The spread of perennial weeds like Johnson Grass into desirable grass areas, such as Fescue turf or pasture, presents a challenge for land managers. This dilemma requires a specialized solution, as most general grass-killing herbicides would destroy the Fescue along with the weed. Selective control is necessary, meaning a chemical must precisely target the weed while leaving the Fescue unharmed. Successful management depends on understanding the weed’s biology, selecting the right chemical tools, and applying them at the optimal time.
Understanding Johnson Grass Persistence
Johnson Grass (Sorghum halepense) is difficult to eradicate because of its perennial nature and robust survival mechanism. Unlike annual weeds, this plant develops an extensive network of underground stems known as rhizomes. These rhizomes store carbohydrates, allowing the plant to survive adverse conditions like winter cold, drought, and mowing.
The energy stored in the rhizomes allows the plant to rapidly sprout new shoots, even after the above-ground foliage is damaged. A single plant can produce many feet of rhizomes and thousands of seeds, making it a prolific spreader. This capacity for underground regrowth requires control methods to employ a systemic approach that translocates the chemical deep into the root system.
This growth habit contrasts sharply with Fescue, a desirable cool-season grass, which relies on a fibrous root system and tillers. Johnson Grass is a warm-season species, actively growing and translocating resources when Fescue is often under summer stress or semi-dormant. This physiological difference creates a window of opportunity for selective control using systemic herbicides.
Selective Chemical Control Options
Selective herbicides are divided into two categories: turfgrass applications and pasture/hay field applications.
Turfgrass Applications
For established Fescue turf, the most common selective post-emergent option contains the active ingredient fenoxaprop-P-ethyl (e.g., Acclaim Extra). This herbicide is an ACCase inhibitor that disrupts lipid synthesis in susceptible grasses. Fenoxaprop-P-ethyl is generally safe for tall Fescue and manages seedling Johnson Grass. However, it usually offers only suppression, not complete eradication, of established Johnson Grass with extensive rhizomes. While the chemical moves throughout the plant, the sheer size and energy reserves of mature rhizomes often allow for regrowth.
Pasture and Hay Field Applications
In Fescue pasture or hay fields, broadcast application options are more limited and may risk temporary Fescue injury. The most effective selective chemicals belong to the sulfonylurea class of herbicides, which inhibit the acetolactate synthase (ALS) enzyme.
Specific active ingredients include:
- Nicosulfuron (e.g., Cryder), which provides selective control in established perennial pastures without significant Fescue injury when applied correctly.
- Sulfosulfuron (e.g., Outrider), which offers excellent Johnson Grass control. Applications may cause initial discoloration or temporary stunting of the Fescue, but it typically recovers.
The required application rate must be chosen carefully based on whether the land is used for turf, pasture, or hay, as label restrictions vary greatly.
Wiper Application
A highly effective, low-impact method, particularly for pasture, involves the precise application of the non-selective herbicide glyphosate using a rope-wick or wiper applicator. This technique capitalizes on the height difference: the taller Johnson Grass is wiped with the chemical while the shorter Fescue remains untouched. This avoids broadcast application but requires the weed to be significantly taller than the desirable grass.
Critical Timing and Application Methods
The success of systemic herbicide application depends heavily on timing. The herbicide must be applied when the plant is actively growing and moving carbohydrates from the leaves down to the rhizomes for storage. This translocation process delivers the chemical to the root system.
The optimum application window is when Johnson Grass reaches 18 to 25 inches tall, typically in mid-to-late summer. Applying the herbicide too early means insufficient leaf surface area for absorption and less downward movement to the rhizomes. Applying it too late, near the end of the season, may mean the plant has already begun to senesce and is no longer actively moving resources.
Eradicating established Johnson Grass often requires sequential treatments rather than a single application. The first application targets the main flush of growth, and a follow-up application is needed two to three weeks later. This second treatment controls new shoots emerging from rhizomes and is important for depleting stored carbohydrate reserves.
To enhance effectiveness, a non-ionic surfactant (NIS) or a crop oil concentrate (COC) should be added to the spray mixture. These adjuvants help the chemical penetrate the waxy cuticle of the leaves, improving absorption and translocation. Always consult the specific herbicide label for the correct adjuvant type and concentration, as failure to include one can significantly reduce weed control.
Safe Handling and Preventing Recurrence
The application of any selective herbicide requires strict adherence to safety protocols to protect the applicator, the Fescue, and the environment. Applicators must wear the Personal Protective Equipment (PPE) specified on the product label, including chemical-resistant gloves, protective eyewear, and long-sleeved clothing. Following all label directions is mandatory, as the label details proper rates, methods, and restrictions.
Preventing drift is a primary concern, especially when applying chemicals near water sources or sensitive areas. Using larger spray droplets, applying during periods of low wind, and avoiding high temperatures will minimize the chance of the chemical moving to non-target areas. The label will also specify the maximum application rate per year and the required interval between treatments.
Long-term management relies on cultural practices that favor the Fescue and prevent weed re-establishment. Johnson Grass spreads vigorously by seed, so preventing seed production through timely mowing or spot-spraying is important. Maintaining a dense, healthy stand of Fescue is the best defense against invasion, as a robust canopy outcompetes emerging Johnson Grass seedlings for light, water, and nutrients.