Horsenettle

Images above: Upper left: Horsenettle with flowers (Antonio DiTommaso, Cornell University). Upper right: Horsenettle fruit (Scott Morris, Cornell University). Bottom: Horsenettle leaf spines (Scott Morris, Cornell University).

Identification

Other common names:  ball-nettle, bull nettle, apple-of-Sodom, wild tomato, sand brier, devil's tomato, devil's potato, Carolina horse-nettle

Family:  nightshade family, Solanaceae

Habit:  Thorny, branched perennial herb spreading by deep horizontal roots. 

Description:  Seedlings have oval to oblong shaped, 0.5” (1.3 cm) long cotyledons.  The cotyledons have a shiny green surface, light green undersides, and hairy margins.  The stem of the seedling is purplish and has short, stiff hairs.  The first pair of true leaves has untoothed, smooth edges and scattered, star shaped hairs on its upper surface.  Later young leaves have wavy, lobed edges, star-shaped hairs, and sharp, stiff, curved prickles on both surfaces.  Mature plants arise from thick, deep horizontal storage roots.  In deep, well-drained soil, horsenettle roots can penetrate to 8 ft (2.4 m) (Robbins et al. 1942).  Semi-woody stems with prickly hairs reach 1-3 ft (30-91 cm).  Stems are erect, angled at leaf nodes, and branch moderately.  Leaves are alternate and 2.5-4.5” (6-11 cm) long by 1-3” (2.5-7.5 cm) wide; they have an egg-shaped outline with two to five shallow lobes on each edge.  Star shaped hairs are present on the stems and both leaf surfaces.  Large, 0.25-0.5” (0.6-1.3 cm), sturdy, yellow or white, curved, painful spines and prickles are present on stems, leaf stalks, leaf veins, and flower stalks.  Small clusters of one to five, white to purple, potato-like flowers sit atop long, sturdy, leafless stalks.  Flowers are star shaped with a central cone of yellow, pollen filled anthers.  Five hairy green sepals occur at the base of the five fused, 0.75-1” (1.9-2.5 cm) diameter flower petals.  Immature, green berry fruits turn yellow and shrivel as they mature; mature berries are 0.5-0.75” (1.3-1.9 cm) across and hold 40 to 170 flat, round seeds.  Seeds are 0.1” (0.3 cm) across, smooth, shiny, and range from pale to dark yellow or orange.

Similar species:  Buffalobur (Solanum rostratum Dunal) cotyledons have a clear central vein when viewed from below and young leaves have deep, round lobes that indent nearly to the midvein.  Clammy groundcherry (Physalis heterophylla Nees) has densely hairy stems, single yellow or greenish flowers arising in leaf-stem junctures, and berries covered in a thin, papery membrane; they do not have prickly or spiny leaves.  

Management

Summer tillage is a key management strategy.  Since the species emerges relatively late, it is likely to increase during a sequence of spring planted long season crops like corn and soybean.  On grain farms, rotation into a small grain crop allows midseason tillage following harvest.  If possible, allow the plants to re-sprout and then till again before planting a winter grain or fall cover crop.  This will not eliminate the weed, but will reduce density.  On vegetable farms, a series of short season crops, for example, spring spinach, summer lettuce, and fall brassica, allow for repeated tillage during times when horsenettle is most sensitive to disturbance.  When spring planted full season crops are unavoidable, cultivate more deeply than usual, for example, 4” (10 cm).  If horsenettle is not well established in a field, repeated hoeing in the row to supplement cultivation can eliminate the weed in a couple of years.  Unlike most perennials, minimal supplemental hoeing will be required because this weed is slow to re-sprout and relatively slow to begin replenishing root reserves.

