The following description of Johnson grass provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g., [72,93,94,96,111,149,176,194]).
Morphology: Johnson grass is a nonnative, warm-season perennial [49,71,122,184,201]. It is usually rhizomatous, but is a highly variable species with many ecotypes [96]. It may grow as an annual in hot, arid climates and at the northern limits of its range [61,197,198]. Johnson grass rhizomes form a dense, tangled, tough sod [204]. Rhizomes serve as carbohydrate-storing and regeneration organs [6]. Most rhizomes occur in the top 7.9 inches (20 cm) of soil, although rhizomes in soft, deep soil may extend deeper [42,96]. Rhizomes vary in size from a few inches to several feet in length, and in thickness from 0.25 to 0.75 inch (6.4-19 mm) [86]. Leaves and aboveground stems (culms) are coarse [149]. Culms are 1.6 to 4.9 feet (0.5-1.5 m) tall. Total plant height may reach 12 feet (3.7 m) during flowering [5]. The inflorescence is a 4- to 24-inch (10-60 cm) open panicle. Spikelets of Sorghum species are paired: 1 is sessile and perfect; the other spikelet is pedicelled and staminate. Spikelets are 4 to 7 mm in length. There are about 35 to 350 spikelets per panicle, depending upon ecotype. Lemmas are cilate; they may be awnless or have short (1-15 mm), sometimes twisted awns that aid in seed dispersal [49,70,93,122,184,194,196,201]. Seeds are about 2 mm long [149].
Physiology: Several physiological characteristics of Johnson grass aid in its spread. Mature Johnson grass plants are moderately drought resistant [6] and salt tolerant [207]. Johnson grass produces toxins (see Toxicity) that may be allelopathic [96,135,196].
Johnson grass is native to the Mediterranean region of Europe and Africa, and possibly to Asia Minor. Worldwide, its range as a weed extends from 55° N to 45° S in latitude [96,128]. It was widely introduced in North America, Europe, Africa, and southwestern Asia [184], and was also introduced in Brazil, Argentina [155], and northern Australia [78]. In North America it occurs in southern Ontario south through all the contiguous United States except Maine [72,109,206] to the Rio Grande Delta region of Tamaulipas and the Cape region of Baja California Sur in Mexico [204]. Johnson grass also occurs in Hawaii and the Caribbean [109].
In the United States, Johnson grass was introduced in South Carolina from Turkey around 1830. William Johnson, whom the plant is named after, established Johnson grass along the Alabama River in the 1840s as a forage species, and Johnson grass spread rapidly across the South [14,149,170,182]. Johnson grass is now widely escaped from cultivation in much of the United States. It is most invasive in the Southeast, although it is widespread in central California and New Mexico [122,128,206]. Johnson grass is not persistent in the Pacific Northwest, upper northern Great Plains, extreme northern portions of the Great Lake states, the Northeast [70,110,132,194], or in Arizona, Colorado, and Utah [111,200,201]. Plants database provides a state distributional map of Johnson grass.
Johnson grass occurrence is not well documented for all plant communities where it may occur. The following classification lists are not restrictive, but include plant communities where Johnson grass is a documented species.
Fire adaptations: Johnson grass is likely to survive fire by sprouting from rhizomes (review by [71]). Because Johnson grass rhizome depths can reach 8 inches (20 cm) or more below ground [42,96], Johnson grass is likely to survive even severe fire. Rhizome expression in Johnson grass is variable (see Asexual regeneration); therefore, this adaptation may not be applicable to all populations.
Little documentation is available on postfire regeneration of Johnson grass from seed. As a seed banking species that produces abundant, readily dispersed seed (see Sexual regeneration) that establishes well in open, disturbed sites (see Seedling establishment/growth), it is likely that Johnson grass is capable of postfire seedling establishment from both on- and off-site sources. Germination of Johnson grass seed was not affected by exposure to temperatures of 200 °F (90 °C) and 400 °F (200 °C) for 120 seconds. However, germination was dramatically reduced following exposure to 660 °F (350 °C) and was eliminated after exposure to 800 °F (430 °C) and higher [131]. See [131]. See IMMEDIATE FIRE EFFECT ON PLANT for details of this study. More information is needed on postfire establishment potential of Johnson grass.
Fuels: Johnson grass litter reportedly remains on the ground all winter [104]. Johnson [107] provides a simple technique for estimating ratios of live:dead plant materials in Johnson grass.
FIRE REGIMES: As of this writing (2004), there was no published information on how North American FIRE REGIMES affect Johnson grass. In riparian and other areas where Johnson grass is highly productive, Johnson grass may promote fire spread by increasing fine fuel loads above historical levels. Studies are needed on the fire ecology of Johnson grass in North American.
