Palmer amaranth is a weed species that must be thoughtfully and carefully managed; simply attempting to control Palmer amaranth often leads to ineffective herbicide applications, substantial crop yield loss, and increasing weed infestations. Ignored or otherwise not effectively managed, Palmer amaranth can reduce corn and soybean yield to near zero. The threat of Palmer amaranth during the 2014 growing season is very real across a large portion of Illinois.
In January 2014, the weed science program at the University of Illinois developed recommendations for management of Palmer amaranth in agronomic crops. These recommendations were developed in accordance with the somewhat unique growth characteristics of this weed species. The goals of the recommendations are twofold: 1) to reduce the potential for Palmer amaranth to negatively impact crop yield, and 2) to reduce Palmer amaranth seed production that ultimately augments the soil seed bank and perpetuates the species.
Before delineating the specific management recommendations, we present three general principles of Palmer amaranth management:
1) Prevention is preferable to eradication. Prevention refers to utilizing tactics that prevent weed seed introduction and weed seed production. Palmer amaranth is not native to Illinois, so any population discovered in the state originated from seed that somehow was moved into the state. The myriad of ways in which Palmer amaranth seeds can be transported, however, makes preventing seed introduction extremely challenging. Once Palmer amaranth populations become established, utilizing any and all tactics to prevent seed production becomes of paramount importance.
2) 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.
3) Control of Palmer amaranth should not be less than 100 percent; in other words, the threshold for this invasive and extremely competitive species is zero. Female Palmer amaranth plants produce tremendous amounts of seed and in less than five years a few surviving plants can produce enough seed to completely shift the weed spectrum in any particular field.
Species Biology: Germination and emergence of Palmer amaranth
Palmer amaranth seed germination and seedling emergence are best described as continuous. Similar to waterhemp, multiple Palmer amaranth emergence events are possible throughout much of the growing season. 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.
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 not 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 lb ai/acre sulfentrazone or 0.063–0.095 lb ai/acre flumioxazin.
Species Biology: Palmer amaranth growth rate
The growth rate and competitive ability of Palmer amaranth exceed those of other Amaranthus species. Research has demonstrated that Palmer amaranth plants have the highest values for several growth parameters, including plant volume, dry weight and leaf area among the Amaranthus species common to agronomic cropping systems. Perhaps most important with respect to application timing of foliar-applied herbicides, Palmer amaranth demonstrates the fastest rate of height increase. Whereas waterhemp can add one inch of new growth per day under good growing conditions, Palmer amaranth can add two to three inches per day. The effectiveness of most foliar-applied herbicides dramatically decreases when Palmer amaranth plants are taller than four inches.
Do not 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 14–21 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 four inches in height. Reiterating, the effectiveness of most foliar-applied herbicides dramatically decreases when Palmer amaranth plants are taller than four 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 four inches tall.
Species Biology: Palmer amaranth seed production
Palmer amaranth (like waterhemp) is a dioecious species, meaning plants are either male or female. Male plants produce pollen and female plants produce seed, which makes Palmer amaranth an obligate outcrossing species. Outcrossing species tend to have more genetic diversity then self-pollenated species and this can hasten the evolution of herbicide resistance. Transfer of herbicide-resistance traits via pollen can quickly spread these traits across the landscape. Research has demonstrated that female Palmer amaranth plants are capable of producing numbers of seed comparable to that of waterhemp (several hundred thousand to over one million).
Recommendations based on Palmer amaranth seed production:
1) Physically remove any remaining Palmer amaranth plants before the plants reach the reproductive growth stage. Plants should be severed at or below the soil surface and carried out of the field. Severed plants can root at the stem if left on the soil surface, and plants can regenerate from stems severed above the soil surface.
The pdf accompanying this article contains additional information about the management of Palmer amaranth. This publication was made possible through funding by the United Soybean Board and collaboration among several university and private industry partners.