By Emma Matcham
Late summer and early fall bring drier weather and slower growth of our cool-season forages throughout Ohio. This is also a great time of year to start controlling perennial weeds and make a plan for dormant-season applications.
Some of our top perennial weed issues in 2025 were hemp dogbane, milkweed, and Canada thistle, and we expect these to remain problematic in 2026. The root systems of perennial weeds allow them to regrow quickly whenever their aboveground structures are killed. Still, actions taken in fall can help reduce the amount of energy these plants store belowground and slow their regrowth next year. Frequent mowing is a great tool for denser patches of perennial weeds, and using a systemic herbicide in combination with mowing can further reduce the weeds’ ability to grow back in spring.
Many different products are effective for fall control of perennial weeds, especially in grass forage stands. But application timing can greatly impact efficacy. Systemic herbicides should be used when plants are still actively growing, and weeds have regrown to around 6-12 inches since the prior cutting. Optimal timing for most of Ohio is usually in September, since this is a few weeks after the last cutting but before any hard frosts. Systemic herbicides with good control on hemp dogbane and Canada thistle include dicamba (Clarity, others) and triclopyr (Remedy Ultra). Glyphosate is the best option for milkweed, although it will also have partial control with Remedy Ultra. Canada thistle, another aggressive perennial, responds best to fall applications of glyphosate, clopyralid (Stinger), or 2,4-D + dicamba, especially after light frosts when the plant is still translocating nutrients to its roots. Note that these systemic herbicides are generally not safe for clovers and should be used in grass forages, but they can also be used in mixed grass-legume stands where you plan to frost seed later this winter.
Source : osu.edu