South Dakota: Palmer Amaranth – A Growing Concern

Although Palmer Amaranth can be found in South Dakota, it is currently not wide spread. If South Dakota producers and agronomists are vigilant, it may be possible to limit the scope and economic burden this weed could cause.

Palmer Amaranth is a versatile weed whose male and female parts exist on different plants. This means it needs to cross pollinate to produce seed. The process of cross pollination increases the plants genetic diversity and its ability to develop plant types that are resistant to herbicides.

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.

First the stem of Palmer is smooth, where other pigweeds, such as redroot pigweed has hairs. Waterhemp also has a smooth stem, but the leaves on waterhemp usually have a narrow lanceolate shape vs Palmer Amaranth which has leaves that are more diamond shaped or broader across.

Another feature that is unique to Palmer Amaranth is a long petiole. This is indicated in Figure 1 below.

Figure 1. The leaf petiole on Palmer Amaranth is usually longer than the leaf blade.

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
Figure 2. Palmer Amaranth can have a very long terminal seed head.

If producers come across a patch of Palmer Amaranth in their fields during harvest, they should consider bypassing those areas. Combining mature pigweeds is not recommended.

If combine harvest cannot be avoided, leaving those areas till last will help keep from moving those pigweeds to other fields or spreading them further in the field where they are found. Harvest equipment should be thoroughly cleaned before leaving those areas.

Managing areas with no till will keep seeds on the surface where they are exposed to weather extremes and predation from birds and insects. Shallow tillage favors germination.

Producers can make a note of infested areas this fall. Those areas will need to be treated with a strong preplant/preemerge program and a follow-up post emerge application next year. Using herbicides with long residuals and multiple modes of action is recommended.

Crop rotations that include wheat, corn and milo will give producers more control options. It is recommended that producers employ a zero tolerance approach to controlling this weed. This may include hand removal of escapes. Keeping the field clean until the crop canopies will help with control of this weed. Emergence will drop significantly after crop canopy.

This weed has been introduced to South Dakota through seed, equipment, feed and manure.

Producers will need to be vigilant and undertake aggressive control measures to ensure this weed does not become widespread in South Dakota.

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