Is Now a Good Time to Control Perennial Weeds in Grass Forages?

One of the most common weed management questions we often get relates to perennial weed control in grass pastures and hayfields.  Many people want to know when the most optimal time is to control perennial weeds such as Canada thistle, milkweed, hemp dogbane, horsenettle, quackgrass, and others and what herbicides are the most effective. The short answer – late summer and fall with systemic herbicides, but there is more to the story.

Weed Biology and Herbicide Application Timing

With the autumn weather, these plants more actively transport carbohydrates and sugars to underground storage structures such as rhizomes, tubers, and roots to enable them to survive the winter and to provide the necessary energy to begin the next cycle of growth in the spring. During that time of year, systemic herbicides such as 2,4-D, dicamba, triclopyr, glyphosate, and others are readily moved along with the sugars into the root systems, allowing for more effective control. However, since many of these perennials begin to flower and set seed by mid-summer, waiting until fall to spray herbicides can be challenging, if the goal is to reduce weed populations and stop reproduction. Therefore, during the growing season, we typically suggest mowing pastures a couple of times, not only to stop weed seed production, but also to "set up" the weeds for a more effective herbicide application in the fall. Time the late-season mowing to occur early enough to allow perennial weeds to regrow about 12-15 inches (usually about 4-6 weeks after cutting) before a herbicide application in the fall. In general, the herbicide application window runs from early September through October, depending on where you are in the region and what weeds you are targeting. Warm-season perennials such as horsenettle, hemp dogbane, common milkweed, smooth groundcherry, pokeweed, bindweed, poison ivy, and brambles should be sprayed with a systemic herbicide (such as 2,4-D, dicamba, triclopyr, etc.) from September 1 through October 15 or before a hard frost. In general, applications by October 1 may be more effective, but as long as the weeds have green leaf tissue and are actively growing, herbicide applications can be made later in the year. These suggested dates target central PA, so adjust by a week or so forward or backward if you are south or north of I-80 in the region. Canada thistle, quackgrass, and dandelion can be effectively controlled after several light frosts, so an additional 2 to 4 weeks or more application window can exist for these species because of their cool-season habit of growth. Spring (or early summer) herbicide applications are possible, but perennial weeds must be at least 12 inches tall for control. Spring applications must be followed by routine mowing and a fall herbicide application for best control. However, that being said, herbicides can be applied now during the summer to budding or flowering perennials. Depending on the species, different control outcomes may occur. So keep in mind that effective control may not likely happen, and additional applications will be needed later in the fall.

Herbicide Options

The most common herbicides used to control many of the broadleaf weeds during the season or in the fall are 2,4-D and dicamba (Clarity, etc.) for broadleaves. However, other systemic products such as triclopyr (e.g., Crossbow, Candor, Crossroad, Remedy Ultra) or metsulfuron can be options as well. A combination of these products may be the best solution for a mixture of different perennial weeds. Although the new herbicide, NovaGraz, does a pretty good job on many common pasture weeds, it is weak on the aggressive perennials like horsenettle, milkweed, dogbane, Canada thistle, and smooth bedstraw. Below are some best bets when controlling selected perennial weeds in grass forages:

• Smooth bedstraw – Crossbow (late June/early July); PastureGard HL (is showing promise in current Penn State trials – more details to follow about best application timing and rate)
• Canada thistle – Stinger/clopyralid or 2,4-D+dicamba (bud to bloom or mow during the season and spray in fall; best to use higher rates)
• Hemp dogbane– Starane Ultra, Yukon, 2,4-D+dicamba or Crossbow (high rates usually needed, mow during the season and spray in late summer)
• Horsenettle – Crossbow (bud to bloom in June-mid-July or late summer if leaves are not too tattered by insect feeding)
• Johnsongrass – Glyphosate spot spray or wick applicator (spray during September); products like Outrider/Cryder are effective on Johnsongrass but unfortunately are not labeled for use in cool-season forage grass species
• Milkweed – 2,4-D+dicamba, Yukon, or Crossbow (high rates needed, mow during the season and spray in late summer but expect only suppression: <70% control)
• Multiflora rose – Cimarron Plus/metsulfuron, Crossbow (bloom or fall)
• Quackgrass– Glyphosate spot spray or wick applicator (October – November)

Herbicides with Liquid Nitrogen

As tempting as it is to do a one-pass "weed and feed" application this time of year, it is not recommended, especially since it is so hot and dry. Adding herbicides such as 2,4-D, dicamba, triclopyr, and other products to liquid N spray carriers can really burn the foliage of the crop and drastically set it back, especially if it's sunny and the temperatures exceed 80°F. Even using a 50:50 mixture of water and liquid fertilizer still might cause unacceptable foliar burn on the forage during the summer. It is recommended to spread dry fertilizer and spray herbicides in separate passes.

Wiper Applications

With this tactic, the herbicide is wiped across the leaves and stems of weeds. Wiper applicators (weed wipers) are devices that use a herbicide-saturated sponge, rope wick, or roller that physically wipes a concentrated solution of herbicide directly onto weeds by taking advantage of the sufficient height difference between the weeds and crop canopy. Since only the top several inches of the weeds are contacted with the herbicide solution, a translocated herbicide such as dicamba or glyphosate is usually required for effective control. Repeat applications may be required to manage certain perennial weeds. Keep in mind, university data shows that wiper applications have not been as effective as broadcast or spot spray applications on perennial weeds in pastures. However, they can provide some necessary suppression of weeds if spraying is not possible. A bidirectional application is recommended to provide thorough control. Herbicides labeled for wiper application will usually provide specific application instructions on the label. In addition, research trials have indicated that a 5 percent solution applied in a wiper applicator is somewhat comparable to a broadcast application rate of 1 quart per acre. Or use a mix ratio of 1/3 glyphosate to 2/3 water in the tank or this can be tankmixed with Crossbow, 2,4-D, or dicamba.

Grazing and Haying Restrictions

And finally, whenever using herbicides, make sure to follow any grazing and haying restrictions for each of the products. Some have no or minimal waiting periods between herbicide application and crop utilization, while others have periods of up to a month or more. Each product can differ depending on whether the forage is being grazed or harvested for hay. Furthermore, the restrictions can vary upon the kind of livestock and if they are lactating.

Where To Find More Information

More details on any of these topics covered above can be found on the respective product labels or in the 2025 Penn State Agronomy Guide. In particular, refer to Tables 2.6-11 (herbicide effectiveness vs. weeds table); Table 2.6-12 (comments on herbicide use), Table 2.6-14 (grazing and haying restrictions), and Table 2.6-17 (optimal time to spray herbicides on specific weeds).

In conclusion, if perennial weed populations have been growing for many years in a field, it will take a few years or more to get them under control with these repeated measures. Effective control in grass hay/pastures is primarily based on routine mowing, timely systemic herbicide applications, a thick forage stand, and not overgrazing. Over time, these tactics can drastically reduce or eradicate these weed populations.