Waterhemp begins to appear

This past week, the Hancock County Extension Office was asked to identify a weed that "appeared" to pop up overnight. It was waterhemp, which can grow 3 inches in a day under the right conditions. This weed is often seen growing above the soybean canopy in late July but may be seen much sooner this year because of the above normal temperatures. Farmers need to check their fields for this troublesome weed and its more serious cousin: Palmer amaranth. These weeds need to be removed before going to seed. They are difficult to control and have caused major financial losses in other soybean-growing regions in the country. Battling them will significantly increase the cost of an herbicide program. If not controlled, soybean yields may be reduced as much as 40%. Waterhemp and Palmer amaranth can take over a field faster than most weeds. They can grow rapidly under ideal conditions. Both weeds are prolific seed producers. One female plant can produce a million seeds in a growing season. Seeds are very small and can easily be moved in grain, feed and machinery. Fortunately, farmers have been able to keep Palmer amaranth out of the county. However, fields with waterhemp have been increasing since 2016. Each fall the Hancock County Extension Office, with assistance from Legacy Farmers' Cooperative, has surveyed soybean fields for surviving weeds at harvest time. Only 1% of the fields had waterhemp in 2016, but the number increased to about 15% in 2020. Fortunately, in my survey taken in taken in 2021, out of 282 soybean fields observed, only 7% had waterhemp. This lower number shows that farmers in the county are taking this weed serious and either removed plants when first observed before going to seed and/or have used the new technology traits in soybean varieties that have allowed the use of dicamba, 2, 4-D or glufosinate on emerged soybeans, which are effective herbicides on waterhemp and Palmer amaranth. If nothing had been done, waterhemp infested fields could have quickly jumped to 50%, which has happened in other counties in northwest Ohio. Waterhemp is a summer annual that may be 2 to 8 feet tall. Leaves are lanceolate to oblong with smooth margins. Petalless flowers develop from narrow panicles of spikes from the upper branches of the plant. 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 many common soybean herbicides. Waterhemp and Palmer amaranth are species of pigweeds, but are 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 and Palmer amaranth have no hairs on the stem. Palmer is generally a larger and more robust plant than waterhemp. Its leaves will be broader than waterhemp and its flower/seed stalk is longer and rougher (like sandpaper). Now that waterhemp has established populations in some fields, farmers will have to be diligent to limit its spread to new fields. Dr. Mark Loux, the Ohio State University Extension weed specialist, recommends the following steps to keep waterhemp from establishing populations in new fields: • Inspect fields for waterhemp. These weeds will be taller than the soybean canopy. The Hancock County Extension Office will assist in identification. Waterhemp is considerably different in appearance than giant ragweed 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 mature seed 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 through the field. • If the waterhemp population is too dense to hand remove from a field, a farmer will have to consider two options: Do not harvest, but mow areas infested with waterhemp several times to prevent seed production, or; harvest infested field(s) after all others 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 and Palmer amaranth, 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 for waterhemp since 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 preemergent with soil residual activity, and removing any waterhemp plants that is not killed by the herbicide program. They should also consider selecting soybean varieties that have resistance to dicamba, 2, 4-D or glufosinate to use as postemergent herbicides. So far, farmers have been successful in keeping Palmer amaranth out of Hancock County. Unfortunately, waterhemp has established populations in a few fields across the county. Farmers have the next few weeks to remove individual plants in new fields before they go to seed. Additional information may be found on waterhemp and Palmer amaranth at the following Ohio State University website: https://u.osu.edu/osuweeds/super-weeds/palmer-amaranth/ Lentz is Extension educator for agriculture and natural resources for Ohio State University Extension-Hancock County.