving animals to uninfested areas to prevent introduction of seeds.

SIMILAR SPECIES: Buffalobur [Solanum rostratum Dunal.][SOLCU] and sticky nightshade [Solanum sisymbrifolium Lam.][SOLSI] are summer annuals with prickles and star-shaped hairs that are similar to the perennial horsenettles and nightshades described above. Unlike the perennial horsenettles and nightshades, buffalobur and sticky nightshade have leaves that are deeply lobed from halfway to all the way to the midribs, with the lobes again toothed or lobed. In addition, buffalobur berries are completely enclosed in a spiny bur-like calyx. Sticky nightshade has red berries that are halfway to mostly enclosed in a spiny calyx. For a comparison of other characteristics, refer to the table Comparison of nightshades (Solanum spp.): with spiny prickles and star-shaped hairs. Buffalobur and sticky nightshade typically grow in disturbed areas, fields, agronomic and vegetable crops, and along roadsides. Plants usually flower May through October. Fruits disperse with human activities, water, soil movement, and possibly by clinging to the fur of animals. Under windy conditions in fall, buffalobur plants can detach at the base and scatter seeds as they tumble along the ground. Buffalobur can harbor nematodes that affect tomatoes, is the natural host of the Colorado potato beetle (Leptinotarsa decemlineata), and is an alternative host for alfalfa, tomato, and potato mosaic viruses. Buffalobur occurs in the southwestern North Coast Ranges, Cascade Range foothills, Central Valley, southwestern South Coast Ranges, Central Coast, and South Coast, to 500 m (1600 ft). It is widespread in the U.S., from California to the East Coast and Mexico. Buffalobur is introduced from the Great Plains region. Sticky nightshade occurs in the Central Valley to 100 m (330 ft) and is expected to expand its range. It is introduced from tropical South America where the roots are used medicinally and ripe fruits are eaten raw or made into marmalade.

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Prevention: There are six species in the Solanaceae considered noxious in California. They are all deeply rooted perennials with an aggressive growth habit in both cultivated and relatively undisturbed areas. All are thought to be native to the southwestern United States and Mexico, except Solanum marginatum, which is native to Ethiopia. There are numerous concerns regarding invasion and spread of these species in California, as they impact agriculture, range, and wildlands. They may reduce crop yields through direct competition for resources. There are also indications of allelopathic effects by S. elagnifolium on several important crops. The foliage and berries of at least S. carolinense, S. dimidiatum, and S. elagnifolium are toxic to livestock. Finally, many of these species are alternate hosts for disease and insect pests in a broad range of crops in California. Known infestations should be aggressively controlled to prevent the further spread of these noxious plants. Information on the following Solanaceae species in Table 1 is given.

Table 1. Solanaceae species considered noxious in California

Scientific name Common name CDFA rating
Solanum cardiophyllum Heartleaf nightshade A
Solanum carolinense Carolina horsenettle B
Solanum dimidiatum Torrey's nightshade A
Solanum elaeagnifolium Silverleaf nightshade B
Solanum lanceolatum Lanceleaf nightshade B
Solanum marginatum White-margined nightshade B

All of these species can spread by seed or root fragments; or tuber fragments in the case of S. cardiophyllum, which is a wild relative of potato (S. tuberosum). Seed may be spread through tillage or harvesting equipment, by animals, or in contaminated hay. Clean equipment before leaving contaminated fields, and avoid spreading root fragments by cultivation equipment. Check hay for nightshade berries, before feeding to cattle. This will prevent livestock poisoning and the introduction of seed into uninfested areas.

Mechanical: All of these species are deep-rooted perennials and may vegetatively reproduce from rootstocks. There is some variation between species in their ability to regenerate from rootstocks; with silverleaf nightshade regenerating from root cuttings <1 cm in length, Carolina horsenettle >1cm, and Torrey's nightshade >2cm. Information is lacking for the other species. Regardless, tillage may spread rootstocks to new areas, where establishment can occur. Small infestations may be hand pulled or hoed, but must be repeated several times during the growing season. Several of these species have sharp spines and gloves should be used for hand pulling. Any root material that is dug should be collected, dried and burned. Repeated mowing throughout the summer may nearly eliminate seed production. However, the plants will take on a flat, rosette-like growth form that is capable of replenishing root carbohydrate reserves.

Biological: There are no currently registered biocontrol agents for use on any of these weeds. There is a great deal of concern since several other species in the Solanaceae family are important agricultural crops in California, such as potato, tomato, eggplant, and peppers. However, researchers have examined a nematode, Orrina phyllobia, which is host specific for silverleaf nightshade. Augmentative releases of this nematode may eventually help reduce silverleaf nightshade populations.
Livestock will favor these spiny plants by overgrazing the surrounding palatable vegetation. If the seed are ingested by livestock, up to 10% may remain viable in excreted feces. Mature berries of these weeds also contain high levels of solanine and solanosine, which are toxic to livestock. Animals should be removed from infested areas until control is achieved.
Experiments have shown that shading reduces silverleaf nightshade berry production and total nonstructural carbohydrate content of the roots. However, there has been little research examining which native California species would compete well with these nightshades.

Chemical: There are few herbicides that effectively control these nightshades, and their application is dependent upon the land use. Herbicide labels should be read and followed in regards to crop rotations and restrictions following application. Herbicides should be applied late bud to early flower. Glyphosate in a 2% solution can be applied as a spot treatment. Dicamba and 2,4-D can be applied at 0.5-1.0 and 1.0-2.0 lb ae/A, respectively. Triclopyr can be applied at 1-3 lb ae/A. Regrowth will occur with any of these treatments and retreatment will be necessary. Picloram has provided excellent control of many of these species in other states, but is not currently labeled in California. Clopyralid has not provided good control of horsenettle or silverleaf nightshade in other states and is not recommended. Glyphosate is non-selective and will injure or kill any foliage it contacts. Dicamba, 2,4-D, and triclopyr will injure or kill most other broadleaf plants. These factors should be considered when applying these herbicides.

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