As the 2018 growing season is wrapping up and with harvest around the corner, weeds are likely not on the forefront of most farmers minds. However, now is the time to make note of weed issues in fields to plan for the 2019 growing season. This is also the time to scout for Palmer amaranth, and to plan for fall herbicide applications.

While it’s too late now to do much for the weed escapes besides spending some time walking fields and pulling weeds, it’s still important to make note of weed issues in fields to help plan for next year. Knowing what the problem weeds are in a field is important to design an effective weed management program to combat those weeds the next year.

Make note of problem weeds 
In addition to identifying what weed issues exist in a field, it may be possible to determine why the weeds survived. Weed survival might not all be due to herbicide resistance; other factors such as weed size, environmental conditions and application problems, like spray coverage, can also result in weed survival.

If you are concerned that you might be dealing with a herbicide resistance issue, the University of Illinois Plant Clinic does offer a herbicides resistance screening for waterhemp and Palmer amaranth. The clinic provides testing for the most common forms of glyphosate and PPO inhibitor herbicide resistance. More information about this screening service and how to submit a sample is online.

After making note of what weed issues exist, consider whether the problems are significant enough to warrant planning your harvest order around the problem weedy fields. This is an effective tactic to prevent the spread of new weeds from one field to another with the combine. To further avoid spreading seed from one field to the next, a combine cleanout between fields is recommended as well.

Scout for Palmer
Palmer amaranth was first identified in Iowa in 2013 and has since been found in 52 counties across the state. It was added to the Iowa noxious weed list in 2017. While this weed will likely not spread quickly across Iowa, more identifications have been made in crop fields in the state in recent years.

The first step to managing Palmer amaranth requires proper identification. Palmer can easily be confused with waterhemp, especially when the plants are in the vegetative stages. However, once they reach the reproductive stages and start flowering, it becomes much easier to distinguish between the two. Both plants are in the reproductive stages now, so it’s a good time to scout and take a second look at any suspicious looking plants.

Like waterhemp, Palmer amaranth has separate male and female plants. A female Palmer plant has long, sharp bracts surrounding the flowers on the tall terminal inflorescences, whereas the bracts on a female waterhemp plant are much shorter and are much softer to the touch. If you try to put your hand around a female Palmer amaranth inflorescence, you will feel the sharp bracts.

Visit the Palmer amaranth identification resource we have on the ISU Extension website.

If you discover a Palmer amaranth plant in your field, it is essential to take steps to remove all plants from the field and prevent any seed from getting to the soil surface or spreading to other locations.

Eradication of this weed is possible, but it is important to properly identify any infestations early. If you come across a questionable plant and aren’t sure if it is Palmer amaranth or not, contact your local Extension field agronomist to help with proper identification and management practices.

Fall herbicide applications
Fall can also provide a great opportunity to help control some challenging winter annual, biennial and perennial weeds in row crop fields, as well as in pasture and hay fields. 

Marestail (Conyza canadensis) is a common problem weed, especially in no-till soybean fields. Although classified as a winter annual, marestail can emerge in the late summer and fall, as well as in spring. However, most marestail in Iowa seems to emerge in the late summer or fall. Scout fields this fall to help determine which fields could be good candidates for a fall herbicide application.

For a fall herbicide application, use a base treatment of a 1-pound a.e. 2,4-D per acre. Other products may be tank-mixed with the 2,4-D to help provide a broader spectrum control of other winter annual weeds; however, it is not recommended to include a residual herbicide, as it will likely provide little benefit in helping to control emerging weeds next spring. Make herbicide applications when daytime high temperatures are in the mid-50s for best results. A fall herbicide application will also likely not eliminate the need for a burndown application in the spring.

Pasture and forage weed control
On the pasture and forage side, fall is a great time to control biennial weeds like bull and musk thistle and wild parsnip and perennial weeds like Canada thistle and horsenettle as these weeds are moving and storing sugars to their root systems for next spring.

Applying systemic herbicides like 2,4-D, dicamba or aminopyralid in the fall when the plants are bringing sugars and energy reserves to their root systems helps to bring the herbicide down to the roots, as well as provide good weed control. Plan to make herbicide applications when the plants are actively growing for best control.

In Iowa, most biennial and perennial weeds are fairly cold-tolerant and can be controlled with an application even after a few light frosts; however, it is important to have good leaf tissue present to absorb the herbicide.

If desirable broadleaf species like alfalfa or clover are present, be aware that spraying for broadleaf weeds will also control these desirable species. If weeds are more localized, consider spot spraying. Otherwise, focus on controlling the weeds and re-establishing legumes after weeds are under control. Always read and follow label directions for application rates, weeds controlled, grazing or haying restrictions, and crop rotation restrictions.

While it seems early, making plans to scout fields for weed issues, including Palmer amaranth, and taking steps to control winter annual, biennial and perennial weed issues this fall will save you headaches in the 2019 growing season.

Vittetoe is an ISU Extension Field Agronomist covering southeast/south central Iowa. Contact her at [email protected].