Posted by Michelle Vignault

Johnsongrass (Sorghum halepense) is a warm-season perennial weed found near roadsides, rights-of-way, row crops, parks, and industrial sites. Populations may germinate from seed in spring after overwintering in the soil. However, Johnsongrass primarily emerges from dormant rhizomes in areas with a history of infestations. Rhizomes are belowground stems that produce daughter plants and storage reserves for new growth in spring.

The persistence of Johnsongrass is primarily associated with extensive rhizomatous growth that enable populations to spread laterally and dominate areas by preventing desirable vegetation from flourishing.  Johnsongrass may reach 6 to 8 feet in height, which can limit the growth of lower growing grasses in mixed stands.  It also produces several allelopathic compounds that are toxic to other plants when released in the soil.  Therefore, controlling Johnsongrass with integrated management programs can be critical for long-term successful management of desirable vegetation.

Controlling Johnsongrass

Regular mowing of Johnsongrass may deplete carbohydrate reserves in rhizomes over time and help limit the spread of infestations.  However, infrequent mowing can help Johnsongrass spread.  It often has rapid regrowth during summer months that may warrant monthly mowing to control shoot development.  Preemergence herbicides used for grassy weed control, such as the dinitroanilines, may control Johnsongrass establishment from seed.

However, preemergence herbicides do not control Johnsongrass emergence from rhizomes. Controlling other annual grassy weeds with preemergence herbicides could potentially release Johnsongrass due to the limited competition for space and resources required for growth in summer. Johnsongrass may be selectively controlled in roadside vegetation with postemergence herbicides.

Monosodium methyl arsonate (MSMA) is an organic arsenical that may be used to control or suppress Johnsongrass in most major warm- and cool-season grasses on roadsides.

Current buffer restrictions in proximity to water bodies and limitations on the number of treatments permitted in a year can restrict the potential use of MSMA in many areas.  Sulfonylureas, such as Outrider (sulfosulfuron), effectively control Johnsongrass in bermudagrass and bahiagrass roadsides, but cannot be applied to tall fescue.  Sulfonylureas are systematically translocated throughout the Johnsongrass plant, unlike MSMA that is immobile.  The mobility of these herbicides to belowground rhizomes enhances the long-term control of perennial populations compared to contact herbicides like MSMA.

Fall applications of herbicides are generally more effective than spring treatments for long-term Johnsongrass control.  Johnsongrass begins allocating carbohydrates from leaves to rhizomes in fall which enhances the movement of herbicides in this source-to-sink pattern.  Conversely, spring treatments of postemergence herbicides can provide temporary control of Johnsongrass leaves, but rapid regrowth from rhizomes often occurs.  While spring treatments can help release desirable vegetation from competition, restricted herbicide translocation to rhizomes may result in erratic control as Johnsongrass allocates energy to shoot growth.  Glyphosate (Roundup, others) is another systemic herbicide that may benefit from fall applications compared to spring treatments for long-term Johnsongrass control.  Glyphosate is non-selective and should be limited to spot treatments at rates required to control Johnsongrass.

Enhance Control

Map, Spray, and Monitor

Many utility companies and DOTs are using Clearion’s GPS-mapping software to improve the identification, control, and monitoring of Johnsongrass populations in roadsides and industrial vegetation management.  This technology helps end-users make informed decisions about spray programs that improve the precision of management programs necessary for long-term control of Johnsongrass.  For effective integrated management, herbicide applications should be delayed about three weeks after a mowing operation.  This permits adequate regrowth of Johnsongrass plants to absorb the herbicide and help release desirable grasses from competition.

The current adopters of Clearion technology have been successful at monitoring the progress of mowing crews using real-time updates and GPS-tracking.  The Clearion system enables spray crews to coordinate herbicide treatments based on the location and timing of mowing operations.  This information can also be overlaid with the coordinates of Johnsongrass populations based on previous mapping to improve the precision of management programs.  Mapping the Johnsongrass populations over time with Clearion, along with a strategic integrated management program, can help improve roadside safety and reduce inputs necessary for long-term success. Learn more at www.clearion.com

Next month: Italian Ryegrass