More info for the terms: allelopathy, cactus, cover, density, fire management, frequency, invasive species, mesic, natural, resistance, restoration, rhizome
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].