Management of Palmer amaranth in Illinois
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.
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