Johnsongrass is an invasive weed that can cause significant issues on roadsides, affecting motorist safety, infrastructure, and aesthetics. When identified, it is best to use an integrated vegetation management approach to gain control of and prevent further proliferation of this invasive species.
Understanding Johnsongrass: History and Identification
Johnsongrass (Sorghum halepense) is a perennial grass species known for its rapid growth and ability to spread aggressively. Native to the Mediterranean, it is believed to have been originally introduced into the United States around 1830 as a forage plant in South Carolina. In about 1840, Colonel William Johnson brought johnsongrass seeds back from a trip to South Carolina to his farm in Alabama where he planted them in the fertile bottom lands of the Alabama River.
Today, johnsongrass is considered to be a noxious weed in 19 states and can be found in almost every one of the lower 48, plus Hawaii. It is widely considered one of the 10 worst weeds in the world.
Johnsongrass will grow in dense clumps, with mature plants reaching heights of six to eight feet. Along with its tall stature, other identifying characteristics of johnsongrass include: two-foot long, broad leaves with a dominate, white mid-vein and its distinctive, Christmas tree-like seed head, that may take on a purplish hue.
Spread, Threat, and Impact
According to the National Forest Service, a mature johnsongrass plant can produce 80,000 or more seeds in one growing season. Once established, Johnsongrass will spread rapidly through asexual regeneration, where each plant may produce 200-300 feet of rhizomes. Johnsongrass prefers full sun and will grow in almost any soil type. Due to its high propensity for propagation, johnsongrass can be a threat to almost any landscape, especially when the soil has been disturbed or competing vegetation populations are low.
The threat and impact of johnsongrass is dependent on the area in which it is growing and the level of establishment. Johnsongrass, originally introduced as a forage, can be a source of nutrition for grazing cattle in range and pasture settings. However, when plants become stressed due to environmental conditions like cold and drought, they can produce cyanogenic compounds like prussic acid, and when consumed, can be fatal to livestock.
In roadside settings, the threat of johnsongrass establishment is multifaceted. When left unmanaged, it becomes a safety hazard for the traveling public by obstructing driver visibility around turns, blocking signs, and hiding guardrails. While wildlife habitats are important, overgrown johnsongrass along roadsides provide cover for deer and other animals, preventing motorists from recognizing potential hazards. Dense growth also serves as a fire hazard, particularly in dry conditions. Finally, due to its invasive nature, johnsongrass can invade and degrade native plant communities, and increase maintenance costs for departments of transportation.
Preventative Measures to Curb Johnsongrass Growth
Preventing the spread of johnsongrass is critical to managing its impact on roadsides. Regular monitoring and early detection allow for prompt intervention before the weed becomes well-established. Implementing proper roadside maintenance, including an integrated vegetation management plan, can also help deter its spread. Use the following example to help you create your plan of action.
1. Identify Goals
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Maintain infrastructure
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Reduce wildfire risks
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Habitat improvement
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Control invasive species
2. Define Clear and Measurable Objectives
- Reduce invasive populations
- Increased line-of-sight for motorists
- Increase the population of native plants
3. Inventory the Vegetation
- Create a list of existing, desirable vegetation
- Identify johnsongrass presence and level of establishment
4. Identify Environmental Sensitivities including Wetlands, Endangered Species Habitats, or Protected Areas
5. Determine the Most Effective Strategy for Control
- Mechanical
- Herbicide(s)
- Prescribed burning
5. Monitor Progress
- Track effectiveness
- Adapt and adjust as necessary
Mechanical and Chemical Control Methods
Regular mowing can help reduce the seed production of johnsongrass, but it must be done repeatedly throughout the growing season to be effective. It may be necessary to increase the number of mow cycles where large populations of johnsongrass exist.
Herbicides like glyphosate and imazapyr are commonly used to manage johnsongrass populations; however, these are non-selective options, and care must be taken not to cause damage to desirable species. One of Alligare's newest solutions, Outline herbicide, is labeled to selectively control johnsongrass in a range of common, established roadside grasses like Bermuda, Bahia, and tall fescue. Herbicide application should be performed according to the label instructions regarding rates and safety guidelines. Applying herbicides after spring green-up, when the grass is actively growing, and before the seedhead develops increases effectiveness. Combining the mechanical and chemical methods above will provide a more comprehensive approach to control.
Long-term Management and Monitoring
Long-term management of johnsongrass requires consistent monitoring and adaptive strategies. Regularly inspecting roadsides for new infestations and promptly addressing them is crucial to prevent widespread establishment. Keeping detailed records of control measures and their outcomes can help refine and improve management practices over time. Ultimately, maintaining a proactive and integrated management plan is key to johnsongrass control on roadsides.
