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
Silverleaf nightshade or White horsenettle [Solanum eleagnifolium Cav.][SOLEL][CDFA
list: B] Photographs Map
Lanceleaf nightshade [Solanum lanceolatum Cav.][Bayer code: none][CDFA list:
B] Photographs Map
[Solanum marginatum L.][Bayer code: none][CDFA list: B]
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.
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.
and UNDERGROUND STRUCTURES:
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.
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.
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.
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.
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
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
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.