The following table provides fire return intervals for plant communities and ecosystems where Johnson grass may be important. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) maple-beech-birch Acer-Fagus-Betula > 1,000 silver maple-American elm A. saccharinum-Ulmus americana < 35 to 200 sugar maple A. saccharum > 1,000 sugar maple-basswood A. saccharum-Tilia americana > 1,000 [195] California chaparral Adenostoma and/or Arctostaphylos spp. < 35 to < 100 bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium 117,144] Nebraska sandhills prairie A. gerardii var. paucipilus-S. scoparium < 10 bluestem-Sacahuista prairie A. littoralis-Spartina spartinae 144] silver sagebrush steppe Artemisia cana 5-45 [92,122,148] sagebrush steppe A. tridentata/Pseudoroegneria spicata 20-70 [144] basin big sagebrush A. tridentata var. tridentata 12-43 [160] Wyoming big sagebrush A. tridentata var. wyomingensis 10-70 (40**) [192,210] coastal sagebrush A. californica < 35 to < 100 saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus < 35 to < 100 desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100 [144] plains grasslands Bouteloua spp. 122,144] blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii 122,144,159] blue grama-buffalo grass B. gracilis-Buchloe dactyloides 122,144] grama-galleta steppe Bouteloua gracilis-Pleuraphis jamesii < 35 to < 100 blue grama-tobosa prairie B. gracilis-P. mutica 144] sugarberry-America elm-green ash Celtis laevigata-Ulmus americana-Fraxinus pennsylvanica 195] paloverde-cactus shrub Cercidium microphyllum/Opuntia spp. < 35 to < 100 blackbrush Coleogyne ramosissima < 35 to < 100 northern cordgrass prairie Distichlis spicata-Spartina spp. 1-3 [144] beech-sugar maple Fagus spp.-Acer saccharum > 1,000 [195] California steppe Festuca-Danthonia spp. 144,178] black ash Fraxinus nigra 195] cedar glades Juniperus virginiana 3-22 [79,144] creosotebush Larrea tridentata < 35 to < 100 Ceniza shrub Larrea tridentata-Leucophyllum frutescens-Prosopis glandulosa 144] yellow-poplar Liriodendron tulipifera 195] Everglades Mariscus jamaicensis 134] wheatgrass plains grasslands Pascopyrum smithii 122,144,148] pine-cypress forest Pinus-Cupressus spp. 10] shortleaf pine P. echinata 2-15 shortleaf pine-oak P. echinata-Quercus spp. < 10 slash pine P. elliottii 3-8 slash pine-hardwood P. elliottii-variable < 35 sand pine P. elliottii var. elliottii 25-45 [195] South Florida slash pine P. elliottii var. densa 1-5 longleaf-slash pine P. palustris-P. elliottii 1-4 [134,195] longleaf pine-scrub oak P. palustris-Quercus spp. 6-10 [195] pitch pine P. rigida 6-25 [38,90] pocosin P. serotina 3-8 pond pine P. serotina 3-8 loblolly pine P. taeda 3-8 loblolly-shortleaf pine P. taeda-P. echinata 10 to < 35 sycamore-sweetgum-American elm Platanus occidentalis-Liquidambar styraciflua-Ulmus americana 195] galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea < 35 to < 100 eastern cottonwood Populus deltoides 144] mesquite Prosopis glandulosa 124,144] mesquite-buffalo grass P. glandulosa-Buchloe dactyloides < 35 Texas savanna P. glandulosa var. glandulosa 144] black cherry-sugar maple Prunus serotina-Acer saccharum > 1,000 [195] California oakwoods Quercus spp. 10] oak-hickory Quercus-Carya spp. < 35 northeastern oak-pine Quercus-Pinus spp. 10 to 195] oak-gum-cypress Quercus-Nyssa-spp.-Taxodium distichum 35 to > 200 [134] southeastern oak-pine Quercus-Pinus spp. 195] coast live oak Q agrifolia 2-75 [74] white oak-black oak-northern red oak Q alba-Q. velutina-Q. rubra 195] canyon live oak Q chrysolepis <35 to 200 blue oak-foothills pine Q douglasii-P. sabiniana 10] northern pin oak Q ellipsoidalis 195] Oregon white oak Q garryana 10] bear oak Q ilicifolia 195] California black oak Q kelloggii 5-30 [144] bur oak Q macrocarpa 195] oak savanna Q macrocarpa/Andropogon gerardii-Schizachyrium scoparium 2-14 [144,195] chestnut oak Q prinus 3-8 northern red oak Q rubra 10 to < 35 post oak-blackjack oak Q stellata-Q. marilandica < 10 black oak Q velutina < 35 live oak Q virginiana 10 to195] interior live oak Q. wislizenii 10] cabbage palmetto-slash pine Sabal palmetto-P. elliottii 134,195] blackland prairie Schizachyrium scoparium-Nassella leucotricha < 10 Fayette prairie S. scoparium-Buchloe dactyloides 195] little bluestem-grama prairie S. scoparium-Bouteloua spp. < 35 tule marshes Scirpus and/or Typha spp. < 35 southern cordgrass prairie Spartina alterniflora 1-3 [144] baldcypress Taxodium distichum var. distichum 100 to > 300 pondcypress T. distichum var. nutans 134] elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. 52,195] **meanJohnson grass is most common on disturbed sites such as ditch banks, roadsides, fields, and "waste places" [93,111,149,200]. It occurs on all soil textures [86], with best growth occurring on silty bottomland soils [3,86]. On old bottomland fields of the Mississippi and Yazoo rivers, Mississippi, Johnson grass cover is greatest on silty-clay loams [3].
Moisture regime: Although Johnson grass occurs in wet to dry habitats in its native range in southern Europe [184], it is generally restricted to wet or mesic sites in the United States [72,93,201]. It is most common in warm, humid southern climates that receive ample summer rainfall. Johnson grass is a facultative wetland species, frequently occurring on floodplains [18,19]. Johnson grass patches are often extensive along canals and irrigation ditches [96]. In arid sites such as Organ Pipe Cactus National Monument, Arizona, Johnson grass establishes mostly in wet locations including washes, drainages, and stream edges [61,177].
