Palmer Amaranth: A growing concern in S.D.
BROOKINGS, S.D. – Although Palmer Amaranth can be found in South Dakota, it is currently not wide spread. SDSU Extension staff say it may be possible to limit the scope and economic burden this weed could cause if South Dakota producers and agronomists are vigilant throughout harvest and into 2019 growing season.
“If producers come across a patch of Palmer Amaranth in their fields during harvest, they should consider bypassing those areas,” said Ruth Beck, SDSU Extension Agronomy Field Specialist.
Beck explained that combining mature Palmer Amaranth or other pigweeds is not recommended. “However, if combine harvest cannot be avoided, leaving those areas till last will help keep from moving these weeds to other fields or spreading them further in the field where they are found,” Beck said.
Before leaving the infested area, harvest equipment should be thoroughly cleaned.
And, to improve control measures in 2019, Beck encourages producers to make note of infested areas.
“Those areas will need to be treated with a strong preplant/preemerge herbicide program and a follow-up post emerge application next year,” said Paul O. Johnson, SDSU Extension Weed Science Coordinator.
Using herbicides with long residuals and multiple modes of action is recommended.
Field management practices to control Palmer Amaranth
If Palmer Amaranth is found in a field this fall, Beck outlined some best management practices to help control the weed.
No-till: Managing infested areas of the field with no-till will keep seeds on the surface where they are exposed to weather extremes and predation from birds and insects. Whereas, shallow tillage favors germination.
Crop rotation: Crop rotations that include wheat, corn and milo will give producers more control options.
Zero tolerance: It is recommended that producers employ a zero tolerance approach to controlling this weed.
This may include hand removal of escapees. “Keeping the field clean until the crop canopies will help with control of this weed,” Beck said. “Emergence will drop significantly after crop canopy.”
Palmer Amaranth ID tips
Palmer Amaranth was introduced to South Dakota through seed, equipment, feed and manure.
It is a versatile weed whose male and female parts exist on different plants, so, it needs to cross pollinate to produce seed, explained Johnson.
“The process of cross pollination increases the plants genetic diversity and its ability to develop plant types that are resistant to herbicides,” Johnson said.
A prolific seed producer, Palmer Amaranth has the ability to germinate for an extended period during the summer, grow quickly and compete aggressively with crops for nutrients, light and moisture.
It is difficult to identify Palmer Amaranth from other pigweed species when the plants are small.
However, Palmer does have some unique features and these will become more obvious as the plant ages, said Gared Shaffer, SDSU Extension Weeds Field Specialist.
Stem: The stem of Palmer is smooth, where other pigweeds, such as redroot pigweed has hairs.
Leaves: Although waterhemp also has a smooth stem, its leaves usually have a narrow lanceolate shape versus Palmer Amaranth which has leaves that are more diamond shaped or broader across.
Petiole: Another feature unique to Palmer Amaranth is a long petiole. This is indicated in Figure 1.
Spiny bract: Palmer amaranth can have a spiny bract where the petiole attaches to the main stem. This spiny bract is not common in redroot pigweed or in waterhemp. “Once Palmer Amaranth develops a seed head it becomes easier to distinguish, as the terminal seed head is usually very long (Figure 2),” Shaffer said.