Johnsongrass

Images above: Upper left: Johnsongrass plant in flower (Joseph DiTomaso, University of California, Davis). Upper right: Johnsongrass panicle (Scott Morris, Cornell University). Bottom: Johnsongrass rhizomes (Jack Clark, University of California).

Identification

Other common names:  Egyptian millet, means grass, Egyptian grass, false guinea-grass, millet-grass, Morocco millet, Aleppo grass, grass sorghum, Cuba grass, St. Mary's grass, evergreen millet, maiden-cane, Arabian millet, false guinea, Syrian grass

Family:  grass family, Poaceae

Habit:  Perennial grass spreading by thick rhizomes.

Description:  The seedling leaf is roughly horizontal and 0.6-1” (1.5-2.5 cm) long by 0.16-0.24” (0.4-0.5 cm) wide.  The collar region of true leaves is narrow, pale and absent of auricles.  The membranous ligule is translucent with fine teeth.  Seedling true leaf blades are 1.6-7” (4-18 cm) long by 0.1-0.2” (0.25-5 cm) wide, hairless, and rolled in the bud.  The midrib is strong and white near the base of the blade.  Sheaths are green, hairless, smooth, open, and sometimes red at the base.  Seedlings closely resemble domestic corn or sorghum.  Mature plants produce tillers that grow 2-8 ft (0.6-2.4 m) tall.  Stems are pale yellow-green and 0.8” (2 cm) in diameter at maturity.  Sheaths are flattened, hairy, open, ribbed, and slightly toothed; sheaths are maroon to red-brown at the base and pale green near the collar region.  The collar region is broad, smooth, and pale, with a translucent 0.2” (0.5 cm) long membranous ligule on newer leaves and a fringed, hairy ligule on older leaves.  Some scattered hairs are present above the collar on the upper surface of rough-edged blades.  Blades are lance shaped 8-24” (20-61 cm) long by 0.2-2” (0.5-5 cm) wide, but typically less than 1” (2.5 cm) wide, with a strong, wide, white midrib.  Roots are fibrous and grow from an aggressive rhizome system that can exceed 6 ft (1.8 m); rhizomes are white with purple or red splotches and long, scaly, brown sheaths at nodes.   The inflorescence is a purplish, 6-20” (15-51 cm) long, panicle.  Several branches, reaching up to 10” (25 cm) long, are widely spaced and whorled about the nodes.  Branches are further divided into spikelets attached in pairs along the length of the subdivision.  Each spikelet pair has a longer, sterile flower with no awn and a shorter, broader, fertile flower with a twisted and kinked awn.  Fertile flowers produce one 0.1” (0.3 cm) long, oval shaped, dark chestnut brown seed.  A very thin, dry layer of fruit tissue adheres tightly to the seed coat.

Similar Species:  Young plants may be confused with fall panicum (Panicum dichotomiflorum Michx.).  However, johnsongrass does not have a hairy ligule or dense hairs on leaf undersides, sheaths, or collars.  Shattercane (Sorghum bicolor (L.) Moench ssp. verticilliflorum (Steud.) de Wet ex Wiersema & J. Dahlb.) is closely related and seedlings differ from johnsongrass only by their half membranous half hairy ligule and the presence of hairs on both blade surfaces near the collar.  Mature shattercane lacks rhizomes, has purple spotted stems, and can reach 13 ft (4 m) high.

Management

Johnsongrass is considered one of the world's 10 worst weeds (Holm et al. 1977) because it is widely distributed and difficult to control. No-till systems tend to favor the establishment and growth of johnsongrass populations (Govindasamy et al. 2020). Conversely, repeated tillage to break up and exhaust rhizomes dramatically limits the growth of this weed and is the most common method of non-chemical control (King 1966). The rhizome buds will sprout any time the soil is warm so tillage is effective during late spring, summer and early fall. Tillage during cool weather in early spring or late fall is useless since it will not prompt the buds to sprout. Before you begin an eradication attempt, determine how deep the rhizomes go in your soil by digging a few test pits. Deep, rotary tillage with slow forward motion will chop the bulk of the rhizomes into very small pieces that are easier to exhaust. For treating large areas, heavy disks are probably the best implement. If you have many deep rhizomes, follow this with deep chisel plowing. Two passes at a 90° angle to each other will work best. Do the rotary tillage or disking first since otherwise the chisel plowing will loosen the rhizomes and prevent them from getting cut into small pieces. Carbohydrate storage in the rhizomes reaches a minimum 10-30 days after the shoots emerge (McWhorter 1974) and when plants are 6-12” (15-30 cm) high with 4 to 8 leaves (Mitidieri 1984 in Van Esso and Ghersa 1993), so successive tillage events at this time will eliminate shoots before the rhizomes recharge. A field cultivator is ideal for suppressing regrowth after the initial attack on the rhizome system. 