Elevational ranges of Johnson grass in several states are:
Arizona below 6,000 ft (1,800 m) [111] California below 2,600 ft (800 m) [93] Nevada 2,000-5,000 ft (600-1,500 m) [201] New Mexico 3,500-6,000 ft (1,100-1,800 m) [122] Utah 2,800-5,000 ft (850-1,500 m) [201]Fire likely top-kills Johnson grass [86], while most rhizomes probably survive most fires.
Fire scarification appears to have no effect on rate of seed germination, and very high temperatures kill Johnson grass seed. In the laboratory, there were no significant differences in rates of germination between unheated Johnson grass seed and seed heated to 200 °F (90 °C) and 400 °F (200 °C) for 120 seconds; mean germination rate was 52%. Mean germination rate dropped to 17% for seed exposed to 660 °F (350 °C) and to 0% for seed exposed to 800 °F (430 °C) and 1000 °F (538 °C) for 120 seconds [131].
Impacts: Johnson grass is an important agricultural weed that causes serious economic losses [35,80,96,112]. Based upon its nearly worldwide distribution and adverse effect on the global economy, it is described as 1 of the world's worst weeds [96]. Johnson grass was recognized as 1 of the 6 most damaging weeds in the United States by the turn of the 20th Century, and was the 1st weed targeted by the USDA for research on control methods [128]. Johnson grass causes millions of dollars in lost agricultural revenue annually in the United States [129]. For example, Johnson grass infestations reduce yields in Louisiana sugarcane (Saccharum officinarum) fields by 25-50% [128]. In 1 study, 7 tons/acre (16 t/ha) of Johnson grass rhizomes were produced on a Louisiana sugarcane field [86]. Johnson grass also impacts agricultural lands as an alternate host for many of crop-damaging insects, nematodes, fungi, and viruses [128]. It hosts sorghum midges [35,69,162], southwestern corn borers [12], corn leaf aphids [106], sugarcane borers [29], banks grass mites [68], sorghum downy mildew [30], and maize viruses [96,128,161].
Little is documented on Johnson grass's impact in wildlands, and further research is needed on how Johnson grass affects wildland habitats. Generalizations about Johnson grass must always be qualified because of numerous ecotypes [125]. Typically, Johnson grass is a good competitor for nutrients [96,196], space [103], and water [166]. It can outcompete associated species for water by extracting water from lower soil profiles (12 inches (30 cm) or more below ground) [105]. Johnson grass may also negatively impact plant community composition through its reputed allelopathy [96,135,196]. Cyanogenetic glycosides and other toxins in Johnson grass may inhibit germination and growth of associated plant species [96,135,196].
On many sites in the United States, Johnson grass is not invasive in undisturbed wildlands, although it may readily invade disturbed sites (Cox, cited in [135]). Johnson grass is most invasive on moist sites in wet-temperate regions of the southeastern United States [71,72,135]. For example, Johnson grass and Canada thistle (Cirsium arvense) were listed as the 2 most invasive and expensive to control weeds on the Eastern Neck National Wildlife Refuge, Maryland [44]. Johnson grass interferes with conifer seedling establishment and growth on southern pinelands [50], and may interfere with cottonwood (Populus spp.) and willow (Salix spp.) establishment in riparian zones [171].
Johnson grass is not invasive on most sites in the Southwest. At the turn of the last century, Johnson grass was planted in southwestern arroyos and stream channels to stabilize soil [46]. It established on such wet and mesic sites, but failed to spread. Felger [61] reports Johnson grass as only weakly invasive in Organ Pipe Cactus National Monument, Arizona, where the arid climate restricts Johnson grass to roadsides and washes. Johnson grass may grow as an annual, without spreading, in arid southwestern wildlands. Nearby agricultural lands are continual seed sources [61].
Control: Although considerable information is available on controlling Johnson grass in agricultural settings (e.g., see [9,80,84,127,180]), information on controlling Johnson grass in rangelands, natural areas, and other wildlands is lacking. The following information on Johnson grass control is extracted primarily from agricultural literature but may be applied to some wildland settings, particularly old fields. Research is needed on controlling Johnson grass in wildland settings [135].
Johnson grass control involves several steps: 1) preventing seed from ripening and dispersing, 2) killing seedlings, 3) killing existing rhizomes, and 4) preventing growth of new rhizomes [9,80,84,127,180]. Control is most effective before plants have developed 5 leaves [101]. Detailed Johnson grass control procedures and techniques are given in several publications [96,127,135].
Prevention: The most efficient and effective method of managing invasive species such as Johnson grass is to prevent their invasion and spread [164]. Preventing the establishment of nonnative invasive plants in wildlands is achieved by maintaining native communities and conducting aggressive surveying, monitoring, and any needed control measures several times each year. Monitoring efforts are best concentrated on the most disturbed areas in a site, particularly along potential pathways for Johnson grass invasion: roadsides, waterways, and old fields. Large plant size makes monitoring Johnson grass relatively easy in summer, and yearly summer monitoring helps managers assess the effectiveness of control programs. As of this writing (2004), monitoring programs for Johnson grass were in their infancy. As potential contact sources, Newman [135] provides a list of managers who have started monitoring programs for Johnson grass on Natural Areas. The Center for Invasive Plant Management provides an online guide to noxious weed prevention practices.
