How to Identify and Manage Johnsongrass in Missouri

By Blake Jackson

Johnsongrass is making an unwelcome return in northwestern Missouri, according to University of Missouri Extension agronomist Wayne Flanary.

While not new to the state, its spread in areas that are challenging to manage is raising concerns. The Missouri Department of Agriculture added Johnsongrass to its noxious weeds list in 2015.

Reaching heights over 6 feet, this perennial weed thrives along ditches, in crop fields, and pastures, says MU Extension weed scientist Kevin Bradley. Its rapid growth and aggressive nature make it highly competitive with crops and forages.

Once it takes hold, removal is difficult due to underground rhizomes that send up new shoots. Each plant can produce seed heads containing up to 80,000 seeds annually. These seeds can remain dormant for more than a decade and spread via water, machinery, and wildlife.

Johnsongrass is sometimes confused with other weedy grasses like shattercane, barnyardgrass, or fall panicum. However, those are annuals and lack rhizomes. Some Missouri populations have even developed resistance to glyphosate, a commonly used herbicide for this species.

“Although for the most part we do a pretty good job at suppressing Johnsongrass with spring or summer herbicide applications in corn or soybean, this is not the ideal time to treat this species with herbicides,” Bradley explains. “Early fall herbicide applications are more likely to result in herbicide translocation to the rootstocks and longer-term control.”

For effective results, wait until plants regrow after mowing or harvesting before applying glyphosate or Group 1 herbicides like clethodim, sethoxydim, fluazifop, or quizalofop.

Controlling infestations in non-planted areas can also limit spread avoid driving equipment through infested patches.

Johnsongrass also poses livestock health risks. MU Extension veterinary toxicologist Tim Evans warns of prussic acid poisoning, nitrate toxicity, and sorghum cystitis.

To prevent cyanide poisoning, avoid grazing immature or frost-damaged plants until they are at least 24 inches tall and wait 14 days after frost.

Nitrate toxicity, common during dry weather, can persist in hay but is reduced in silage. Sorghum cystitis affects only a small percentage of horses, causing nerve and bladder damage.

Effective control requires integrated strategies, combining herbicides, targeted grazing, and mowing. For detailed guidance, visit the Mizzou weed science website.

Photo Credit: gettyimages-dszc