By CELESTINE DUNCAN
Silverleaf nightshade (Solanum elaeagnifolium) and Carolina horsenettle (Solanum carolinense), also known as horse nettle or bull nettle, are deep-rooted, herbaceous, perennial plants in the nightshade family. Both plants are considered native to the Americas, although silverleaf nightshade may have been dispersed to new locations by Spanish or Portuguese colonists. They are well established throughout much of the U.S. (Figure 1). Carolina horsenettle is considered problematic in the Midwest, eastern U.S., and northeast Canadian provinces, whereas silverleaf nightshade is more problematic in the southern and western U.S.
Figure 1: Distribution of silverleaf nightshade (top) and Carolina horsenettle (above) in the United States (USDA-NRCS Plants).
Silverleaf nightshade is declared noxious in Arkansas, California, Hawaii, Idaho, and Nevada and is an “A”-designated weed for quarantine in Oregon and Washington. Carolina horsenettle is declared noxious in Alaska, Arizona (prohibited weed), Arkansas, California (“B”-list), Hawaii, Iowa, and Nevada (USDA-NRCS, 2018).
Silverleaf nightshade and Carolina horsenettle reproduce by seed, rhizomes, and root fragments. Plant stems die in the fall, and new shoots develop from adventitious buds on roots. Stems are 1 to 3 feet in height. Flowers have five petal-like lobes that are about 3/4 inch across, with bright yellow stamens (Sidebar). Flower color ranges from dark violet to blue for silverleaf nightshade, and white to light violet for horsenettle. Individual plants produce up to about 200 fruit that resemble small tomatoes, and each fruit contains from 40 to 120 seeds. Fruit remain on dead branches where they can be spread by machinery, wildlife, livestock, and wind. Seed remain viable for up to 10 years in soil.
Silverleaf nightshade and Carolina horsenettle have the potential to invade natural areas, pastures, and cropland. Once established, plants form dense colonies from an extensive root system. Infestations reduce crop production, forage quality and quantity, and serve as a host for insects and plant diseases. Plants are adapted to a wide range of habitats, but thrive on disturbed land, including roads, ditch banks, rivers, livestock corrals, and overgrazed lands.
Both plants contain glycoalkaloids, which are toxic to livestock. Although all parts of the plant are toxic, immature fruit have a higher concentration of glycoalkaloids than mature fruit, leaves, or stems. Damage to the intestinal tract and nervous systems in livestock can occur. In severe cases, ingestion can cause hallucinations, paralysis, and death. Cattle and horses are reported to be more susceptible to toxicity than sheep, while goats are apparently unaffected. Most grazing animals and wildlife avoid grazing the plants.
Several management methods have been investigated for both weeds. The key to effective control is stopping seed production and killing the root system. Quarantine and exclusion are regarded as the most important control strategies in areas where the plant is not established.
In field trials, silverleaf nightshade and Carolina horsenettle responded similarly to various broadleaf herbicide treatments. Results from studies conducted in the U.S. show that Milestone®, GrazonNext® HL, or Chaparral® specialty herbicides provided significantly better control 90 to 120 days after application than 2,4-D (Figure 2). On sites where other weedy broadleaf plants are present, GrazonNext HL at 1.5 to 2.1 pints per acre will provide more broad-spectrum control than Milestone at 4 to 7 fluid ounces and would be the most cost-effective treatment. Applications of 2,4-D alone will suppress nightshade shoot growth and fruiting, but result in minimal damage to roots allowing plants to regrow. The optimum time to apply selective herbicides is when nightshades are actively growing at the bud to flower growth stage.
Livestock grazing favors invasive night-shades since animals will graze more desirable forage first. Nightshades are poisonous to most livestock; however, goats appear to be unaffected and may reduce flowering and seed production in a targeted grazing program. Do not graze plants when they are fruiting as seed passing through the gut of animals remains viable. If seed are present on a site with grazing livestock, hold animals for at least 7 days to allow seed to pass through before moving animals to a weed-free pasture.
Frequent mowing and cutting through-out the growing season may prevent flowering and seed production, while reducing root carbohydrate reserves in invasive nightshades. Cutting once during the growing season will not control either plant, since rosettes will grow below the height of mower blades. Tillage is not recommended, since viable rhizome fragments may be moved to new areas and start infestations.
The native leaf-galling nematode (Ditylenchus phyllobius) has been used to reduce silverleaf nightshade density with some measure of success, but it is not host specific. The nematode needs to be integrated with other vegetation management practices. Biological control research is not being conducted due to concerns about possible attacks on non-target crops and closely-related native plants.
The genus Solanum is derived from the Latin word solamen (quieting), referring to the narcotic properties of many species in this genus. Nightshades are rich in solanine, a poisonous glycoalkaloid that causes gastrointestinal, neurological, and coronary problems. However, glycoalkaloids from the nightshade family have been shown to be effective in a variety of medical applications, including limiting growth of certain cancer cells and treating herpes complex viruses.
Native American tribes utilized silverleaf nightshade to treat a variety of ailments, including toothaches, respiratory problems, colds, eye and stomach ailments, sneezing fits, or constipation. The fruit was also mixed with milk, to produce a curdled beverage or cheese, a use that is still reported to occur in the desert southwest and Mexico.
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Milestone, GrazonNext HL, and Chaparral specialty herbicides are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state.
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