Integrated management: A combination of complementary control methods may be helpful for rapid and effective control of Johnson grass. Integrated management includes not only killing the target plant, but establishing desirable species and discouraging nonnative, invasive species over the long term. Johnson grass control is rarely successful with only 1 method of control [141], but a combination of control methods can be effective. For example, in a tallgrass restoration study on the Hear Wildlife Sanctuary, Texas, a combination of early fall glyphosate spraying followed by late fall tillage helped control nonnative grasses on a former Johnson grass-Bermuda grass (Cynodon dactylon) pasture. Early fall spraying targeted Johnson grass while it was still actively growing. After spraying, rhizomes brought to the soil surface by tilling 4 to 6 inches (10-15 cm) deep were killed by winter frost. Johnson grass showed 4.2% cover and 50% frequency 3 years after treatments. Only trace amounts of Bermuda grass were present [172].
Fire: See Fire Management Considerations.
Biological: Biological control of Johnson grass is problematic, as known control agents that kill Johnson grass also kill crop grasses such as corn and sorghum [128,143,145]. As of this writing (2004), there are no biocontrol agents approved for Johnson grass [183]. Several biological agents are being tested for possible use. A smut (Sphacelotheca holci) has helped control Johnson grass in Louisiana croplands [130]. In Florida field trials, a mixture of native fungal pathogens controlled Johnson grass and other weedy grasses in citrus (Citrus spp.) groves [43].
Heavy grazing over 2 or more years reduces Johnson grass by depleting rhizome reserves [3,8,89]. Rhizome development is greatly reduced when plant height is kept below 12 to 15 inches (30.5-38 cm) [127]. Best control is offered when herbicide or winter plowing treatments follow grazing treatments [3]. For example, in an unpublished study at the Patagonia/Sonoita Creek Preserve, Arizona, cow and horse summer grazing reduced density of Johnson grass. After 4 years of summer grazing, Johnson grass stem density had decreased 75% compared to pretreatment levels. Plots were then sprayed in late spring with glyphosate. Posttreatment restoration plantings gave mixed results. One to two months after spraying, native bunchgrasses were transplanted onto the study sites. Broadleaf weeds invaded the study plots after Johnson grass density was reduced by the grazing and herbicide treatments. After mowing treatments to control the broadleaved weeds, native bunchgrasses on some test plots were showing good growth. Other plots experienced Johnson grass reinvasion and pocket gopher herbivory, to the detriment of native bunchgrasses. Preserve managers are continuing weed control treatments to promote the native bunchgrasses [183].
Geese are sometimes used for Johnson grass control in croplands. Geese prefer young shoots, and do not graze Johnson grass over about 7 inches (18 cm) in height [9,86].
Chemical: Herbicides may provide initial control of a new invasion or a severe infestation, but used alone, they are rarely a complete or long-term solution to invasive species management [40]. Herbicides are most effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change the conditions that allowed the invasion to occur in the first place (e.g., [211]). See The Nature Conservancy's Weed Control Methods Handbook for considerations on the use of herbicides in Natural Areas and detailed information on specific chemicals.
The most effective chemical control of Johnson grass involves using systematic herbicides that translocate the active chemicals to rhizomes [127]. A single application of herbicide generally does not control large infestations, and follow-up measures are needed for long-term control [169]. Johnson grass control can be obtained using glyphosate [7,21,104,127], phenoxy (e.g., 2,4-D, fluazifop), [108,115], or halogenated aliphatic (e.g., dalapon) herbicides [8,86,127]. Spot spraying with sodium chlorate [82,86] or dalapon has been effective for small infestations [153]. Spot control is not effective in the long term unless surrounding seed sources are also eliminated [104]. Experiments in agricultural fields in Argentina showed best control when the herbicide (dalapon) was applied when rhizome biomass was low. Ghersa and others [67] provide a model for predicting optimal spraying time based on minimum rhizome biomass. Although based on South American seasons, the model is easily adjustable for use in the northern hemisphere.
Postemergent herbicides are the most common method of Johnson grass control in agricultural systems, and are probably the best herbicide choice for wildland settings as well, since postemergent herbicides cause less damage to nontarget species. In a Maryland old-field study, foliar application of postemergence herbicide (DPX-V9360) was more effective in late-growth stages (>5 leaves) than early-growth stages (<5 leaves) when rhizomes had not fully expanded [138]. Rosales-Robles and others [158] discuss the relative effectiveness of several postemergent herbicides as influenced by application rate and Johnson grass growth stage. Application procedures for postemergent herbicides effective on Johnson grass are given in these publications: [20,51,119,208].
Ecotypes may show differential response to herbicides [128]. Populations in Kentucky and Mississippi show genetic resistance to fluazifop and other phenoxy herbicides [15,137,168]. Virginia populations have resistance to enzyme acetyl-coenzyme A carboxylase inhibitors [33]. In Greece, some populations show resistance to glyphosate [113].
