Upcoming opportunities for weed control in perennial forages

By Emma Matcham, PhD, CCA, Ohio State University

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.

Just as fall is a useful weed control window for grass pastures, the dormant season is a useful weed control window for alfalfa. Chemical formulations are limited in mixed alfalfa-grass stands, and metribuzin is really the only option. But it doesn’t have great control of many of our problematic perennials (quackgrass, Canada thistle, yellow nutsedge). There are more options for dormant-season herbicides in legume forages when they are not planted with a companion grass. We’ve had some increasing challenges with quackgrass throughout Ohio, and products containing pronamide (Kerb or other brand names) can be a useful dormant-season product in this case. It is sensitive to timing, though, and most pronamide products perform best when soil temp is below 55* F but not yet frozen. More mature alfalfa stands may be suitable for dormant season applications of hexazinone (Velpar), which is a little more effective on thistles and sedges than other alfalfa-safe dormant season products. Another dormant season option for mature alfalfa is terbacil (Sinbar), which has some activity on quackgrass and thistles but not much on nutsedges. Just be sure to check the label to make sure that the age of your alfalfa stand is appropriate for the product(s) you’re considering, and be careful with plant-back intervals for fields that may be rotated to a grass crop soon.

This article focuses on perennial weeds, but fall can also be a good time of year to control problematic biannuals and winter annuals, too. Poison hemlock is in its rosette stage in late fall and can be controlled in grass pastures with many of the same systemic herbicides mentioned above, including triclopyr and dicamba. Poison hemlock removal is much tougher in alfalfa, and spot-sprays are generally the best option. Crestleaf groundsel in grass pastures and hayfields responds to fall applications of 2,4-D or 2,4-D + Dicamba mixes, and pre-mix formulations of aminopyralid + 2,4-D (GrazonNext) are also an effective option. Terbacil and hexazinone are the best options for crestleaf groundsel on dormant alfalfa, but those generally aren’t labeled for use on seedlings. Crestleaf groundsel can be suppressed, but not necessarily eliminated, from seedling alfalfa using imazethapyr (Pursuit) in the late fall or early spring.

As always, scout your fields before purchasing products so that you understand which weeds are problematic in your fields. Read all herbicide labels before spraying, and pay close attention to any restrictions on treated hay, plant back intervals, and application timings.