Common names: Palmer amaranth, Palmer pigweed, carelessweed, and dioecious amaranth.
Latin name: Amaranthus palmeri.
- “Amaranthus” is derived from the Greek word “amarantus” meaning “everlasting flowers”. This is a reference to the showy bracts (modified floral leaves) of Amaranthus species that persist well after the plants have flowered and set seed.
- “palmeri” is a reference to English botanist Edward Palmer, who collected plants in North America in the late 1800s.
Family: Amaranthaceae – the pigweed family
Life cycle
Palmer amaranth is a summer annual weed. Although it can germinate in soils as cool as 63° F, it germinates best between 86 and 99° F and can germinate all summer long. Later emerging plants may pose less of a risk to crop yield but are still capable of producing viable seeds before a killing fall frost/freeze.
Identification
- Seedlings: Palmer amaranth has two long and narrow seed leaves (“cotyledons”) (Figure 1). The first true leaves are “egg-shaped”- wider at the base and narrower at the tip. The leaf tips of young seedlings are notched (Figure 2). Seedling leaves are hairless.
Figure 1. A Palmer amaranth seedling with linear seed leaves and elliptic-shaped true leaves (Photo by: A. Mueth)
Figure 2. Palmer amaranth leaves with a notch at the tip (Photo by A. Mueth).
- Mature plants can reach 6 feet tall or more (Figure 3) and can grow several inches in a day under ideal growing conditions. The form of a mature Palmer amaranth plant can vary. At lower densities, Palmer amaranth plants will have many branches and grow wider (Figure 4). We have recorded Palmer amaranth plants nearly 5 feet wide! However, at higher densities, plants tend to be narrower and taller. Leaves are alternately attached to the stem, but when viewed from above, they appear whorled, like a poinsettia (Figure 5). Petioles (“leaf stalks”) of Palmer amaranth plants are at least as long as the leaf blade and often longer (Figure 6). Leaf blades of mature plants are egg- or diamond-shaped and may have a whitish chevron-shaped watermark on the upper leaf surface. Mature plants have a thick taproot with an extensive secondary fibrous root system.
Figure 3. The second author stands next to a Palmer amaranth plant in North Carolina, circa 2007 (Photo by K.M. Jennings).
Figure 4. A Palmer amaranth plant growing in a sweet potato field (Photo S.L. Meyers).
Figure 5. Above view of alternate leaves looking like whorled (Photo by A. Mueth).
Figure 6. Palmer amaranth petioles are longer than their leaf blades (Photo by A. Mueth).
- Reproduction: Palmer amaranth plants are dioecious, meaning that male and female flowers appear on separate plants. The entire flowering structure (inflorescence) of female plants is prickly. However, male inflorescences are soft to the touch. Individual flowers are small and inconspicuous and are densely arranged in upright, slender, branched spikes (0.5 to 1.5 ft long) that appear at the ends of shoots (Figures 7 and 8). A single female Palmer amaranth plant can produce hundreds of thousands of glossy, reddish-brown to black, 1 mm seeds.
Figure 7. Individual flowers are arranged in upright, slender, branched terminal spikes (Photo by A. Mueth).
Figure 8. Palmer amaranth can grow to more than 6 ft tall and produce over 100,00 seeds (Photo by A. Mueth).
Integrated weed management strategy
Cultural and mechanical control
Scout and rogue: Although Palmer amaranth is present in Indiana, it is not uniformly distributed. In fields with no history of Palmer amaranth, scouting for them and removing them before they set seeds will be helpful to delay infestation.
Sanitation and exclusion: Remove soil from equipment and implements that have come from a field or location with Palmer amaranth before using them in a field or location with no history of Palmer amaranth. Consider harvesting infested fields last to limit their spread and decrease machinery cleaning time.
Inversion tillage: Because Palmer amaranth seeds are so small, only seeds near the soil surface can germinate and emerge. Plowing several inches deep can invert the soil and bury small weed seeds. Note, however, that yearly plowing will return buried weed seeds to the soil surface. So, it is best to limit this method to no more than once every 5 years or so.
Cultivation: Cultivation can be highly effective but must target small seedlings. Because Palmer amaranth can emerge all growing season, repeated cultivation is necessary if this is your chosen method of control. Ensure that implements such as rolling cultivators are operated in a manner that provides weed control as close to the crop as possible (Figure 9).
Figure 9. A rolling cultivator used between rows of sweet potato did not control emerged Palmer amaranth seedlings in the planted row (Photo by S.L. Meyers).
Crop rotation: Consider rotating to crops that are more competitive or have more registered effective herbicides, including row crops.
Cover crops: Cereal rye planted in the fall and terminated by roller-crimping the following spring can provide suppression of Palmer amaranth but usually not complete control. Fields not currently in production can be planted with cover crops to limit Palmer amaranth’s ability to grow and reproduce. Summer annual cover crops include buckwheat and sorghum-sudangrass. The Midwest Cover Crops Council (MWCCC) maintains a cover crop selection tool that can help you select the best option: MWCCC (midwestcovercrops.org)
Hand-weeding: Hand-weeding escaped Palmer amaranth plants is common. Make sure to hand-weed before the plant makes viable seeds then properly dispose of the weeds by composting or burning them. If kept on the soil, they will re-root along their stem and continue to grow and produce viable seeds.
Herbicides
Burndown: Consider using a burndown herbicide to kill all Palmer amaranth before planting your crop. Since most populations are glyphosate-resistant, consider using a tank mix of glyphosate and a Group 4 (dicamba or 2,4-D) herbicide (be aware of the planting restrictions of burndown herbicides).
Pre-emergence: Use preemergence (AKA soil-applied or residual herbicides) at full rates for your cropping system and soil type as close to planting as the label allows. This will help to reduce the selection pressure of the few post-emergence herbicide options available later in the season. These herbicides must be activated by either overhead irrigation or rainfall to move them into the soil where Palmer amaranth seeds are germinating. Most preemergence herbicides will only provide control for a month or so. When possible, consider “overlapping residuals” by making a second application of a registered pre-emergence herbicide before the first application has worn off. For a list of effective herbicides, visit the Midwest Vegetable Production Guide’s Weed Management Chapter by clicking this link (18-Herbicide-Tables_2023-12-20-141131_wjyh.pdf (mwveguide.org)) or scanning this QR code and looking in the “Pigweed/Amaranth” column:
Midwest Vegetable Production guide QR code
Post-emergence: Most vegetable crops will have few registered, effective, postemergence herbicide options for Palmer amaranth. For those that do, timely postemergence applications are essential. Applications should target Palmer that are less than three inches in height. Late-emerging weeds might not impact yields but could increase the seed bank. For this reason, adding a residual herbicide to the postemergence application can help control later-emerging Palmer amaranth.
Rotate chemistries: Palmer amaranth populations have documented resistance to numerous herbicide modes of action. Learn which herbicides are effective against the population you have, but rotate among effective herbicides to delay the onset of more resistance.
References
Legleiter T and Johnson B, 2013. Palmer Amaranth Biology, Identification, and Management. Available online: Palmer Amaranth Biology, Identification, and Management WS-51 (purdue.edu)
Neal J, Uva RH, DiTomaso JM, DiTomaso A, 2023. Weeds of the Northeast 2nd edition. Cornell University Press, Ithaca, (NY) 14850. p 118. ISBN 9781501755729.