Kelly Nichols, Agriculture Agent Associate
University of Maryland Extension, Frederick County
This summer, we have gotten a few calls about Palmer amaranth and waterhemp, two pigweed species that are unfortunately becoming more common. These two pigweeds are difficult to control, mostly due to their herbicide resistance and fast growth (especially in hot weather). Often, these pigweeds are not noticed until they are seen growing up over the crop canopy, especially in soybeans. By then, it is too late to control them.
As we head into harvest, if you have Palmer or waterhemp – or want to make sure you don’t have them – scout your fields to be certain. Know how to identify Palmer (Figure 1) and waterhemp (Figure 2). The main characteristic is that these two troublesome pigweeds are completely hairless. Redroot and smooth pigweed, our most common pigweed species, have hairs on the stems and leaves. Another characteristic of Palmer is that the petiole (the little stem that attaches the leaf blade to the main stem) is longer than the leaf blade itself. (Note: Spiny amaranth, or spiny pigweed, is another pigweed that is common in pastures. It also does not have hairs; however, it will have spines on the stem. Palmer and waterhemp do not have these spines).
While you are scouting, pull out the Palmer or waterhemp plants (as many as you can). Palmer and waterhemp can produce hundreds of thousands of seeds per plant. At this point in the season, this is the best way to reduce the number of seeds that could germinate next year. Consider taking a paper bag with you to put the plants in, as smaller plants can re-root. Take the plants out of the field and bury or burn them.
Harvest infested fields last. The biggest concern with harvesting infested fields is the spread of the seeds – not only throughout the currently infested field, but also to other fields and possibly other farms. If there is only a small section of the field that is infested, consider not harvesting that section to avoid spreading the seeds. If you are not running the combine, be in communication with the person who is to ensure that Palmer or waterhemp seeds are not brought onto your farm and/or spread around your fields. If the infested harvested crop is to be fed to livestock, the processes of grinding, roasting, and ensiling can destroy weed seeds and prevent the seeds from being spread in the manure.
After harvest (or in between fields if necessary), clean out the combine. Pigweed seeds are tiny (about the size of a pencil point), and it is difficult to perfectly clean out a combine. However, cleaning can still reduce the number of weed seeds in the combine. Use compressed air and start at the front of the combine, working up to the grain tank and auger, and then to the back. Running straw through the combine can also help to clean it out. Research from the University of Delaware has shown that using compressed air in combination with running straw through the combine can potentially reduce the number of weed seeds in the combine by thousands.
For next year, consider using these strategies to control Palmer and waterhemp: plant a cover crop to provide weed suppression in the spring; rotate to corn for more effective herbicide options (compared to soybeans) or a perennial forage; use the full recommended labeled rate; use residual herbicides in both the pre- and post-emergence applications, as Palmer and waterhemp seeds can germinate throughout the growing season; use multiple effective modes of action; and rotate modes of action. More information on Palmer and waterhemp, as well as herbicide resistance weed management, can be found at www.integratedweedmanagement.org, a website run by Extension Weed Specialists from across the U.S.