Johnsongrass populations can also be badly damaged by overheating or freezing the rhizomes. Naturally, the former works better in subtropical areas and the latter works better in temperate areas of this weed’s range. For either procedure, use tillage that breaks the rhizome into coarse fragments and work the pieces to the soil surface with a spring tooth harrow. To freeze kill the rhizomes, the frost must penetrate into the soil. If you are treating a small area, push snow off to ensure deep frost penetration. When the soil freezes to a crust, chisel plow to break up the surface and allow deeper penetration of the frost and thorough freezing of the surface chunks. Freeze killing Johnsongrass depends entirely on your coldest snow free winter weather conditions. The procedure may fail completely one year but work well the next. 

Heat killing works best on dark colored soils. Do this during the hottest, sunniest part of the summer. Direct sunshine is more critical than high air temperatures for heating soil. After the rhizomes have cooked for a few days (see Drought tolerance below), stir the soil with a field cultivator or springtooth harrow to bring new rhizomes to the surface. Remember that summer soil temperatures drop rapidly with depth. Small areas can be treated more effectively by covering the soil with black or clear plastic. Remove the plastic periodically so you can stir the soil to expose more rhizomes to killing temperatures. Narrow-windrow burning of soybean residue destroyed Johnsongrass seeds (Norsworthy et al. 2020).

Flooding also can be an effective strategy for Johnsongrass control during fallow periods in the deep South, but timing is critical (Camargo Silva et al. 2024). Disking in fall plus flooding for 14 days when temperatures were warm was effective, whereas flooding after freezing temperatures was ineffective. Rotation with flooded crops such as rice can be an effective means of employing this approach.

 Persistence is the key to Johnsongrass management. Once you have set the population back to manageable levels with the procedures above, continue to hammer it whenever a break in your cropping allows a clean cultivated fallow period or heat killing.

Ecology

Origin and distribution: Johnsongrass is native from southern Europe through India.  It was introduced into the southeastern U.S. in the early 1800's as a forage crop and subsequently spread through most of the United States (USDA Plants).  It has been introduced into most of the temperate and tropical areas of the world, but is best adapted to the humid summer rainfall areas of the subtropics.  A variety that overwinters only as seeds was reported to be spreading northward into southern Canada (Warwick et al. 1984).

Seed weight:  2.6 mg (Barroso et al. 2012), 5.1 mg (Benvenuti et al. 2001), 6.2 mg (Mohler unpublished data).

Dormancy and germination:  Johnsongrass seeds are dormant when shed from the plant due to persistent chaff and tannins in the seed coat that prevent uptake of water (Monaghan 1979, Warwick and Black 1983).  Water stress of the mother plant during seed development reduces seed dormancy through enhanced permeability to oxygen diffusion (Benech Arnold et al. 1992).  After 4-5 months or a period of cold treatment, for example 20 days at 50 °F (10 °C), seeds tend to lose dormancy (Taylorson and McWhorter 1969, Warwick and Black 1983).  Germination response to temperature, light and nitrate varies between populations (Monaghan 1979), but these factors tend to promote germination of seeds from many populations provided they have not experienced a few weeks of cold (Taylorson and McWhorter 1969).  Extreme day/night temperature fluctuations, such as 95/59 °F (35/15 °C), stimulated the highest germination (Benech Arnold et al. 1988), although after-ripening reduced the magnitude of the response to temperature fluctuation (Benech Arnold et al. 1990a).  In the absence of dormancy, seed germination rate was highest at 97 °F (36 °C), whereas rhizome bud break was fastest at 90 °F (32 °C) (Holshouser et al. 1996, Monaghan 1979).  In one study, percent germination was highest after chilling, followed by fluctuating temperatures of 104/75 °F (40/24 °C), light, and a weak nitrate solution.  Johnsongrass germination is turned on by red light (such as direct sunlight) and off by far red light (such as light passing through a plant leaf canopy) (Monaghan 1979).  The leaf canopy also reduces germination in soil by reducing the difference in soil temperature between day and night (Benech Arnold et al. 1988).  Dormancy in Johnsongrass rhizomes is disputed, but there is evidence for winter dormancy and apical dominance (Monaghan 1979).  The temperature optimum for rhizome sprouting is 82 °F (28 °C) (Horowitz 1972a).