Herbicide treatments greatly decreased Johnson grass cover in an Illinois bottomland old field. Restoration treatments included tillage, pre- or postemergent herbicide applications (sulfometuron or glyphosate, respectively), and green ash (Fraxinus pennsylvanica) plantings. Tillage had no significant impact on Johnson grass cover. Mean Johnson grass cover (%) was significantly lower after the 1st postspray year [76]:
Treatment Year 1 Year 2 Year 3 No herbicide 27.4 by* 0.5 ax 0.01 ax sulfometuron 1.2 ay 1.3 ay 0.01 ay glyphosate 7.3 ay 2.4 ay 0.01 ay *Columns followed by the same letter (a or b) are not significantly different. Rows followed by the same letter (x-z) are not significantly (P=0.05) different.Cultural: Little information is available on cultural methods of control for Johnson grass. An Arizona study using integrated pest management, including native bunchgrass plantings, showed some success in controlling Johnson grass (see grazing in the Biological control section above). Additional studies incorporating cultural control of Johnson grass are needed.
Physical/mechanical: Johnson grass can be controlled by tilling, mowing, and flooding [6,127,169]. Individual small plants or small clumps may be controlled by hand-pulling or solarization [13,54,169].
A consistent tillage program may provide effective control [6,42,80,125]. Tilling is not practical on most wildlands due to damage to desirable native plant species, uneven terrain, erosion, and cost constraints [104]. Tilling can be used on some sites such as bottomlands and old fields. Shallow plowing helps control Johnson grass by breaking up rhizome systems, exposing rhizomes to the sun or killing frosts, and depleting carbohydrate reserves [6,42,80,125]. Optimal plow depth is 8 to 12 inches (20-30 cm). Several treatments are needed in hot climates [86,101]. Killing sprouts early, before they form 5 leaves and start developing new rhizomes, gives best control [101]. First plowing is in spring (May), followed by similar plowings every 3 weeks (in rainy weather) to 6 weeks (in dry weather). Plant heights of 12 or more inches (30 cm) are recommended before plowing again [9]. In cold climates, Johnson grass is plowed in late October to expose rhizomes to frost [19]. An exposure of 24 or more hours to temperatures below 25 °F (- 4 °C) kills rhizomes [80,102,125,126]. A single plowing, or long intervals between plowings (>4 years), is generally not effective because it stimulates growth [104,170], buries and protects rhizomes [42], and exposes deeply buried seeds to upper soil levels where they may germinate [64].
Because rhizomes may extend more than 20 inches (51 cm) below ground, cultivation alone may fail to kill Johnson grass rhizomes [42]. After plowing, close grazing or mowing (so that the grass stays <12-15 inches (30-38 cm) tall) helps further reduce Johnson grass cover [86].
Even on old fields, tilling is a major soil disturbance that provides a favorable seedbed for pioneer species. Unless further rehabilitation efforts that include planting native herbaceous species are taken, it is likely that tilled fields will succeed to other invasive nonnatives.
Repeated, close mowing has the same inhibitory effect on growth as grazing [104,169]. In Mississippi, mowing seedlings 13 days after emergence killed them [125]. In an Alabama field experiment, multiple cuttings, starting when plants were 1 foot (0.3 m) high, slowed Johnson grass rhizome development. At the end of the growing season, plots cut 8 times averaged 15 dry-weight ounces (431 g) of Johnson grass top-growth and 0.3 dry-weight ounces (10 g) of rhizomes. Plants cut only twice had 67 ounces (1,909 g) top-growth and 26 ounces (739 g) of rhizomes. Plots were 4 à 5 feet²[179].
Flooding for 3 to 6 weeks in early spring, before rhizomes sprout, can effectively control Johnson grass. Replacing open irrigation ditches with culverts or pipes helps prevent reinfestation [127].
Hand-pulling Johnson grass usually leaves rhizome pieces behind in the soil, stimulating sprouting. It is not an effective control method unless all rhizomes are removed or new sprouts are controlled [104,169]. Best results are obtained in early spring when soil in moist and rhizomes are least likely to break [169].
Repeated solarization treatments (using a clear polyethylene tarp to trap solar heat in the soil) can control small Johnson grass infestations [13].
Seeds: Solarization of moist soil at 140 °F to 150 °F (60-70 °C) for 7 days kills most Johnson grass seeds. Solarization of dry soil does not kill Johnson grass seed [54]. In Davis, California, soil watered and solarized for 9-12 weeks supported no Johnson grass. Untreated control plots showed 58% Johnson grass cover [55]. For established plants, 30 days of solarization kills most Johnson grass. Remaining plants have grown rhizomes through and above the landscape fabric, but rhizomes above the landscape fabric were easily removed by hand-pulling [121].
Composting Johnson grass seeds in cow manure for 3 days killed the seeds. Temperatures in the compost reached 120 °F (49 °C) [203]. Ensiling for 21 days also killed Johnson grass seed [212].Although Johnson grass can be an undesirable species, it can also provide good forage for wildlife and livestock under most growing conditions [31,47,87,91].
Palatability/nutritional value: Johnson grass is moderately palatable and nutritious [47,49]. Deer make light to moderate use of Johnson grass [114,163], grazing all aboveground portions of the plant [47]. Rodents also graze Johnson grass. In honey mesquite (Prosopis glandulosa var. glandulosa) plains of Texas, Heerman's kangaroo rat and Great Basin pocket mouse used Johnson grass frequently (6.7% volume, 61% frequency; and 5.9% volume, 36% frequency, respectively) [2]. Quail, geese, and wild turkey consume Johnson grass seeds [31,47].
