Witchgrass

Panicum capillare L.

Images above: Upper left: Witchgrass seedling (Antonio DiTommaso, Cornell University). Upper right: Witchgrass hairs on stem and leaf (Scott Morris, Cornell University). Bottom: Witchgrass plant (Randall Prostak, University of Massachusetts).

Image above: Witchgrass panicle (Scott Morris, Cornell University).  

Identification

Other common names:  old-witch grass, tickle-grass, witches-hair, tumble weed-grass, fool-hay, witches'-hair

Family:  grass family, Poaceae

Habit:  Sprawling summer annual grass.

Description:  Seedlings are upright.  The seed leaf is lance-shaped, parallel to the ground, and up to 0.5” (1.3 cm) long by 0.13” (0.3 cm) wide.  Subsequent leaves are rolled in the bud, 0.6-1.6” (1.5-4 cm) long by 0.16-0.0.4” (0.4-1 cm) wide, pointed at the tip, and lack auricles.  The collar is pale green to white, and the ligule is a 0.04-0.06” (0.1-0.15 cm) long fringe of hairs.  Long, silky hairs densely coat the sheath, collar, and both leaf surfaces.  The hairs are often thickened at the base.  Mature plants are 8-36” (20-90 cm) tall and usually produce several tillers.  Stems are round in cross-section, and either erect or horizontal and spreading with upturned tips.  The sheath is distinctly veined, green to purplish, and open.  Leaf blades are flat, pale green, 2.3-10” (6-25 cm) long by 0.25-0.75” (0.6-1.9 cm) wide, and have a conspicuous white midvein.  The ligule and collar of seedlings and mature plants are similar.  Both the sheath and the leaves are densely hairy.  The roots are shallow and fibrous.  Tillers root only at the base, and not at stem joints.  The inflorescence is a terminal, open, diffusely branching, 8-16” (20-40 cm) tall and 3-9” (8-23 cm) wide panicle.  The panicle branches are rough, straight to wavy and 4.5-12” (11-30 cm) long.  Each branch splits into several smaller branchlets, each of which has a single 0.08-0.13” (0.2-0.3 cm) long, beaked spikelet at the tip.  Each spikelet produces a single smooth, shiny, dark brown or gray seed.  As with all grasses, the seed includes a thin, tight coating of fruit tissue.  The seeds are oblong to egg-shaped and 0.06-0.13” (0.15-0.33 cm) long.    
Similar species:  Crabgrasses (Digitaria spp.), fall panicum (Panicum dichotomiflorum Michx.), wild-proso millet (Panicum miliaceum L.) have a similar appearance to witchgrass.  Crabgrass seedlings are folded in the bud, lack sheath hairs, and have membranous ligules, while witchgrass seedlings are rolled in the bud, have hairy sheaths, and have a fringe of hairs as the ligule.  The leaves of fall panicum seedlings are hairy only on the underside, while witchgrass leaves are hairy above and below, and fall panicum usually is a larger, more robust mature plant.  Wild-proso millet also is a larger plant than witchgrass, reaching up to 4.25 ft. (1.3 m) in height with spikelets that are 0.18-0.2” (0.45-0.4 cm) long.

Management

Since soil disturbance promotes almost 100% of witchgrass seeds near the surface to germinate (Baskin and Baskin 1986), spring tillage can make this species highly susceptible to stale or false seedbed management that kills seedlings before planting summer crops.  In contrast, late summer or fall fallow will usually prompt little germination.  Because seedlings mostly emerge from the top 1” (2.5 cm) of soil and establish relatively slowly, tine weeding and other shallow, in-row weeding methods can be highly effective against this species.  Due to their drought tolerance, burying young plants during cultivation may be more effective than partially uprooting them. 

Avoid excessive N fertility since this will favor the weed relative to crops, particularly less vigorous crop species.  Compared with many other weeds, witchgrass is a poor competitor (Clements et al. 2004), and it can be suppressed by a dense, vigorous crop or cover crop. 