Horsenettle is common in overgrazed pastures.  If the infestation is severe, renovate the pasture through deep tillage and reseeding to develop a dense stand of palatable species.  Horsenettle sprouts back vigorously following mowing early in the season.  Recovery from mowing later in the season is slower.  Unfortunately, horsenettle responds to repeated mowing by producing a rosette of leaves close to the ground that largely escapes further damage (Ilnicki et al. 1962).  Mowing is effective at preventing flowering and fruit production (Ilnicki et al. 1962), and thereby reducing the likelihood of livestock poisoning.  Because horsenettle is relatively slow to re-sprout from roots after elimination of the shoots, intensive rotational grazing is effective for controlling this species.  If possible, grazing episodes should be timed to catch the weed while stems are still soft and easily damaged by trampling,

Ecology

Origin and distribution:  Horsenettle is native to the southeastern U.S. (Bassett and Munro 1986), but has been spread northward into southern Canada and westward through most of the U.S.A., but it is uncommon between the western corn belt and the far West (USDA Plants).  It has been introduced into Japan, South Asia, Australia, New Zealand and parts of Latin America.

Seed weight: 1.1-1.9 mg (Ilnicki et al. 1962).

Dormancy and germination:  Many seeds are dormant in the fall, but lose dormancy after a few months of storage (Ilnicki et al. 1962).  Seeds do not need cold to break dormancy (Ilnicki et al. 1962).  The optimum temperature for germination is 68-86 °F (20-30 °C) (Ilnicki et al. 1962).  Germination is increased by nitrate and fluctuating temperatures but not by light (Ilnicki et al. 1962).

Seed longevity:  Seeds remain viable for at least three years when buried at 3-5” (8-13 cm.) (NAPPO)

Season of emergence:  Seedlings begin emerging in mid-May (Ilnicki et al. 1962).  Shoots sprouting from roots begin emerging in mid-spring and continue emerging through summer (Doll 2002, Ilnicki et al. 1962, Nichols et al. 1991).  The temperature range most suitable for sprouting from roots is 59-86 °F (15-30 °C) (Follak and Strauss 2010).  Horsenettle emergence is reduced in areas where horsenettle previously grew, suggesting that populations are self-regulating (Solomon 1983a).

Emergence depth:  Seedlings emerge well from 0.5-2” (1.3-5 cm) and some seedlings can emerge from 3-4” (7.5-10 cm), but no seedlings emerge from 5-6” (12.3-15 cm) depths.  Emergence is low for seeds at 0.25” (0.6 cm) or less from the soil surface.  All root cuttings emerge when buried 12” (30 cm) or less.  (Ilnicki et al. 1962)

Photosynthetic pathway:  C3

Sensitivity to frost:  Shoots are killed by the first frost (Follak and Strauss 2010).  Only 10% of seedlings established in fall survive winter (Ilnicki et al. 1962).  Freezing also kills roots (Wehtje et al. 1987), but roots below the frost line survive (Bassett and Munro 1986).

Drought tolerance:  Horsenettle thrives in hot weather and is drought resistant due to deep penetration of the root system (Bassett and Munro 1986, Follak and Strauss 2010).  Root segments on the soil surface do not tolerate 3 days of drying (Ilnicki et al. 1962).

Mycorrhiza:  Horsenettle exhibited moderate levels of mycorrhizal infection in a prairie habitat (Dhillion and Friese 1994).

Response to fertility:  Horsenettle tolerates infertile soil.  It is among the few early colonizers of extremely poor, eroded soil of abandoned fields in the Carolina Piedmont (Hursh 1935).  Horsenettle vegetative and fruit biomass is doubled by supplemental nitrogen fertilizer (Cipollini et al. 2004).  Root length is doubled by a balanced fertilizer when plants are grown from seed, but not when grown from root segments (Ilnicki et al. 1962).  This suggests that root segments have sufficient nutrients for establishment without reliance on soil nutrients.

Soil physical requirements:  Horsenettle grows on a wide range of soil types from sand and gravel to clay.  It establishes best and grows most rapidly, however, on coarse textured soils.  (Bassett and Munro 1986, Ilnicki et al. 1962)

Response to shade:  Horsenettle tolerates moderate shade, but growth and fruit production are suppressed by heavy shade (Bassett and Munro 1986).

