Palmer amaranth is a tough weed that continues to march across the peanut belt. While imazapic (Cadre, Impose) was our main line of defense against this weed for many years, some populations are now resistant. In this case, we have very few options for applications postemergence if the weeds are greater than four inches tall. Considering that Palmer amaranth can grow in excess of one inch per day, this is not a very wide window for timing a postemergence application.
A Formidable Opponent
Palmer amaranth is a highly competitive weed that roots very deeply and pulls water from several feet away, provides deep shade to the peanut crop and drops up to 400,000 seeds per plant. These facts indicate that Palmer can decimate a peanut field in a short amount of time, and it will cause lasting effects due to the heavy seed output.
Peanut is particularly sensitive to weed competition. In the absence of weeds, peanut can harvest a large amount of light energy and efficiently convert it to yield. But when weeds are present, the peanut enters a defensive mode and becomes more interested in survival than high yield. Even if the weeds are removed, the peanuts never recover and stay in the defensive mode for the rest of the season.
Have A Pre-Season Plan
Many states have labels that allow the use of Gramoxone Inteon in wiper-type applicators. These applicators must wipe the herbicide solution on approximately 50 percent of the plant in order to be effective. In most cases this will not occur until late in the season when the weeds are quite tall. Although it can be important to use the wiper to remove Palmer to reduce seed production, by this time peanut yield has already taken a hard hit.
Controlling Palmer amaranth is a difficult proposition that requires a high level of pre-season planning and dedication. It is essential to use a strong preemergence herbicide program followed by timely postemergence applications. If escapes occur, then the weed wipers should be employed to kill these plants before they make seed.
Just remember, Palmer that is greater than six inches tall is difficult, if not impossible, to stop. Even if we eventually control these large plants, they have already done severe damage. Prevention is the key. PG