Ecology

Origin and distribution:  Witchgrass is native to eastern North America from southern Canada to Florida, and a western variety extends to the Pacific.  Prior to agriculture, the species persisted on beaches, riverbanks and similarly open, disturbed habitats.  It has been widely introduced into temperate areas of the world, including Europe, Asia, Australia, New Zealand and southern South America.  (Clements et al. 2004)

Seed weight:  0.15 mg (Gross et al. 1992), 0.29 mg (Shipley and Parent 1991), 0.30 mg (Gaba et al. 2019), 0.65 mg (Stevens 1932).

Dormancy and germination:  Seeds are dormant when shed from the parent plant and require a period of cold, wet conditions before they are ready for germination (Baskin and Baskin 1969).  As seeds lose dormancy in spring, they require relatively high temperatures of 86-95 °F (30-35 °C) in the day and 59-68 °F (15-20 °C) at night for germination.  As spring progresses, the temperature required for germination declines (Baskin and Baskin 1986, Clements et al. 2004).  Light is required for germination, and when present with suitable temperatures, up to 100% of seeds will germinate (Baskin and Baskin 1986).   Alternating temperatures also are required for high levels of germination (Clements et al. 2004).  If seeds do not germinate in the spring, they enter secondary dormancy during mid-summer (Baskin and Baskin 1986).  Since these dormant seeds require another period of chilling, seedling emergence declines substantially during the summer and fall.  Dilute solutions of nitrate or ammonium promote germination (Clements et al. 2004).

Seed longevity:  The seeds can live at least several decades in undisturbed soil (Clements et al. 2004) but there are no reports on estimated annual mortality rates.  Given its propensity to germinate in light, longevity of seeds near the soil surface in regularly tilled soil is probably much lower than that of seeds buried deeper in soil.

Season of emergence:  Witchgrass emerges primarily in late spring and early summer (Clements et al. 2004).

Emergence depth:  Seeds emerge best from the top 0.5” (1.3 cm) of soil, and none emerge from 2” (5.1 cm) or deeper (Brecke 1974).

Photosynthetic pathway:  C4 (Clements et al. 2004)

Sensitivity to frost:  Plants are killed by the first frost (Uva et al. 1997).

Drought tolerance:  Witchgrass is relatively drought tolerant and probably out-competes most crops during dry periods.  It is more efficient than C4 crops like corn at using water for growth, and much more efficient than C3 crops (compare Dillman 1931 cited in Clements et al. 2004 with Loomis and Connor 1992, p. 255).  Young plants have a herring-bone root arrangement that is highly effective at exploiting soil moisture (Gross et al. 1992).

Mycorrhiza:  Witchgrass was rated as having moderate levels of mycorrhizal infection (Dhillion and Friese 1994).  Witchgrass biomass and P uptake were increased by inoculation with vesicular-arbuscular mycorrhizae at low available P levels (Clements et al. 2004).

Response to fertility:  Witchgrass is highly responsive to nitrogen.  Under high fertility it can accumulate nitrogen to the point of becoming toxic to livestock.  It is also highly responsive to P, but tolerates K and Ca deficiencies well (Brecke 1974).  It is highly salt tolerant and its ability to tolerate very high pH is indicated by its natural occurrence on alkali flats in the western U.S.A. (Clements et al. 2004).

Soil physical requirements:  The species tolerates a wide variety of soil conditions, but does best on sandy to loamy soils (Clements et al. 2004).  It tolerates compaction.

Response to shade:  Witchgrass tolerates moderate shade, but shade slows development and reduces dry matter and tillering (Vengris and Damon 1976).  It does not thrive in areas with dense shade (Clements et al. 2004).

Sensitivity to disturbance:  Witchgrass exists in primarily disturbed and cultivated habitats, resists trampling, and sometimes invades overgrazed pastures (Clements et al. 2004).  Its drought tolerance probably allows survival after partial uprooting during cultivation or hoeing.  Because the inflorescence develops slowly, mowing before seed set can eliminate or greatly reduce seed production.

