Johnson grass (Sorghum halepense) is an invasive, perennial weed that poses a serious threat to the productivity of hay fields. It was originally introduced as a forage crop but quickly became an agricultural pest due to its rapid growth and ability to form dense, competitive stands. Effective management requires a multi-year, integrated approach. A successful control program must combine cultural, mechanical, and chemical strategies to deplete the weed’s reserves and prevent new generations from establishing.
Understanding the Spread of Johnson Grass
Johnson grass reproduces through two distinct mechanisms: extensive underground rhizomes and prolific seed production. The thick, cream-colored rhizomes are underground stems that store carbohydrates, allowing the plant to survive harsh conditions and emerge earlier in the spring. These rhizomes spread laterally, forming large colonies and are the source of perennial regrowth after mowing or cutting.
The plant is a heavy seed producer, with a single plant potentially generating over 80,000 seeds in one season. These seeds are easily shattered and can remain viable in the soil seed bank for more than ten years, leading to continuous new seedling emergence. Tillage or mowing equipment can spread both rhizome fragments and seeds to uninfested areas, further complicating control efforts.
Non-Chemical Management Techniques
Cultural and mechanical controls are essential for weakening the Johnson grass stand and reducing its energy reserves before chemical treatment. Repeated mowing and harvesting are primary tools. Mowing frequently, specifically before the plant reaches the seed head stage, prevents new seed production and depletes the rhizome carbohydrate reserves.
Timely cutting starves the plant because new rhizomes begin to develop only after the plant has grown five to seven true leaves. Rotating the hay field into a different crop, such as corn, allows for deep tillage in the fall. This exposes rhizomes to winter temperatures that can weaken or kill them.
Planting highly competitive forage species, such as dense stands of alfalfa, can also suppress Johnson grass growth. Alfalfa is a broadleaf crop, allowing for the use of selective grass-targeting herbicides that would injure a pure grass hay stand. These methods must be paired with targeted chemical application for long-term control.
Herbicide Selection and Application Timing
Chemical control involves using selective post-emergent herbicides that target Johnson grass without destroying the established forage crop. In legume hay fields, such as alfalfa or clover, ACCase-inhibiting herbicides are used because they selectively kill grasses while leaving the broadleaf crop unharmed. These systemic herbicides move throughout the Johnson grass plant, including down to the rhizomes, which is necessary for perennial control.
For pure grass hay fields (fescue or orchardgrass), selective broadcast herbicide options are limited because Johnson grass is also a grass. In these cases, a rope-wick applicator or spot-spraying with a non-selective herbicide like glyphosate is recommended. This method applies the herbicide only to the taller Johnson grass plants, which must be at least six inches taller than the desired forage to avoid injury.
Timing is crucial to ensure the herbicide translocates effectively to the rhizomes. The optimal time for application is when the Johnson grass is actively growing and is approximately 8 to 15 inches tall, before seed heads begin to form. Growers should delay harvesting hay for two to three weeks after treatment to allow sufficient time for the herbicide to move throughout the plant. Always consult the product label for specific pre-harvest intervals (PHI) to ensure compliance and forage safety.
Preventing Reinfestation
Successful long-term control requires vigilance in managing the soil seed bank and preventing the spread of new plant material. After initial control efforts, continuous scouting for new Johnson grass seedlings is necessary. These seedlings must be spot-treated or hand-pulled immediately, before they develop new rhizomes, which can occur as early as three to six weeks after emergence.
A major source of reinfestation is the mechanical transfer of seeds and rhizome fragments. It is important to clean all tillage, planting, and harvesting equipment thoroughly before moving between fields. Managing fence rows and field margins with spot treatments is also important, as these areas often harbor seed sources that can reintroduce the weed to the hay field.