Fall panicum

Images above: Upper left: Fall panicum plant and inflorescence (Antonio DiTommaso, Cornell University). Upper right: Fall panicum plant (Randall Prostak, University of Massachusetts). Bottom: Fall panicum seedling (Randall Prostak, University of Massachusetts).

Identification

Other common names:  spreading witch-grass, sprouting crab-grass, smooth witchgrass, western witchgrass

Family:  Grass family, Poaceae.

Habit:  Large, erect, summer annual grass.

Description:  Seedlings often have a purple hue.  Seed leaves and first true leaves are parallel to the ground, lanceolate to linear in shape, and densely hairy on their undersides.  True leaves are rolled in the bud and hairless after the fourth or fifth leaf stage.  Hairy ligules are 0.06” (0.15 cm) long; auricles are absent.  Blades are 0.75-1.5” (1.9-3.8 cm) long by 0.25” (0.6 cm) wide and rough edged.  Sheaths are compressed and hairy.  Mature plants are largely hairless, reaching up to 5 ft (1.5 m) tall.  Stems are waxy, with irregularly spaced, swollen nodes that give the stem a zigzagged appearance.  Red-purple sheaths are compressed, smooth, hairless, and open at the top.  Collars are wide; ligules are densely hairy and 0.06” (0.15 cm) long.  Blades are dull green with red edges, glossy undersides, and lance to linear, 4-20” (10-51 cm) long by 0.25-0.75” (0.6-1.9 cm) wide.  Leaves have a pale green to white prominent midvein.  The root system is fibrous with spontaneous roots developing at lower stem-sheath joints.  The inflorescence is a widely-spaced, 4-16” (10-41 cm) long panicle with long, subdividing branches.  Panicles are purplish at maturity and are located at stem ends and in leaf joints.  Narrow, stiff stems support stalked, narrow spikelets at the branch tips.  Spikelets are dull yellow to purple-hued, 0.10-0.12” (0.25-0.3 cm) long by 0.08” (0.2 cm) wide, and oval to thumb shaped.  Spikelets each produce one dull, yellow-brown, 0.06” (0.15 cm) long seed. 

Similar species:  Yellow foxtail [Setaria pumila (Poir.) Roem. & Schult.] is similar at the seedling stage, but it has hairless sheaths and long wispy hairs near the blade base.  Witchgrass (Panicum capillare L.) seedlings have hairs on both blade surfaces.  Wild-proso millet (Panicum miliaceum L.) is distinguished from fall panicum by its half membranous, half hairy ligule. 

Management: Crop rotation is a key element in controlling fall panicum.  Since it does not germinate until the soil warms, either winter grains or early planted spring grains are highly competitive with this species.  Also, leaf cover from well-established grain crops will keep soil temperatures cool and relatively constant, which will decrease germination (Fausey and Renner 1997).  Moreover, both fall and spring sown grains are harvested before fall panicum sets seed.  Post-harvest cleanup or a competitive interplanted cover crop like red clover can prevent seed set, particularly if the cover crop is mowed when fall panicum begins to flower.  Set the mower at about 4-5” (10-12 cm); the goal is to sever the young flowering stalks but leave the cover crop enough tissue to regrow quickly and smother the weed.  Since the seeds do not persist well in the soil, rotation with hay is helpful in suppressing this species.  In vegetables, any short season crop that is harvested before fall panicum begins to set seed in late summer will disrupt the life cycle, provided the field is cleaned promptly after harvest.

For row crops, tine weed pre- and post-emergence, and throw soil into the row to cover seedlings if the crop will tolerate such a procedure.  If possible, cultivate shallowly close to the row.  Severing the root from the shoot with minimal soil disturbance may be more effective than excavating or burying this weed.  Unless cultivation is effective, the environment in long season row crops is likely to favor fall panicum.

 Avoid soil compaction since this favors fall panicum.  Similarly, if the field has drainage problems, drain tile or other measures to improve drainage will increase the competitiveness of crops relative to the weed.  Flooding during rice production significantly reduces growth and reproduction of fall panicum, particularly at water depths greater than 4 inches (10 cm) (Chiruvelli et al. 2023).

Ecology

Origin and distribution:  Fall panicum is native to North America.  It occurs throughout the U.S.A., Mexico and the West Indies (Newman 2006), and it has been introduced throughout the more humid parts of southern Canada (USDA Plants Database).  Its status as a native in much of the western U.S.A. seems doubtful.

Seed weight:  0.57 mg (Mohler unpublished), 0.2 to 0.9 mg (Gross and Smith 1991).