Although intolerant of heavy grazing, Johnson grass is a good pasture grass and makes fair-quality hay when cut in the boot stage [170,191]. Livestock make moderate to good use of fresh Johnson grass [87,91]. In a comparison of Texas range grasses, Johnson grass showed greatest in-vitro digestibility (45%-69%, depending on the digestion media) of 5 grasses tested [116]. Dairy cattle in Alabama showed good weight gain and milk production on Johnson grass pasture [87].
Nutritional content: In a western Texas study, Johnson grass had highest summer crude protein content (13.2%) of 9 rangeland grasses. Spring, fall, and winter crude protein values were 6.62%, 8.06%, and 3.81%, respectively [37]. In a greenhouse study comparing relative mineral content of 15 grass species, Johnson grass scored significantly higher in phosphorus content than other grass species (P=0.1). Cobalt, manganese, and copper contents were moderate compared to other grasses [29]. Energy value of Johnson grass grown in Texas was 3,900 kcal/g [136]; in India, seasonal fluctuation in energy value varied from 3,684 kcal/g in October to 4,578 kcal/g in April [167]. Nutritional content of fresh Johnson grass in eastern Texas was [62]:
Growth stage No. samples Protein (%) Ether extract (%) Crude fiber (%) N-free extract (%) young 6 9.22 2.28 28.51 42.35 bloom 3 5.43 1.87 30.00 43.19 mature 2 5.36 1.40 32.36 44.01Seasonal changes in forage quality of Johnson grass on the Edwards Plateau of Texas were [103]:
Plant part Date Water (%) Ash (%) Cell wall (%) P (%) Protein (%) Digestible organic matter (%) leaves 5/24/73 71 10 55 0.38 15 73 leaves and stems 6/28/73 68 9 60 0.21 12 70 leaves 10/25/73 76 9 66 0.16 10 63When harvested at its peak, Johnson grass makes fair-quality hay, similar to timothy (Phleum pratense) hay in nutrient content [146]. In a 1928 study in an Alabama coal mine, draft horses and mules were fed oat (Avena sativa) grain and either Johnson grass or timothy hay for 3 months. The equines maintained their weight on both diets under "moderate" workloads. The animals lost weight on both diets under "heavy" workloads, but lost less weight on timothy hay and grain compared to Johnson grass hay and grain (mean losses of 10.71 and 21.78 lbs., respectively). Digestible nutrient means were [75]:
Total dry matter per 100 lbs.
Digestible nutrients per 100 lbs.
Crude protein Carbohydrates Fat Total Johnson grass hay 89.9 2.9 45.0 1.0 50.1 timothy hay 88.4 3.0 42.8 1.2 48.5Toxicity: Johnson grass is generally a good forage grass [31,47,87,91]. However, at certain developmental stages or under some adverse environmental conditions, Johnson grass may form cyanogenetic glycosides that can poison livestock. Phenologically, Johnson grass is most toxic when leaves and culms are actively growing. Seedlings and sprouts generally have higher levels of glycosides than plants that have reached the flowering stage. Secondary growth, produced after mature plants are mowed or heavily grazed, can also have high levels of glycosides. Environmentally, Johnson grass is most toxic after drought, extreme heat, frost, or when plants are wet with dew or light rain. Glycoside levels can vary considerably among Johnson grass populations. Ruminants, especially cattle, are more susceptible to glycoside poisoning than monogastric herbivores like horses [157,173]. As well as fresh plants, hay cut when Johnson grass is young or experiencing adverse environmental conditions such as drought can also be toxic [81,133,170,170]. Livestock poisoning can be prevented by waiting until new growth is 15 to 18 inches tall (38-46 cm) tall after drought, or deferring grazing until plants have dried after frost [128].
Johnson grass may sequester selenium or other elements that are toxic at high doses when growing in soils with high concentrations of toxic elements. In the Dead Sea area of Jordan, for example, selenium concentrations in Johnson grass samples were high enough to poison livestock [1].
Prolonged consumption of fresh Johnson grass can cause nitrate poisoning in ungulates [173]. Most livestock can graze Johnson grass safely when plants are at least 18 inches (46 cm) tall [170].
Cover value: No information is available on this topic.
Fire may promote Johnson grass growth. Spring prescribed burning increased Johnson grass biomass in a short-term study in Georgia. Old fields were burned on 5 March, 1970. In the 1970 postfire growing season (March-October), Johnson grass net productivity averaged 27.42 g/m² on burned plots and 0.20 g/m² on unburned control plots. Prescribed burning significantly reduced the litter layer during the first 5 postfire months, and plants on burned plots showed increased spring nitrogen uptake compared to control plots (P=0.05) [140]. Reduced litter and increased nitrogen uptake probably enhanced Johnson grass growth on burned plots.
Published information on postfire seedling establishment of Johnson grass is lacking. Studies are needed on the ability of Johnson grass to establish from seed in postfire environments.
Johnson grass reproduces from rhizomes and from seed [42,86,96].
Asexual regeneration: Once a population of Johnson grass is established, most population growth is from asexual regeneration by rhizomes [96]. Throughout most of their North American range, Johnson grass populations are strongly rhizomatous [49,71,122,184,201]. Some Johnson grass populations are weakly rhizomatous or nonrhizomatous, especially at the species' distributional limits [61,197,198]. Rhizome expression in Johnson grass is apparently controlled by multiple, dominant genes, resulting in variable degrees of rhizome development in both Johnson grass and its hybrids [209]. Extreme temperatures also inhibit Johnson grass's ability to produce rhizomes [85,175].
