Palmer amaranth is becoming an increasingly common and troublesome weed in the Southeast. With its rapid growth rate and enormous capacity to produce seed, this weed can go from limited to a severe infestation in one or two seasons.

Several herbicides have activity on Palmer amaranth, but season-long control can still be tricky. The millions of seeds produced will germinate throughout the entire growing season. It has been observed that, “a new flush of seedlings will come after every rainfall.”

Effectiveness Of Pre-Emergence Herbicides
Generally speaking, Cadre is highly effective, easy to apply and controls small and large plants alike. However, every year brings new reports of Palmer amaranth resistance to Cadre. With the loss of Cadre, we will be forced to rely on pre-emergence herbicides, such as Valor and Dual Magnum, or early postemergence contact herbicides, such as Cobra or Ultra Blazer. Though Cobra and Ultra Blazer are effective options, it must be applied to Palmer amaranth that is approximately two inches in height for reliable control. Considering that these herbicides have no residual activity and must be applied within a narrow window of effectiveness, it is imperative that we document which pre-emergence herbicides are most effective. Additionally, we also need to understand approximately how long each of the pre-emergence herbicides will likely last to better plan post-emergence applications.

The herbicides listed below (Table 1) were all applied the day after peanut planting. After the applications were made, we visited the treated areas weekly and counted how many weeds were present in each plot. We considered that each herbicide had lost its effectiveness when the Palmer amaranth population reached one plant per three feet of peanut row. This threshold was set because that is when we considered that a post-emergence herbicide application would be necessary.

Control Can Vary Greatly
The results showed that the effectiveness of different pre-emergence herbicides on Palmer amaranth control can vary greatly. Prowl H2O and Solicam were the least effective with control ranging between just a few days and approximately one week. Dual Magnum was better with three to four weeks of control, but Valor provided control for up to two months.

We also found that the rye cover crop at this
location did little to suppress Palmer amaranth
growth. However, the rye at this location was
relatively thin. Other research has shown that
dense rye cover can greatly enhance weed control.

Prowl H2O is highly effective on annual grasses,
Florida pusley and many of the pigweed species.
However, we have observed over the past several
years that it is largely ineffective against Palmer
amaranth. Therefore, if Prowl H2O is the only pre-
emergence herbicide in the program, plan to treat for
Palmer escapes within the first week after planting.
Conversely, Valor will generally provide excellent pre-
emergence control, while also being somewhat less
expensive to apply than Dual Magnum.

For all pre-emergence herbicides, it is important to remember that rainfall or irrigation within seven to 10 days after application is essential to activate the herbicide. We had excellent incorporation at this location, but prolonged drought will likely result in each of these products failing to control Palmer amaranth.

PG

Research funded by the Southern Peanut Research Initiative.

Factors To Improve Pre-Emergence Herbicide Applications:

Incorporation. Pre-emergence herbicides require incorporation into the soil to become activated. Incorporation can be conducted with light tillage, rainfall or irrigation.

Timing. Herbicides that are not incorporated will either turn to a gas and dissipate or be degraded by sunlight. The “yellow herbicides,” like Sonalan, require incorporation as soon as possible, while products like Dual Magnum can persist for five to 10 days.

Depth. Herbicides should be incorporated as shallow as possible. Most weeds, particularly grasses and pigweeds, germinate within the top one-fourth of an inch of soil. Therefore, it is important to concentrate the herbicide in that shallow zone to maximize weed control.

Equipment. If mechanically incorporating, it is best to use a field cultivator. This ensures good mixing and generally will not move the herbicide too deep into the soil. Conversely, a disc harrow commonly slices the herbicide deep into the soil in a streaked pattern, which distributes the herbicide too deep into the soil and fails to achieve good mixing.

Rain or Irrigation. If rainfall is not predicted, irrigating with one-half inch of water will consistently and effectively incorporate the herbicide.