Johnsongrass (Sorghum halepense) is one of the world’s most aggressive and difficult-to-control agricultural weeds, known for its rapid growth and prolific reproductive capabilities. This warm-season perennial grass can quickly infest a wide range of environments, from cultivated fields to lawns and roadsides. Its presence can lead to significant yield losses in crops. Effective long-term management involves the strategic use of chemical controls, which must be precisely applied to overcome the weed’s robust survival mechanisms.
Understanding Johnsongrass Biology
Johnsongrass is difficult to eradicate because of its dual reproductive system, utilizing both seeds and an extensive underground network of rhizomes. The rhizomes are horizontal underground stems that function as a perennial energy storage system, allowing the plant to survive harsh conditions like winter or drought. These rhizomes produce new shoots earlier in the spring than those from seeds, giving them a competitive advantage over desirable plants.
Johnsongrass is also a massive seed producer, with a single plant potentially yielding over 80,000 seeds in a single growing season. These seeds can remain viable in the soil for several years, creating a persistent seed bank that leads to annual re-infestation. Any successful management plan must therefore target both the active growth emerging from the rhizomes and the new seedlings emerging from the soil seed bank.
Pre-Emergent Prevention Methods
Pre-emergent herbicides are designed to prevent the establishment of new Johnsongrass plants by targeting the seeds as they germinate. These chemicals are considered residual herbicides because they create a chemical barrier in the top layer of the soil that inhibits the growth of the emerging seedling’s root or shoot. They are effective only against plants developing from seed and have no effect on established plants emerging from rhizomes.
Common chemical classifications used for pre-emergent control include the dinitroanilines, with active ingredients such as pendimethalin and trifluralin. For these herbicides to be effective, they must be properly incorporated into the top few inches of the soil either through mechanical means or by irrigation or rainfall shortly after application. This incorporation is necessary to prevent the herbicide from breaking down in sunlight and to ensure the chemical is present where the Johnsongrass seeds germinate.
Post-Emergent Chemical Solutions
Post-emergent herbicides are the primary tool for controlling actively growing Johnsongrass. These chemicals are systemic, meaning they are absorbed by the leaves and then translocated throughout the plant, including down into the underground rhizomes. This action weakens or eliminates the plant’s energy reserves.
A widely used, non-selective option for fallow ground, fence rows, or spot treatments is glyphosate. Glyphosate is highly effective against Johnsongrass because it moves throughout the plant, disrupting a fundamental metabolic pathway. However, it will injure or kill any plant it contacts, and resistance has been confirmed in some Johnsongrass populations.
For selective control within desirable grasses or ornamental areas, the ACCase-inhibiting herbicides are preferred. These grass-specific chemicals, known as graminicides, include active ingredients like fluazifop, sethoxydim, and clethodim. They work by inhibiting an enzyme necessary for lipid synthesis in grasses but do not harm most broadleaf plants. In non-crop areas, other options include ALS-inhibiting herbicides like sulfosulfuron or imazapic, which are also systemic and move to the rhizomes for long-term control.
Optimal Application Timing and Safety Guidelines
The timing of post-emergent herbicide application is the most important factor determining control success. Systemic herbicides are most effective when Johnsongrass is actively growing and has sufficient leaf area to absorb the chemical, typically when the plants are 15 to 25 inches tall but before the seed head emerges. This growth stage maximizes absorption and ensures the plant is actively transporting the chemical down to the rhizomes.
Fall applications, before a killing frost, are particularly effective for long-term control. The plant naturally moves carbohydrates from the leaves down to the rhizomes for winter storage, enhancing the downward movement of the herbicide. Avoid applying herbicides during periods of drought or extreme heat, as plants under stress will not actively translocate the chemical.
Adhering to safety guidelines is mandatory and starts with reading the entire product label. Personal Protective Equipment (PPE) is required to minimize exposure. Always apply chemicals on calm days to prevent drift onto non-target plants, and ensure all application equipment is properly calibrated.
Personal Protective Equipment (PPE)
Chemical-resistant gloves made of nitrile or neoprene
Long-sleeved shirts
Long pants
Socks and closed-toe footwear
Eye protection such as goggles