Palmer amaranth may have originated as a desert plant, but the aggressive weed is now making itself at home farther north. Illinois, Iowa, Missouri and even Michigan have reported nasty outbreaks over the past few years.
Weed specialists are urging farmers to be on the lookout this spring for the weed seedling. A prolific seed producer, the weed also spreads by seed movement. So controlling the weed before it sets seed is of primary importance.
“The thing that is going to hurt us on controlling Palmer is that farmers don’t know what it is and often won’t recognize it until it is too late,” said University of Illinois weed scientist Aaron Hager. “Trust me, we are not ready for Palmer in this country.”
In a news release last week, Hager said it is not uncommon for annual herbicide costs to at least double once Palmer amaranth becomes established. “There are simply no soil- or foliar-applied herbicides that will provide sufficient control of Palmer amaranth throughout the entire growing season. At least three to five herbicide applications per growing season are common in areas where Palmer amaranth is well established,” he said.
Not even waterhemp, a weed that has become increasingly resistant to herbicides in the Midwest, can stand up to its weedy cousin. “Similar to waterhemp, multiple Palmer amaranth emergence events are possible throughout much of the growing season,” Hager said. “However, previous research has demonstrated that Palmer amaranth seed has a higher germination rate than most other Amaranthus species (including waterhemp), and demonstrates a germination percentage higher than waterhemp at both low and high temperatures.
“These germination and emergence characteristics help explain why Palmer amaranth can seemingly ‘displace’ waterhemp from a field within only a few years after Palmer’s introduction. Palmer amaranth that emerges before waterhemp in the spring and later in the growing season after waterhemp emergence has stopped, gives the species a competitive advantage over waterhemp and most other weed species,” Hager said.
Recommendations based on Palmer amaranth germination and emergence characteristics:
1) Be certain to control all emerged Palmer amaranth plants before planting corn or soybean. Burndown herbicides or thorough tillage are effective tactics to control emerged Palmer amaranth plants before planting. Keep in mind, however, that glyphosate will not control glyphosate-resistant Palmer amaranth and growth regulator herbicides (such as 2,4-D or dicamba) are most effective on Palmer amaranth plants less than 4 inches tall. If preplant scouting (which is especially important prior to planting soybean) reveals Palmer amaranth plants taller than 4 inches, consider using tillage instead of herbicides to control the plants.
2) Apply a full rate (based on label recommendations for soil texture and organic matter content) of an effective soil-residual herbicide no sooner than seven days prior to planting, nor more than three days after planting. Many soil-residual herbicides that are effective for controlling waterhemp are also effective for controlling Palmer amaranth.
Soil-applied herbicide families that demonstrate control or suppression of Palmer amaranth include the triazines (atrazine, simazine, metribuzin), dinitroanilines (trifluralin, pendimethalin), chloroacetamides (metolachlor, acetochlor, dimethenamid, etc.), and protox inhibitors (flumioxazin, sulfentrazone, saflufenacil). Do not apply less than the rate recommended by the product label. In soybeans, products containing sulfentrazone (Authority) or flumioxazin (Valor) have provided effective control of Palmer amaranth. Application rates of products containing these active ingredients should provide a minimum of 0.25 pound ai/acre sulfentrazone or 0.063-0.095 pound ai/acre flumioxazin.
SPECIES BIOLOGY: PALMER AMARANTH GROWTH RATE
Perhaps most important with respect to application timing of foliar-applied herbicides, is that Palmer amaranth demonstrates the fastest rate of height increase. Keep in mind that while waterhemp can add 1 inch of new growth per day under good growing conditions, Palmer amaranth can add 2 to 3 inches per day. The effectiveness of most foliar-applied herbicides dramatically decreases when Palmer amaranth plants are taller than 4 inches.
Again, Hager reminds growers not to rely solely on glyphosate to control Palmer amaranth. Molecular assays have indicated resistance to glyphosate appears to be relatively common among recently identified Palmer amaranth populations in Illinois.
Recommendations based on Palmer amaranth growth rate:
1) Begin scouting fields within 14 to 21 days after crop emergence. We recommend this interval even for fields previously treated with a soil-residual herbicide applied close to planting.
2) Foliar-applied herbicides must be applied before Palmer amaranth plants exceed 4 inches in height. Reiterating, the effectiveness of most foliar-applied herbicides dramatically decreases when Palmer amaranth plants are taller than 4 inches. Postemergence herbicides that demonstrate control or suppression of Palmer amaranth include synthetic auxin herbicides (dicamba, 2,4-D), diphenylethers (acifluorfen, lactofen, fomesafen), glufosinate, glyphosate, and HPPD inhibitors (mesotrione, tembotrione, topramezone).
Palmer amaranth can germinate and emerge over an extended period of time, so there is often a wide range of plant sizes by the time postemergence herbicides are applied. This can present problems with spray interception by smaller plants under the protective canopy of larger plants. Adjustments in spray volume and pressure can help to overcome some of the challenges with coverage.
3) Consider including a soil-residual herbicide during the application of the foliar-applied herbicide. A soil-residual herbicide applied with the foliar-applied herbicide can help control additional Palmer amaranth emergence and allow the crop to gain a competitive advantage over later-emerging weeds.
4) Fields should be scouted 7-14 days after application of the foliar-applied herbicide to determine:
- Herbicide effectiveness.
- If the soil-residual herbicide included with the POST application is providing effective control.
- If additional Palmer amaranth plants have emerged.
If scouting reveals additional Palmer amaranth plants have emerged, make a second application of a foliar-applied herbicide before Palmer amaranth plants are 4 inches tall.
