Western Ragweed (Ambrosia psilostachya) is a persistent perennial weed that commonly infests pastures, rangelands, and disturbed areas across North America. It spreads aggressively through an underground network of creeping root systems called rhizomes. Controlling this weed requires a chemical strategy to target the established root network and prevent its rapid spread and seed production. All chemical applications must comply strictly with local, state, and federal regulations regarding herbicide use.
Understanding Western Ragweed and Treatment Timing
Western Ragweed is a member of the sunflower family, often growing as an erect, hairy forb up to three feet tall with deeply lobed or divided leaves. Since it is a perennial, cutting the plant or treating it superficially will not eliminate the established rhizomes, which are typically found in the top five to ten inches of soil. Therefore, a post-emergent herbicide application is necessary to target the plant after it has begun its active spring growth.
The timing of treatment is the most important factor for achieving high efficacy. Treatment is most effective when the plants are young, actively growing, and small, ideally between three and five inches in height. This stage typically occurs in late spring or early summer, well before the plant has developed flowering stalks and entered the more resistant pre-bud stage. Applying herbicide during this vegetative phase allows the chemical to be efficiently translocated down to the rhizomes, ensuring a complete kill of the underground system.
Selective Herbicides for Control
Selective herbicides are the preferred choice for controlling Western Ragweed in areas like turf or established rangeland where desirable grasses must be preserved. These chemicals are broadleaf killers that work as auxin agonists. They disrupt the plant’s natural growth hormones, causing uncontrolled and abnormal growth that leads to death. The most common active ingredients recommended for this purpose include 2,4-D, dicamba, and triclopyr.
Products containing 2,4-D provide reliable control when applied to small, actively growing plants. For tougher infestations or where a broader spectrum of control is needed, combination products that pair 2,4-D with dicamba, such as Weedmaster, are effective. These dual-ingredient herbicides enhance the disruption of the plant’s physiological processes, leading to a complete kill rate for the deep root system.
Triclopyr, often found in formulations designed for brush control, can also be used on Western Ragweed in established grass stands. To maximize the absorption of any selective product, especially on the hairy leaves of Western Ragweed, it is recommended to include a non-ionic surfactant in the spray mixture. Users must consult the product label for specific concentration rates and application guidelines.
Non-Selective and Spot Treatment Options
In areas where preserving surrounding vegetation is not a concern, non-selective herbicides offer an effective control option. The most widely used non-selective chemical is glyphosate, which kills virtually all green, growing plants it contacts. Glyphosate is particularly useful for spot treatments along fence lines, driveways, or in non-crop areas where the goal is total vegetation removal.
Glyphosate has no residual soil activity, meaning it only kills the plant tissue it touches and does not prevent new seeds from germinating later. For longer-term control in non-crop areas, herbicides with soil residual properties may be considered. Certain formulations containing picloram are effective against deep-rooted perennials like Western Ragweed and can prevent regrowth for an extended period. However, these products often carry restrictions and may be classified as restricted-use pesticides due to their persistence and potential for off-site movement.
Safe Handling and Application Guidelines
All herbicide application begins with reading and following the product label. Personal Protective Equipment (PPE) is mandatory and includes long-sleeved shirts, long pants, chemical-resistant gloves made of nitrile or PVC, and eye protection. Respiratory protection may also be required, depending on the specific product formulation and application method.
Mixing should be done outdoors or in a well-ventilated area, measuring the product according to the label’s dilution instructions. To prevent herbicide drift, application should be avoided when wind speeds exceed five to ten miles per hour. Spraying during the early morning or late evening hours often provides calmer wind conditions and cooler temperatures. Once the application is complete, all empty containers must be triple-rinsed, and the rinse water should be added to the spray tank for application, not poured onto the ground.