Sensitivity to disturbance:  Starch stored in the roots reaches a minimum from one to two months after stems emerge and when flowering is initiated (Bassett and Munro 1986, Ilnicki et al. 1962, Nichols et al. 1991).  Thus, destroying the shoots at that time is particularly effective for decreasing vigor of the plants.  Mowing only temporarily reduced above-ground shoots for a month, but often reduced flowering and fruiting for the duration of the summer (Ilnicki et al. 1962, Nichols et al. 1991).  Disturbance of established plants, even if they survive, will reduce their capacity to sprout and produce shoots in the subsequent year (Ilnicki et al. 1962).  New plants can establish from root sections as short as 0.4-0.8” (1-2 cm), and 6” (15 cm) sections can produce shoots when buried 18” (46 cm) deep (Ilnicki et al. 1962, Wehtje et al. 1987).  Root segments require about 1 month to produce new shoots (Ilnicki et al. 1962).  Most shoots sprout from tap root rather than lateral root segments (Ilnicki et al. 1962).  Seedlings become capable of regenerating from the root following shoot removal when 15 to 20 days old (Ilnicki et al. 1962).

Time from emergence to reproduction:  Shoots begin flowering about 5 to 8 weeks after emergence, peak in early summer, and continue until the fall (Doll 2002, Nichols et al. 1991).  Berries and seeds mature 1 to 3 months after flowering (Bassett and Munro 1986, Nichols et al. 1991).

Pollination:  Although all flowers have both male and female parts, flowers near the top of the plant function as males whereas those near the base function as females (Elle 1999).  Individuals are normally self-incompatible (Elle 1999), but flowers can self-pollinate when crossing pollen is scarce (Travers et al. 2004).  The flowers are pollinated by bumblebees and carpenter bees (Bassett and Munro 1986).  Feeding damage by larvae of a host specific moth, Frumenta nundinella, can cause fruits to develop without pollination (Bassett and Munro 1986).

Reproduction:  Most populations are maintained primarily by vegetative reproduction, but seeds are important for spread of the species (Ilnicki et al. 1962).  The tap root can grow 4 ft (120 cm) (Ilnicki et al. 1962) and horizontal roots can spread more than 3 ft (91 cm) per year (Bassett and Munro 1986).  Tillage increases vegetative reproduction by cutting up and moving roots.  Even short root segments (0.4” or 1 cm) have a high probability of producing a shoot when near the soil surface and longer pieces can produce a shoot from the bottom of the plow layer (Ilnicki et al. 1962, Wehtje et al. 1987).  Horsenettle berries contain 13-160 seeds (average of 86) (Ilnicki et al. 1962), and single shoots produce up to 5,000 seeds (Bassett and Munro 1986, Elle 1999, Travers et al. 2004).

Dispersal:  Horsenettle seeds pass through cattle unharmed (Nishida et al. 1998) and are dispersed in manure.  Probably many species of mammals eat the fruits and subsequently disperse the seeds in their feces (Bassett and Munro 1986, Cipollini et al. 2004).  Root fragments can be spread by harvesting and tillage equipment in agricultural fields or earth moving equipment in non-agricultural areas (Follak and Strauss 2010). 

Common natural enemies:  Eggplant lacebug (Gargaphia solani) causes leaf yellowing and early leaf drop, and potato bud weevil (Anthonomus nigrinus) can destroy a substantial proportion of the flowers (Wise and Cummins 2006).  First generation larvae of the host specific moth, Frumenta nundinella, can cause substantial damage to leaves and flowers.  Second generation larvae can heavily damage fruits and seeds (Bassett and Munro 1986, Nichols et al. 1992, Solomon 1983b).  Populations of the moth are normally kept in check, however, by a parasitoid wasp (Bassett and Munro 1986).  A downy mildew caused by Erysiphe cichoracearum can infect foliage in fall (Nichols et al. 1992).

Palatability:  Both leaves and fruit of horsenettle are toxic to people and livestock (Bassett and Munro 1986, Ilnicki et al. 1962), and immature berries are particularly poisonous (Burrows and Tyrl 2006).  Toxicity increases in autumn (Bassett and Munro 1986).  Toxicity is unaffected by fertility and moisture conditions, suggesting the importance of toxic compounds as a deterrent to insects and pathogens under all growing conditions (Cipollini 2004).