Seed longevity:  Johnsongrass seeds can survive for up to five years (Brunch 1961). In one study, johnsongrass seeds were able to survive for six years in undisturbed soil when buried at a depth of 9” (22.5 cm), but less than two years when buried more shallowly (Leguizamon 1986).  The majority of overwinter seed losses when seeds remain on the soil surface was attributed to seed predation (Bagavathiannan and Norsworthy 2013, Scopel et al. 1988, Van Esso and Ghersa 1989).  Viability of buried seeds decreased an estimated 12 to 17% per year in Mississippi (Egley and Chandler 1978, 1983), but 52% in one year in Arkansas (Bagavathiannan and Norsworthy 2013). 

Season of emergence:  Sprouts from rhizomes and seedling emerge in spring, with rhizome sprouts generally emerging earlier and growing more rapidly than seedlings (Warwick and Black 1983).  In Texas, johnsongrass seedlings began emerging at 61 °F (16 °C) (Prostko et al 1998).  Minimum temperature for seedling emergence of johnsongrass in Argentina was 47 °F (8.5 °C) (Benech Arnold et al. 1990b), which is similar to the minimum temperature of 49 °F (9.4 °C) required for emergence of shoots from rhizomes in Italy (Loddo et al. 2012). 

Emergence depth: Seedlings emerge well from the top 2” (5 cm) of soil, and a few seedlings can emerge from as deep as 4” (10 cm) (McWhorter 1972a, Prostko et al 1997) or 6” (15 cm) (Holm et al. 1977). Reduction of day-night temperature differences with depth was identified as an important factor in limiting germination of seeds deeper in the soil profile (Ghersa et al. 1992). Shoots from rhizome fragments emerge best from the surface 0-4” (10 cm), but can emerge from 6” (15 cm) (McWhorter 1972a).

Photosynthetic pathway:  C4 (Elmore and Paul 1983).

Sensitivity to frost:  The shoots are frost sensitive. Rhizomes die if they freeze (King 1966, Monaghan 1979). Freezing typically occurs if rhizomes are exposed to temperatures of 21 to 27 °F (-6 to -3 °C) for 8 hr in the lab (McWhorter 1972a), or daily minimum soil temperatures of less than 16 °F (-9 °C) in the field (Stoller 1977).

Drought tolerance: Long rhizomes resist dehydration more than short rhizomes (Monaghan 1979), and long rhizomes can withstand long periods of drought. Johnsongrass responds to drought by allocating more resources to growth of fine roots that create a higher root surface area for uptake of water (Acciaresi and Guiamet 2010). Exposure of rhizome pieces to high temperature on the soil surface, however, kills them within a few days even in moist soil. They
die in one to three days at 122 to 140 °F (50 to 60 °C) (McWhorter 1972a) and in about seven days at 86 to 95 °F (30 to 35 °C).

Mycorrhiza:  Johnsongrass can form mycorrhizal associations (Entry et al. 1999).

Response to fertility:  Johnsongrass is highly responsive to nitrogen.  For example, dry weight nearly doubled when 48 lb/A (54 kg/ha) of N was applied (Bennett 1973).  The species tolerates pH from 5-7.5 (Warwick and Black 1983).

