Johnsongrass is an extremely competitive perennial grass prevalent in the southern half of Ohio, although it has been observed as far north as Wood County in northwestern Ohio. It reproduces both by seed and overwintering rhizomes (large, white, scaly, underground stems). Control of rhizome johnsongrass is an ongoing process that should include both cultural and chemical methods.
Most rhizome production occurs when johnsongrass plants reach 2 or more feet in height and begin producing seed heads. Close grazing or mowing to keep johnsongrass less than a foot tall will greatly reduce rhizome production.
Noncrop/Fallow Areas. Glyphosate provides excellent control of johnsongrass that is in the boot to head stage or anytime prior to frost. For spot treatment, use a 2% solution. For broadcast application, apply 0.75 lb ae/A glyphosate plus ammonium sulfate (17 lbs/100 gallons of water) in a spray volume of 5 to 10 gpa.
Corn. Postemergence application of Accent (2/3 ounce/A), Beacon (3/4 ounce/A), or glyphosate (0.75 lb ae/A - Roundup Ready corn) provides the most effective control. Rhizome johnsongrass plants should be at least 8 inches tall at the time of Accent or Beacon application, and as close to the boot stage as possible for Roundup. Apply Accent and Beacon with crop oil concentrate or methylated seed oil (1% v/v) plus nitrogen fertilizer solution for best results. As a single application, Accent has generally provided more effective rhizome johnsongrass control than Beacon. Both herbicides are labeled for split applications, which may improve johnsongrass control, especially under unfavorable environmental conditions. In a long-term johnsongrass control program, glyphosate, Accent, and Beacon will reduce the population of rhizome johnsongrass plants. In heavily infested fields, Accent may be the better choice than Beacon in the first few years, since it tends to be more effective on rhizome johnsongrass plants. Once the population has been reduced to a low level, Beacon may be a better choice since it is more effective on ragweeds and some other broadleaf weeds. Seedling johnsongrass plants are more susceptible to Accent and Beacon than rhizome plants, and it may be possible to reduce herbicide rates once the population of rhizome plants is reduced. Exceed (1 ounce/A), Spirit (1 ounce/A), Basis Gold (14 ounce/A), Accent Gold (2.9 ounce/A), Steadfast (3/4 oz/A), (Lightning (1.28 ounce/A - Clearfield corn) and Liberty and Liberty ATZ (28 ounces/A and 48 ounces/A - Liberty Link corn) will also control seedling johnsongrass and suppress rhizome plants.
Soybeans. Postemergence application of Assure II (10 ounces/A), Fusilade DX (12 ounces/A), Fusion (12 ounces/A), or Select (8 to 16 ounces/acre) provides good to excellent control. Application should be delayed until johnsongrass reaches a height of about 10 to 20 inches (labels vary with regard to minimum height at the time of application consult individual labels for more information). Glyphosate (Roundup Ready soybeans) will control seedling and rhizome johnsongrass. For most effective control of rhizome johnsongrass, apply after plants are in the boot stage. For any of these herbicides, a second application at a lower rate may be necessary to control regrowth.
For lateseason control in soybeans without tolerance to glyphosate, a ropewick or sponge applicator can be used to apply glyphosate when weeds are sufficiently taller than soybeans (weeds usually 3 to 4 feet tall).
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