References

• Bassett, I. J., and D. B. Munro.  1986.  The biology of Canadian weeds. 78. Solanum carolinense L.  Canadian Journal of Plant Science 66:977-991.
• Burrows, G. E., and D. J. Tyrl.  2006.  Handbook of Toxic Plants of North America. Blackwell: Ames, IA.
• Cippolini, M. L., E. Paulk, K. Mink, K. Vaughn, and T. Fischer.  2004.  Defense tradeoffs in fleshy fruits: Effects of resource variation on growth, reproduction, and fruit secondary chemistry in Solanum carolinense.  Journal of Chemical Ecology 30:1-17.
• Dhillion, S. S., and C. F. Friese.  1994.  The occurrence of mycorrhizas in prairies: Applications to ecological restoration.  Thirteenth North American Prairie Conference 13:103-114.
• Doll, J.  2002.  Knowing when to look for what: Weed emergence and flowering sequences in Wisconsin.  https://extension.soils.wisc.edu/wcmc/knowing-when-to-look-for-what-weed-emergence-and-flowering-sequences-in-wisconsin/
• Elle, E.  1999.  Sex allocation and reproductive success in the andromonoecious perennial Solanum carolinense (Solanaceae). I. Female success.  American Journal of Botany 86:278-286.
• Follak, S., and G. Strauss.  2010.  Potential distribution and management of the invasive weed Solanum carolinense in central Europe.  Weed Research 50:544-552.
• Hursh, C. R.  1935.  Plant indicators of soil conditions in recently abandoned fields.  U. S. Forest Service Appalachian Forest Experiment Station, Technical Note No. 17.  3 pp.
• Ilnicki, R. D., T. F. Tisdell, S. N. Fertig, and A. H. Furrer Jr.  1962.  Life History Studies as Related to Weed Control in the Northeast.  3 – Horse Nettle.  Agricultural Experiment Station Bulletin 368.  University of Rhode Island: Kingston, RI.
• Nichols, R. L., J. Cardina, and T. P. Gaines.  1991.  Growth, reproduction and chemical composition of horsenettle (Solanum carolinense).  Weed Technology 5:513-520.
• Nichols, R. L., J. Cardina, R. L. Lynch, N. A. Minton, and H. D. Wells.  1992.  Insects, nematodes, and pathogens associated with horsenettle (Solanum carolinense) in bermudagrass (Cynodon dactylon) pastures.  Weed Science 40:320-325.
• Nishida, T., N. Shimizu, M. Ishida, T. Onoue, and M. Harashima.  1998.  Effect of cattle digestion and of composting heat on weed seeds.  Japan Agricultural Research Quarterly (JARQ) 32:55-60.
• Robbins, W. W., A. S. Crafts, and R. N. Raynor.  1942.  Weed Control: a Textbook and Manual.  McGraw-Hill: New York.
• Solomon, B. P.  1983a.  Autoallelopathy in Solanum carolinense: Reversible delayed germination.  American Midland Naturalist 110:412-418.
• Solomon, B. P.  1983b.  Compensatory production in Solanum carolinense following attack by a host-specific herbivore.  Journal of Ecology 71:681-690.
• Travers, S. E., J. Mena-Ali, and A. G. Stephenson.  2004.  Plasticity in the self-incompatibility system of Solanum carolinense.  Plant Species Biology 19:127-135.
• USDA Plants.  USDA, Natural Resources Conservation Service, Plants Database.  http://plants.usda.gov
• Wehtje, G., J. W. Wilcut, T. V, Hicks, and G. R. Sims.  1987.  Reproductive biology and control of Solanum dimidiatum and Solanum carolinense.  Weed Science 35:356-359.
• Wise, M. J. and J. J. Cummins.  2006.  Strategies of Solanum carolinense for regulating maternal investment in response to foliar and floral herbivory.  Journal of Ecology 94:629-636.