Dormancy and germination:  Seeds are dormant when shed due to the tight, hard chaff (Baskin and Baskin 1983, Brecke and Duke 1980).  Four to five months of after-ripening or 2 weeks of cold, moist conditions breaks this dormancy (Brecke and Duke 1980).  Even when this first level of dormancy is relieved, the seeds still require light and either high (95 °F = 35 °C) (Taylorson 1980) or fluctuating day/night temperatures ranging from 68/50 °F to 95/68 °F (20/10 °C to 35/20 °C) to germinate (Baskin and Baskin 1983, Fausey and Renner 1997).  Seeds are only capable of dark germination when exhumed in mid-summer months and exposed to 95/68 °F (35/20 °C) temperature fluctuation (Baskin and Baskin 1983).  Fall panicum seeds require high soil moisture for germination.  For example, germination of this weed is more sensitive to moderately dry soil than is corn or common lambsquarters (Barrett and Peters 1976).

Seed longevity:  Two percent of seeds germinated after 10 years of burial at 9” (23 cm) under sod in Nebraska (Burnside, et al., 1981).  Seeds buried at 8” (20 cm) in silt loam and gravelly loam in Ontario declined by 39% and 56% per year over a 4.5 year period (computed from Alex 1980).  Seeds buried more shallowly disappeared faster, partially due to emergence.

Timing of emergence:  Fall panicum emerges primarily in mid- to late-spring in the South (Newman 2005) and mid-spring to mid-summer in the North (Doll 2002, Alex 1980, Vengris 1975), but some plants emerge throughout the growing season.

Emergence depth:  Fall panicum emerges best from the soil surface to 1” (2.5 cm), but a few seedlings can emerge from as deep as 3” (7.6 cm) (Alex 1980, Brecke and Duke 1980, Fausey and Renner 1997, Vengris 1975).

Photosynthetic pathway:  C4 (Elmore and Paul 1983)

Sensitivity to frost:  This species is sensitive to frost.  Seeds fall from the plant after the first frost (Newman 2006). 

Drought tolerance: This species is favored by moist conditions (Hilty 2006, Newman 2006).  Growth and reproduction decline approximately linearly as soil water content declines, but fall panicum is still capable of producing seeds under dry conditions (Chiruvelli et al. 2022).  Emergence and seedling survival are enhanced by supplemental irrigation that alleviates droughty conditions (Gross and Smith 1991).  It is well adapted to survive and reproduce under shallow (2-4” or 5-10 cm) flooded conditions as is found in rice paddies (Chiruvelli et al. 2023).

Mycorrhiza:  There are no reports of the mycorrhizal status of fall panicum, but other Panicum species are mycorrhizal (Dhillion and Friese 1994).

Response to fertility:  Increasing rates of 10-10-10 fertilizer promoted growth of fall panicum between corn rows.  Conditions were favorable for corn growth in this experiment and competition from the crop prevented a noticeable fertility response by the weed within rows.  Potassium concentration was lower in plants growing in the row than between the row.  (Peters and Barrett 1976)

Soil physical requirements:  Fall panicum occurs on soils with a wide range of textures, but it does best on fine textured (Hilty 2006) and organic soils (Newman 2005).  Fall panicum is an indicator of anaerobic, compacted soils.  It tolerates waterlogged and salty soils (Newman 2006).

Response to shade:  Shade from crops can substantially decrease biomass, height, seed production and increase maturation rate (Vengris 1975, Vengis and Damon 1976).

Sensitivity to disturbance:  Cutting seedlings just below the base of the shoot kills them (Alex 1980), so shallow tillage that severs seedlings should be effective at controlling this species.

Time from emergence to flowering:  Plants emerging in early May in Wisconsin flowered in about 3 months (Doll 2002).  Time to flowering and seed set decreases with later emerging plants.  For example, in Massachusetts, plants emerging in late May required 116 days to set seed whereas plants emerging in late July required only 74 days (Vengris 1975).  

Pollination:  Fall panicum is wind pollinated (Hilty 2006).

Reproduction:  Typical plants growing in a field in Massachusetts produced 500,000 seeds per plant (Vengris 1975).  Production of 10,000 to 100,000 seeds per plant may be more usual for larger plants (Govinthasamy and Cavers 1995), while seed production under competition from a mixed community of weeds will range from 100 to 300 seeds per plant (Gross and Smith 1991).

Dispersal:  As plants commonly grow near water and the seeds float (Newman 2006), the seeds probably disperse in waterways.  Fall panicum seeds are spread with cattle manure (Mt. Pleasant and Schlather 1994).  Given the high density of seeds sometimes found in the soil (Alex 1980), seeds probably also spread with soil clinging to tires, shoes and machinery.

Common natural enemies:  The specialist smut fungus (Ustilago destruens) can greatly reduce seed production of fall panicum (Govinthasamy and Cavers 1995).

