Pigweed plants that escaped POST applications or emerged after can now be seen above soybean canopies. Especially important are waterhemp and Palmer amaranth, as these species pose increased economic and management concerns. Waterhemp and Palmer plants can produce upwards of one million seeds per plant in certain situations. Managing these weeds often starts with preventing introductions. Anything we can do from now through harvest to prevent seed from being deposited into the soil seed bank will pay dividends down the road. At this point there are limited control options beyond scouting and hand pulling. Just a few plants left in the field can lead to a total infestation if they produce seeds.
Viability of pigweed seed is greatly reduced after 3-5 years. Management over a couple of growing seasons can drastically reduce populations. Aside from tremendous seed production, fast growth rates, and lengthy emergence windows, what makes us most nervous about these weeds is their propensity to develop herbicide resistance. In other states, waterhemp has exhibited the ability to resist up to seven different herbicide sites of action, and Palmer amaranth up to nine. Resistance to more than one site of action within a single population is not uncommon. Metabolic herbicide resistance may increase the prevalence of populations with resistance to multiple herbicide groups. Experience would tell us it's only a matter of time until we have these types of resistance issues in Ohio. The status of herbicide resistance in Ohio waterhemp populations was covered in this article.
We have a ton of resources that can be helpful for scouting, including a pigweed ID guide, pigweed management fact sheet, and YouTube video. More helpful information on the management of pigweeds can be found on the OSU weed science website.
Late-season scouting will allow us to evaluate how well our programs worked this year and forecast issues for next year. Below are some guidelines for scouting as we approach harvest.
- Scout all fields at some point between now and harvest. Evaluations of the weed species present and level of infestation can take place from the road or field edge. This can also be a good use of drones and other technology. At this time of year corn mostly hides weed infestations, but weeds can be seen above the soybean canopy.
- Fields that are suspected to have any level of Palmer amaranth or waterhemp should be evaluated more closely. If you are unsure of whether or not you are dealing with one of these problematic pigweeds, send pictures to us or your local extension educator.
- Remove waterhemp and Palmer amaranth plants by hand. Cut plants at the soil surface, and where mature seed is present, bag seed heads before removing them from the field. This will help reduce the spread of seed.
- Where there are severe infestations and hand removal is not realistic, the decision then becomes whether it is best to mow or harvest, both between and within fields. Harvesting fields with waterhemp or Palmer with mature seed heads will contaminate equipment and increase the likelihood of spreading seed to other fields or operations. Mowing before seed is mature can help reduce future populations. Where the decision is made to harvest infested areas, harvest these areas last and thoroughly clean equipment afterwards.
- Waterhemp is increasing in prevalence across the state. If you discover waterhemp, feel free to reach out to the OSU weed science program or your local extension educator for management recommendations. Palmer amaranth is not quite as widespread at this time. If you discover Palmer amaranth, please reach out to us so we can monitor its location in the state.
Feel free to reach out to Alyssa Essman (Essman.email@example.com, 614-247-5810) for questions regarding this topic or other concerns related to the identification and control of weeds.