Time from emergence to reproduction:  Early emerging witchgrass flowers about 8-13 weeks after emergence, with seeds maturing about 3.5 weeks later (Doll 2002, Vengris and Damon 1976).  Plants emerging in midsummer develop more rapidly but are smaller and produce fewer seeds.

Pollination:  Witchgrass self-pollinates but probably also cross-pollinates by wind (Clements et al. 2004).  Under stressful conditions, the flowers often self-pollinate without opening.

Reproduction:  A well-developed plant grown without competition in North Dakota produced 11,000 seeds (Stevens 1932).  A very large plant produced 56,000 seeds (Clements et al. 2004).

Dispersal:  The entire inflorescence breaks off as a unit and rolls as a tumble weed (Clements et al. 2004).  Witchgrass is a common contaminant of forage seed like timothy and white clover, and contaminated forage and grain seed have been a major source for spreading this weed throughout the world.  The seeds float and commonly contaminate surface irrigation water, especially since the species often grows on stream banks.  Seeds also disperse on tires, shoes and farm machinery.  The seeds pass intact through horses, cattle, sheep, and pigs and are dispersed in feces and manure that is spread on fields (Clements et al. 2004).

Common natural enemies:  The species is infected by a great variety of fungi and viruses but their impact in agricultural situations has not been evaluated.

Palatability:  No part of the plant makes a desirable food for humans.  The plant is unpalatable to livestock except when very young, and can contain toxic levels of nitrate when growing on highly fertile soils (Clements et al. 2004).

References

  • Baskin, C. C., and J. M. Baskin.  1969.  Germination and dormancy in Cedar Glade plants. IV. Isanthus brachiatus, Panicum capillare, Cyperus inflexus, Eragrostis spectabilis, and Ruellia humilis.  Journal of the Tennessee Academy of Science 44:69-70.
  • Baskin, J. M., and C. C. Baskin.  1986.  Seasonal changes in the germination responses of buried witchgrass (Panicum capillare) seeds.  Weed Science 34:22-24.
  • Brecke, B. J.  1974.  Life cycle studies of Panicum dichotomiflorum Michx. and Panicum capillare L.  Masters Thesis, Cornell University.
  • Clements, D. R., A. DiTommaso, S. J. Darbyshire, P. B. Cavers, and A. D. Sartonov.  2004.  The biology of Canadian weeds. 127. Panicum capillare L.  Canadian Journal of Plant Science 84:327-341.
  • 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.  Weed Science University of Wisconsin.  https://extension.soils.wisc.edu/wp-content/uploads/sites/68/2016/07/Doll-2.pdf
  • Gaba, S., P. Deroulers, F. Bretagnolle, and V. Bretagnolle.  2019.  Lipid content drives weed seed consumption by ground beetles (Coleoptera, Carabidae) within the smallest seeds.  Weed Research 59:170-179.
  • Gross, K. L., D. Maruca and K. S. Pregitzer.  1992.  Seedling growth and root morphology of plants with different life-histories.  New Phytologist 120:535-542.
  • Loomis, R. S. and D. J. Connor.  1992.  Crop Ecology: Production and Management in Agricultural Systems.  Cambridge University Press: Cambridge, U.K.
  • Shipley, B., and M. Parent.  1991.  Germination responses of 64 wetland species in relation to seed size, minimum time to reproduction and seedling relative growth rate.  Functional Ecology 5:111-118.
  • Stevens, O. A.  1932.  The number and weight of seeds produced by weeds.  American Journal of Botany 19:784-794.
  • Uva, R. H., J. C. Neal and J. M. DiTomaso.  1997.  Weeds of the Northeast.  Cornell University Press: Ithaca, NY.
  • Vengris, J., and R. A. Damon, Jr.  1976.  Field growth of fall panicum and witchgrass.  Weed Science 24:205-208.