Soil physical requirements:  Johnsongrass tolerates a wide range of soil conditions but it does best on porous lowland soils and least well on poorly drained clay soils (Warwick and Black 1983). Compaction restricts growth. Flooded soil will kill rhizome buds at 86-104 °F (30-40 °C), but not as effectively at cooler temperatures (McWhorter 1972b). Rhizomes grow closer to the surface in clay soils (down to 2-3” or 5-7 cm) than in sandy loam (down to 4-6” or 10-15 cm) (McWhorter 1972a).

Response to shade:  Johnsongrass can grow rapidly in shade, and the shoots are tall enough to overtop most crops, thereby avoiding shade.  When shaded, the plants put more energy into leaf growth and less energy into storage in the rhizomes (McWhorter and Jordan 1976).

Sensitivity to disturbance:  Most Johnsongrass rhizomes lie in the plow layer where they can be disturbed by tillage, though one study found >10% of rhizomes below 12” (30 cm) (Horowitz 1972b). Short rhizome fragments (1-4” or 2.5-10 cm) were less vigorous and required 20-30 days longer to initiate new rhizome growth than fragments 6-8” (15-20 cm) (Lolas and Coble 1980). Periodic mowing when Johsongrass shoots reached 1 ft (30 cm) high reduced rhizome
biomass and length after two years (Camargo Silva et al. 2024).

Time from emergence to reproduction:  Johnsongrass begins flowering about seven weeks after emergence and continues flowering until frost (Bridges and Chandler 1989, Monaghan 1979, Scopel et al. 1988).  Seed shed begins 3 months after planting and can continue for an additional 3 to 4 months (Barroso et al. 2012, Scopel et al. 1988).  Johnsongrass flower development was most rapid at 90 °F (32 °C), but was nil at 54 or 104 °F (12 or 40 °C) (Holshouser et al. 1996).  The minimum temperature for rhizome formation was between 59 and 68 °F (15 and 20 °C) (Monaghan 1979).  New rhizome initiation begins approximately 30 to 60 days after planting depending on size of initial rhizome fragments (Lolas and Coble 1980).

Pollination:  Johnsongrass normally self-pollinates, but can also cross-pollinate by wind (Warwick and Black 1983).

Reproduction:  Johnsongrass reproduces by both seeds and rhizome sprouts.  Two-year old plants grown in a garden experiment in Mississippi produced 28,000 seeds per plant (Monaghan 1979).  Seed production by individual plants decreased from 2,350 to 87 seeds as plant densities increased from moderate to high levels (Barroso et al. 2012).  Plants in a dense stand of Johnsongrass produced 66 seeds and 40 rhizome buds per plant (Scopel et al. 1988).  Only 4% of seeds were shattered at the time of soybean harvest in Texas (Schwartz-Lazaro et al. 2021).  

New rhizomes grow from the overwintering rhizomes in early summer and form new shoots.  Rhizomes formed after flowering will overwinter.  Single plants of Johnsongrass produced 18 lb (8 kg) and 200-300 ft (60-90 m) of new rhizomes per year (Monaghan 1979).  In an Israeli study, patches from single individuals covered 183 square ft (17 square m) with 18 shoots per square ft (190 per square m) (Warwick and Black 1983).

Dispersal:  Johnsongrass seeds are dispersed by water, on machinery, in contaminated grain and hay, and for, short distances, by wind (Barroso et al. 2012, Monaghan 1979).  They pass through cattle digestive tracts and are spread with manure.  They are also dispersed by birds.  Within a field, johnsongrass disperses from the primary source by movement of rhizomes in the direction of tillage (Andújar et al. 2012). 

Common natural enemies:   Sphacelotheca cruenta (loose kernel smut) infected 55% of johnsongrass panicles, but had little impact on the competitiveness of this weed (Millhollon 2000).

Palatability:  Johnsongrass is highly palatable to livestock and is grown as a forage species in the southeastern U.S.A (Bennet 1973).  Young growth and plants stressed by frost or drought may develop toxic levels of cyanide producing compounds (Burrows and Tyrl 2006, Holm et al. 1977).

Note:  Johnsongrass produces allelopathic compounds that suppress the growth of many crops and other weeds (Monaghan 1979, Warwick & Black 1983, Vasilakoglou et al. 2005).

References:

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