Palatability:  People do not eat fall panicum.  At the vegetative stage, fall panicum can provide ruminants with nutrient levels similar to cultivated grasses (Bosworth et al. 1980).  The species can cause nitrate poisoning and photosensitivity in livestock (Newman 2006).  Dichotomin, a furostanol saponin isolated from fall panicum, may account for photosensitivity (Munday et al. 1993). 

References:

• Alex, J. F.  1980.  Emergence from buried seed and germination of exhumed seed of fall panicum.  Canadian Journal of Plant Science 60:635-642.
• Barret, M., and R. A. Peters.  1976.  Germination of corn, lambsquarters, and fall panicum under simulated drought.  Proceedings of the Northeastern Weed Science Society 30:98-103.
• Baskin, J. M., and C. C. Baskin.  1983.  Seasonal changes in the germination responses of fall panicum to temperature and light.  Canadian Journal of Plant Science 63:973-979.
• Brecke, B. J., and W. B. Duke.  1980.  Dormancy, germination, and emergence characteristics of fall panicum (Panicum dichotomiflorum) seed.  Weed Science 28:683-685.
• Bosworth, S. C., C. S. Hoveland, G. A. Buchanan, and W. B. Anthony.  1980.  Forage quality of selected warm-season weed species.  Agronomy Journal 72:1050-1054.
• Burnside, O. C., C. R. Fenster, L. L. Evetts, and R. F. Mumm.  1981.  Germination of exhumed weed seed in Nebraska.  Weed Science 29:577-586.

• Chiruvelli, V. S. D. K. S., H. S. Sandhu, R. Cherry, and D. C. Odero.  2022.  Influence of soil water content on growth and panicle production of fall panicum (Panicum dichotomiflorum).  Weed Technology 36:678–684.

• Chiruvelli, V. S. D. K. S., H. S. Sandhu, R. Cherry, and D. C. Odero.  2023.  Effect of depth of flooding on growth and fecundity of fall panicum (Panicum dichotomiflorum).  Weed Technology 37:76–83.

• Dhillion, S. S., and C. F. Friese.  1994.  The occurrence of mycorrhizas in prairies: application to ecological restoration.  Thirteenth North American Prairie Conference: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/
• Elmore, C. D., and R. N. Paul.  1983.  Composite List of C4 Weeds.  Weed Science 31:686-692.
• Fausey, J. C., and K. A. Renner.  1997.  Germination, emergence, and growth of giant foxtail (Setaria faberi) and fall panicum (Panicum dichotomiflorum).  Weed Science 45:423-425.
• Govinthasamy, T., and P. B. Cavers.  1995.  The effect of smut (Ustilago destruens) on seed production, dormancy, and viability in fall panicum (Panicum dichotomiflorum).  Canadian Journal of Botany 73:1628-1634.
• Gross, K. L., and A. D. Smith.  1991.  Seed mass and emergence time effects on performance of Panicum dichotomiflorum Michx. across environments.  Oecologia 87:270-278.
• Hilty, J.  2006.  Illinois Wildflowers. Fall panicum: Panicum dichotomiflorum.  http://www.illinoiswildflowers.info/index.htm.
• Mt. Pleasant, J., and K.J. Schlather.  1994.  Incidence of weed seed in cow (Bos sp.) manure and its importance as a weed source for cropland.  Weed Technology 8:304-310.
• Munday, S. C., A. L. Wilkins, C. O. Miles, and P. T. Holland.  1993.  Isolation and structure elucidation of dichotomin, a furostanol saponin implicated in hepatogeneous photosensitization of sheep grazing Panicum dichotomiflorum.  Journal of Agricultural Food Chemistry 41:267-271.
• Newman, S. D.  2006.  Natural Resources Conservation Service, Plant Guide: Fall Panicgrass Panicum dichotomiflorum Michx.  http://plants.usda.gov/plantguide/pdf/pg_padi.pdf
• Peters, R. A.,  and M. Barrett.  1976.  Competition between corn and fall panicum or large crabgrass at three fertility levels.  Northeastern Weed Science Society 30:104.
• Taylorson, R. B.  1980.  Aspects of seed dormancy in fall panicum (Panicum dichotomiflorum). Weed Science 28:64-67.
• USDA Plants Database.  Painicum dicotomiflorum. http://plants.usda.gov
• Vengris, J.  1975.  Field growth habits of fall panicum.  Proceedings of the Northeastern Weed Science Society 29 (Supplement):8.
• Vengis, J., and R. A. Damon, Jr.  1976.  Field growth of fall panicum and witchgrass.  Weed Science 24:205-208.