Rhizome development: Johnson grass plants begin growing rhizomes in the seedling stage. Primary rhizomes are initiated at the 5-leaf stage, when plants are about a foot tall. Rhizome growth continues slowly until the 10-leaf stage, then accelerates greatly. Rhizomes are well developed by 6 to 7 weeks [6,101,125]. In a greenhouse experiment, Anderson and others [6] noted extensive rhizome development on 4.5-month-old plants, with over 5,200 rhizome nodes/plant.
In older plants, last-year or primary rhizomes produce new, secondary rhizomes in spring. Secondary rhizomes in turn produce tertiary rhizomes. Secondary and tertiary rhizome growth slows or stops during flowering, then resumes with seedhead development [126]. Rhizome production peaks at seed ripening [78,97], when a single plant may produce 200 to 300 feet (60-90 m) of rhizomes [127]. Secondary and tertiary rhizomes continue growth and carbohydrate accumulation until late fall, then go dormant over winter. Primary rhizomes die each fall. In spring, secondary and tertiary rhizomes become current-year primary rhizomes [4,42,80,86,96,101].
Rhizome sprouting: Small or broken rhizomes, especially secondary rhizomes, can form new plants [6,9,42,173]. Plows spread Johnson grass by breaking up, dispersing, and replanting rhizomes [3] (see Physical/mechanical control). Small rhizomes are more likely to sprout when shallowly buried, while large rhizomes are more likely to sprout when deeply buried. In the greenhouse, 3-inch (7.6-cm) rhizome sections sprouted best when planted less than 3 inches deep. Longer, 6-inch (15.2-cm) sections sprouted best when buried deeper than 3 inches ([39] and references therein). In a Mississippi field experiment, McWhorter [125,126] found that with shallow burial (<2 inches (6 cm)), short rhizomes (<3 inches (7.6 cm)) produced more sprouts than long rhizomes (6 inches (15.2 cm)). The opposite trend occurred when rhizomes were planted deeper than 2.4 inches (6 cm). Rhizomes usually grow to a depth of 10 to 20 inches (25-50 cm) [127]. Loose, sandy or loamy soils generally allow for best rhizome expansion [126]. Clay tends to inhibit rhizome expansion [86]; however, rhizomes may penetrate several feet down cracks in clay soil [126]. Deeply buried rhizomes that do not sprout do not survive more than a year [4,80,96].
Rhizomes are somewhat drought-resistant, remaining viable after drying to 40% of initial harvest weight [6]. They are sensitive to extreme temperatures. In northern climates, rhizomes must be deeply buried in order to overwinter. In an Illinois field experiment, Johnson grass rhizomes did not survive winter temperatures less than 1.4 °F (-17 °C) unless buried 7.9 inches (20 cm) or more below ground [175]. In southern Ontario, rhizomes must be 10 inches (25 cm) or more inches below ground to overwinter ([85] and references therein).
Sexual regeneration: Although growth of established populations is primarily through rhizomes, Johnson grass establishes new populations through seed spread [96].
Breeding system: Sorghum species are mostly self-crossed, although some outcrossing occurs ([196] and references therein).
Pollination: Johnson grass is primarily self-pollinated [196]. Some pollination is effected by wind, especially when plants are <425 feet (130 m) apart [196,202].
Seed production: Johnson grass is a short-day plant, requiring 8 to 16 hours of daylight to flower [66,135]. It is a good seed producer under favorable growing conditions. A single plant may produce 80,000 or more seeds in 1 growing season [3,84]. Two seed crops may be produced under good conditions. In agricultural fields in Argentina, Johnson grass produced a large seed crop in early summer (Jan-Feb.; 60% of total seed production for the year) and a smaller seed crop in late summer-early fall (mid-March-early April; 40% of annual seed production) [66]. Johnson grass seed production is estimated at 90 gallons/acre (855 l/ha) on good sites in the South. Field trials in Mississippi showed mean seed production of 84 g/plant and 28,000 seeds/plant [127,196]. Resources are allocated to rhizomes at the expense of seeds under poor growing conditions [22,23].
Greenhouse trials using Johnson grass seed from the Northeast showed populations that grow and reproduce as annuals have faster growth rates, more rapid development, more and larger seeds, and fewer rhizomes compared to populations that sprout from overwintered rhizomes [198].
Seed dispersal: Wind, water, machinery, and animals disperse Johnson grass seeds [3,65,84,85,182]. Spikelets are readily deciduous [201] and usually disperse as a unit beneath the parent plant [3,65,70,72]. Strong winds disperse seeds longer distances. In Argentina, 28- to 31-mile/hr (45- to 50-km/hr) winds that occurred during May thunderstorms carried Johnson grass seeds 2,950 to 3,300 feet (900-1,000 m) from parent plants [65]. Water has dispersed seeds along many waterways of the United States [3,84,182]. Farming equipment also spreads seeds [65,84]. Viable Johnson grass seed is a common contaminant in hay, harvested crops, and commercial seed [3,142]. Johnson grass seed retains viability after passing through the digestive tracts of livestock [9,84,126]. The relative importance of agents that disperse Johnson grass seed is unclear [128].
