COLUMBUS, Ohio – Waterhemp is prolific – it can produce up to a million seeds per plant. That can create a real problem in a farm field. The Ohio State University has been tracking the plant for many years. Its numbers started to ramp up in 2020 and 2021.

“It is now just about the No. 1 problem weed that we are having issues controlling,” said Alyssa Essman, assistant professor and weed science extension specialist at OSU. “It is native to the Midwest, and across this whole region we have seen it increase in prevalence.”

Part of the problem is that waterhemp was sensitive to many residual herbicides, Essman said. When the market moved away from residual products and started relying more on post applications and then herbicide-tolerant crop varieties, as they came on the market, waterhemp began to develop resistance to those products. 

“Waterhemp has prolific seed production, a fast growth rate and lengthy emergence window,” Essman said. “These factors, combined with tricky spring and summer weather patterns, make it difficult to manage. What makes us most nervous about these weeds is their ability to develop herbicide resistance.”

Interestingly, on a per-plant basis, waterhemp is not the most competitive plant, Essman explained. Other weed species are bigger. Waterhemp’s strategy is to produce so many seeds that it can quickly establish in an area if there are not good control options.

“It basically overwhelms an operation with the sheer number of plants,” Essman explained. “It can take over an area. As it gets taller, it gets more difficult to manage. Once the plants can produce seed, it is really hard to slow that down.”

Late-season scouting allows farmers to evaluate how well programs worked this year and forecast problem areas for next year. The following are some guidelines for scouting, as plants start to develop seed and harvest time nears, that Essman suggested in the C.O.R.N. Newsletter.

Scout all fields between now and harvest. Evaluate weed species present and level of infestation from the road, field edge or with drones. Corn can hide weed infestations, but waterhemp can be seen above the soybean canopy at this time of year.

Fields with a history of waterhemp or suspected introductions should be evaluated closely. If you are unsure of what pigweed species you’re dealing with, send pictures to OSU or your local extension educator.

Few herbicide options exist late in the season and are unlikely to be effective in reducing waterhemp populations or viable seed on large plants. Removing plants by hand is the best approach when late-season escapes occur. Cut plants at the soil surface and remove from the field. If mature seed is present, bag seed heads before removing them to reduce the spread of seed.

Other options include harvest weed seed control or weed zapping, and rely on the availability of specialized equipment.

Where infestations are severe, and hand removal is not realistic, the decision becomes whether it is best to mow or harvest, Essman explained. Harvesting fields with dense waterhemp populations that have 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 infested fields are harvested, do these areas last and very thoroughly clean equipment afterward.

If farmers are struggling to control waterhemp in corn or soybean fields, Essman suggested they reach out to the OSU weed science program of their local extension educator for recommendations.

Waterhemp is an edible plant and poses no risk to human or animal health, Essman said. It is a member of the amaranth or pigweed family.