By Blake Jackson
As the growing season winds down, fall presents an ideal opportunity for farmers to evaluate and improve their weed management programs.
According to Dwight Lingenfelter, Extension Associate in Weed Science at Penn State Extension, late summer and early fall are among the most effective times to control many biennial and perennial weeds in pastures, hayfields, and crop fields because plants are actively moving nutrients to their root systems.
This process also helps systemic herbicides move into underground plant tissues, improving long-term control.
Biennial weeds such as common burdock, wild carrot, musk thistle, bull thistle, and plumeless thistle are most vulnerable while in the rosette stage before overwintering. Once these weeds resume growth in spring and begin reproductive development, they become much more difficult to manage.
Perennial weeds, including horsenettle, common milkweed, smooth groundcherry, multiflora rose, Canada thistle, quackgrass, and dandelion, can also be controlled more effectively during the fall application period.
Early September through October generally provides the best treatment window, although timing may vary depending on weed species and local weather conditions.
Common herbicides used for fall broadleaf weed control include 2,4-D, dicamba, triclopyr, and metsulfuron, while products containing picloram can provide additional control of difficult perennial species in permanent pastures.
Healthy green foliage is essential for successful herbicide uptake, so applications should be made before weeds begin turning yellow or entering dormancy.
Japanese stiltgrass is another concern in pastures. Since this invasive annual grass produces seeds later in the season than many other weeds, mowing before seed drop can significantly reduce future infestations. Spot treatments may also help when plants remain actively growing.
For newly established forage seedings, producers should carefully select herbicides and ensure crops have reached an appropriate growth stage before treatment. In mixed alfalfa-grass stands, some herbicides may damage desirable forage species.
Fall also remains the best time to terminate declining hay stands and manage perennial weeds following silage harvest. Fields should be monitored for invasive species such as Palmer amaranth and waterhemp, which can continue producing viable seed even after harvest if plants are not properly controlled.
Weed Management Recommendations
- Target biennial weeds in the rosette stage before winter for easier and more effective control.
- Apply systemic herbicides during fall when perennial weeds are actively transporting nutrients to their roots.
- Treat only healthy, green weeds to ensure maximum herbicide absorption and effectiveness.
- Monitor harvested fields for Palmer amaranth and waterhemp, preventing late-season seed production.
- Consider crop rotation restrictions and future planting plans when selecting herbicides to avoid carryover injury to subsequent crops.
Categories: Pennsylvania, General