Seed banking: Johnson grass builds up a soil seed bank [189]. The seeds are dormant and may remain viable for several years, although most soil-stored seeds germinate in their 1st or 2nd year [3,96,135]. In Mississippi, 1st-year stratified seed showed 82% viability in the field. After burial in the field for 2.5 years, the same seed lot showed 62% viability [53]. In California, 5-year-old buried seed showed >50% viability, but viability dropped to 2% by age 6 [120].
Germination: Johnson grass has 2 mechanisms of dormancy: mechanical dormancy imposed by the seed hull and seedcoat, which requires weathering or scarification to break; and chemical dormancy, which requires oxygen to break [100]. Diurnal fluctuations in temperature, afterripening, or both are needed to overcome both types of dormancy [26,26,64,99,181]. Seed from water-stressed plants is generally less dormant than seed from amply watered plants [23]. Benech and others [25] present a model predicting loss of seed dormancy and consequent seedling emergence based on soil temperature.
Light improves germination rate with warm temperatures (>93 °F (34 °C)) and inhibits germination with cold temperatures (<72 °F (22 °C) [100]. In the greenhouse, Taylorson and McWhorter [181] found a 63% increase in germination rate for Johnson grass seed exposed to light vs. seeds kept in the dark. Deeply buried seed remains dormant for at least 7 years [84] but does not germinate [27,64,99]. Soil upheaval such as cultivation, which brings seed closer to the soil surface, usually increases germination rates [64,99]. In the greenhouse, best germination (60-75%) occurred with surface-scattered to shallowly buried (0-1.6 inches (0-4 cm)) seed. Less than 5% germination occurred with seed buried >3 inches (8 cm) below the soil surface [27]. Litter cover or shallow burial may aid germination in the field. Prostko and others [147] present a model to predict Johnson grass seedling emergence based upon temperature and seed burial depth.
Seedling establishment/growth: Best establishment occurs on open, disturbed sites. Seed dispersed away from parent plants may show better establishment compared to seed falling beneath the parent. In an old field in Argentina, most Johnson grass seed fell near parent plants on undisturbed plots. Only 1% of seed beneath a parent plant established. On tilled plots mowed every 1 to 2 weeks by a corn (Zea mays) harvester, seed was carried 3 to 82 feet (1-25 m) from parent plants. Recruitment of tilled seed neared 100% [65]. On favorable sites, plants may produce 80 or more culms in their 1st year [3].
In areas where Johnson grass grows as a facultative annual, it shows variable ability to regenerate from seed. Johnson grass annuals in rural-interface wildlands of southern Arizona rarely reproduce either from on-site seed or from rhizomes. Seed dispersed from adjacent agricultural lands provide continual sources of seed [61]. However, some annual populations in the northern portion of Johnson grass's range successfully reproduce from seed. In southern Ontario, northern Ohio, and northern New York, annual populations have larger leaves, inflorescences, and seeds compared to perennial Johnson grass populations [198].
Johnson grass seedlings may show faster 1st-year growth than plants started from rhizome fragments. On the Mississippi Delta near Stoneville, Mississippi, Johnson grass started from seed showed greater biomass and more rapid height gain than plants started from rhizome pieces. At flowering, seed plants were producing 0.75 to 3 feet (0.23-0.9 m) of new rhizome growth per day. Plant growth patterns were as follows (data are means) [125]:
Biomass and height of Johnson grass seedlings
Date (1959)
Days after emergence
Green weight (g)
Height (in)Rhizome length (ft)
Leaves Rhizomes Roots Seedhead May 20 20 12 2 3 ----* 12 ---- May 27 27 190 30 30 ---- 23 <1 June 15 46 690 90 100 ---- 52 7 July 1 62 1990 750 220 180 74 35 Sept. 1 124 2950 5050 360 500 74 153 Sept. 29 152 3140 8070 430 680 74 212Biomass and height of Johnson grass rhizome sprouts
May 20 19 9 3 3 ---- 9 ---- May 27 26 80 14 20 ---- 13 <1 June 3 33 530 100 40 ---- 30 1 June 15 45 610 110 130 ---- 47 5 June 19 49 590 310 160 9 70 9 June 24 54 950 220 160 74 72 14 *Not present.Johnson grass is a pioneer species, and is often found on old fields [71], frequently inundated, or otherwise disturbed sites [71,83,169]. Johnson grass is not restricted to disturbed sites, however; it also invades undisturbed tallgrass and coastal prairies, savannas, and riparian zones [45,169]. In an Oklahoma study of succession in little bluestem (Schizachyrium scoparium) prairie, Johnson grass was most common in midsuccessional seres, when other weeds and woody species were succeeding to tallgrass prairie species [45]. Johnson grass's spread through rhizomes may slow succession, especially in grassland ecosystems [169].
Old fields: Johnson grass is particularly common on old bottomland fields in the South [3,17,18]. Unlike most crop weeds, which tend to decrease in the absence of irrigation and fertilization, Johnson grass tends to persist on abandoned fields. For example, in Georgia it was prevalent in 1-, 4-, and 8-year-old fertilized crop fields, but was also prevalent in 8-year-old fallow fields [139].
Shade tolerance: Johnson grass requires open sites and does not persist under closed canopies [77,190]. In a honey mesquite (Prosopis glandulosa) Texas savanna, Johnson grass associated with Texas wintergrass (Nassella lecotricha) in open areas but was not found under honey mesquite or other trees [190]. In Argentina grasslands, canopy removal increased Johnson grass germination and establishment compared to closed-canopy sites [25].
