Late Season Weed Scouting Resources – Ohio Ag Net
By Alyssa Essman, Ohio State University Extension
Aquatic Hemp and Palmer Amaranth plants that have escaped POST applications or emerged after are now beginning to develop mature seeds. These plants can produce over a million seeds per plant in some situations. When it comes to dealing with these weeds, the best offense is a good defense. Anything we can do from now until harvest to prevent seeds from being deposited in the soil seed bank will pay dividends later. At this point in the season, control options beyond scouting and hand pulling are limited. A few plants left in the field can lead to a total infestation within a few years. The viability of Aquatic Hemp and Palmer Amaranth seeds is greatly reduced after 3-5 years.
Some diligence over a few growing seasons can significantly reduce populations. Aside from the huge seed set, rapid growth rates, and long emergence windows, what makes us most nervous about these weeds is their propensity to develop resistance to herbicides. In other states, hemp has shown the ability to resist seven different sites of herbicide action, and Palmer pigweed can now resist nine.
Resistance at more than one site of action within a single population is not uncommon. Metabolic resistance to herbicides can increase the prevalence of populations resistant to several groups of herbicides. As of today, Ohio seems to be a little behind the southern and western states in resistance issues, although experience tells us it’s only a matter of time. The status of herbicide resistance in Ohio water hemp populations has been discussed in this article.
OSU Weed Science has a plethora of resources that can be helpful in scouting, including a pigweed identification guide, amaranth management fact sheet, and a YouTube video that covers seed maturity assessment. More useful information on amaranth management can be found on the OSU Weed Science website.
Below are some guidelines from Dr. Mark Loux for end-of-season scouting from the 2018 newsletter on this topic:
We recommend the following as we progress through harvesting from now on:
• Take some time in late summer to scout fields, even if it’s from the road or the edge of the field with a pair of binoculars. Now would be a good time to have a friend with a drone that provides real-time video, or your own personal satellite. Scouting from the road applies primarily to soybean fields, as corn often hides weed infestations.
• Walk the field to check for any weeds that might be Palmer pigweed, water hemp or otherwise mysterious. If you need help with identification, send us photos or tear plants out and bring them to someone who can identify them. Palmer and waterhemp are considerably different in appearance from giant ragweed and marestail, the more common late season offenders.
• When the presence of Palmer’s pigweed or water hemp is confirmed, check plants for mature seeds (in Palmer’s infestations these are the rough female heads), by shaking/crushing parts of the seed head in your hand or any other surface that will provide contrast. Mature seeds will be small and very dark.
• Plants without mature seeds should be cut just below the soil surface, and ideally removed from the field and burned or composted. Plants with mature seeds should be cut and bagged (at least the seed heads) and removed from the field, or removed by any other method that prevents seed dispersal across the field.
• If the Palmer pigweed or water hemp population is too dense to remove from the field, some decisions must be made about whether or how to mow or harvest. Harvesting through infested plots or fields will result in further spread throughout the field and also contamination of the combine with weed seeds which can then be dispersed to other fields. So consider: 1) not harvesting areas of the field infested with Palmer pigweed or water hemp, and instead mow several times to prevent seed set, and 2) harvest the infested field(s)( s) after all other fields have been harvested, and clean the combine thoroughly before further use. This also applies to any infestation discovered during harvest.
• Locate field edges and adjacent roadsides, flooded or manure-spreading areas, and CREP/wildlife area seedings. The latter can be infested due to contaminated seed produced in states where Palmer pigweed and water hemp are endemic and not considered harmful. Reminder – ODA will test any seed used for these purposes for the presence of Palmer’s Pigweed.
Please feel free to contact Alyssa Essman ([email protected], 614-247-5810) with any questions regarding this topic or other concerns related to weed identification and control.