Written by Ed Letnz, Professor, Ohio State University Extension in Hancock County
In the past, farmers were not too concerned about a few weeds going to seed in their soybean field. They would be able to control them when they planted corn the following year. But that was before many of them started planting continuous soybean, selected Roundup Ready varieties, relied heavily on glyphosate for weed control, and moved away from tillage. A new weed appeared in our area adapted to no-till and soon became resistant to glyphosate and other commonly used soybean herbicides. Before 2016, a farmer would not have seen waterhemp in Hancock County fields. Today, it has become the number one problem weed in Ohio and continues to spread in our county.
Each fall the Hancock County Extension Office surveys soybean fields for surviving weeds at harvest time. From this survey, only 1% of the fields had waterhemp in 2016, 15% in 2020, and 38% in 2024. This number could quickly jump to 50% if farmers do not take steps to prevent the spread to new fields. Waterhemp is difficult to control once it has established a population in a field. It has caused major financial losses in other soybean growing regions, such as Illinois and Kansas. Battling infested fields will significantly increase the cost of an herbicide program. If not controlled, soybean yields may be reduced 40%. As a result, farmers that do not have an established population must control its spread the old fashion way — by removing any plants that escaped their weed control program by hand before it goes to seed.
Waterhemp can take over a field faster than most weeds. It can grow 2 to 3 inches per day under ideal conditions. It is a prolific seed producer. One female plant can produce a million seeds in a growing season. Seeds are exceedingly small and can easily be moved in grain, feed, and machinery. Waterhemp is a summer annual that may be two to eight feet tall at maturity. Leaves are lanceolate to oblong with smooth margins. Petal-less flowers develop from narrow panicles of spikes from the upper branches of the plant https://www.illinoiswildflowers.info/weeds/plants/water_hemp.htm. Plants are dioecious, meaning a plant will only have male or female flowers, but not both. This ensures that pollen will come from another plant, which may have resistance to certain herbicides. Thus, waterhemp is known to quickly develop resistance to soybean herbicides.
Waterhemp is a type of pigweed, but much more difficult to control than the common redroot pigweed. Identifying the type of pigweed in the field is relatively easy by looking for hairs on the main stem. Redroot pigweed will have fine hairs like peach fuzz. Waterhemp has no hairs on the stem. Waterhemp stems are usually green but may also be red.
Now that waterhemp has established populations in some fields, farmers will have to be diligent to limit its spread to new fields. The Ohio State University Extension recommends the following steps to keep waterhemp out of new fields:
- Inspect fields for waterhemp. These weeds will be taller than the soybean canopy in late July to early August prior to flowering. The Hancock County Extension office will assist in identification. Waterhemp is considerably different in appearance than giant ragweed, velvetleaf, and marestail – other late-season tall weeds.
- Scout field borders, adjacent roadsides, flood areas, manure applied fields, and conservation/wildlife area seedings. The latter may contain waterhemp that grew from seed sources collected from states that do not prohibit this weed from being harvested with conservation species.
- Where waterhemp is confirmed, check to see whether plants have mature seed by shaking/crushing parts of the seedhead into your hand or other surface that will provide contrast. Mature seed will be small and very dark. Plants with seeds and flower stalks should be cut off and bagged (at least the seedheads) and removed from the field, or removed via any other method that prevents seed dispersal in the field.
- If the waterhemp population is too dense to hand remove from a field, a farmer will have to consider two options:
1) do not harvest, but mow areas infested with waterhemp several times to prevent seed production, or 2) harvest infested field(s) after all other fields have been harvested and clean the combine thoroughly before further use. Harvesting through patches of infested fields first may further spread weed seed throughout the field and contaminate the combine, which can then disperse weed seeds to other fields.
Farmers need to take zero tolerance to waterhemp, which means individual plants need to be removed by hand before they go to seed if found in soybean fields. Once infested a farmer will have serious weed issues in future years. Farmers will have to change their weed control program since waterhemp it is likely resistant to glyphosate and other commonly used soybean herbicides. Even if a farmer does not have waterhemp now, they should adjust their herbicide program in the future as if they did. This would include using a burndown program with a mix of herbicides with different modes of action; a pre-emergent with soil residual activity; switching to dicamba, 2,4-D, or glufosinate tolerant soybean varieties for post herbicide applications; and removing any waterhemp plants that are not killed by the herbicide program. Farmers have the next few weeks to remove individual plants in new fields before they produce mature seed.
Additional information may be found on waterhemp at the following Ohio State University website: https://ohioline.osu.edu/tags/waterhemp.