Peas are planted as early as ground can be worked in late February or early March in the mid-Atlantic states. Planting continues into late April to stagger the harvest. This is especially important for peas grown for processing. Peas planted later, in April, germinate and emerge quickly in the relatively warm soil, but early planted peas emerge more slowly. This exposes the new root and shoot emerging from the seed to the herbicide in the soil for a longer period of time. In addition, the new seedling’s ability to detoxify a herbicide is temperature dependent, working more slowly in cold weather. These conditions make the crop more susceptible to herbicide injury during cold weather that can reduce the stand or delay harvest.
Treflan is not recommended in the mid-Atlantic states due to the likelihood of delayed maturity of crop when the herbicide is preplant incorporated two to three inches deep prior to planting, placing the Treflan under the seed. Treflan’s site of action is the emerging root where it inhibits growth and encourages root branching resulting in a bottle brush appearance. Treflan has a very low solubility in water, so it is not moved down into the soil by rainfall. Placement of the herbicide above the seed, and therefore above the root, would reduce the risk of injury, but Treflan is broken down by sunlight, so applications left on the soil surface provide poor weed control.
Dual Magnum is absorbed by the shoot as it emerges through the soil. Early planted peas often germinate more slowly, increasing the likelihood that rainfall will move the herbicide into the soil prior to crop emergence. The pea shoot, emerging more slowly through the cold weather, is exposed to the herbicide for a longer time, increasing the risk of injury and delayed harvest. Due to the risk of injury, Dual Magnum is only recommended for use on later planted peas when the weather is more favorable for rapid emergence.
Command 3ME applied preemergence is also more likely to cause injury in early planted peas. Like with Dual Magnum, there is increased risk of exposure to the herbicide in early plantings when the weather is cold. In addition, Command injury can be intensified by cold weather due to the way it works. Command inhibits the plants ability to synthesize compounds that the plant needs to protect the chlorophyll in the leaves from damage by sunlight, like suntan lotion! When sunlight damages the large and complicated chlorophyll molecule in the leaf it no longer appears green. Instead the leaf looks white and bleached out. Crops where Command can safely be used can detoxify the herbicide, but the detoxification of Command in the plant is an enzyme reaction that is temperature dependent, working more slowly when it is cold. The injury to the chlorophyll is a light reaction and relatively temperature independent, so crop injury is most likely to be observed when planting is followed by rainfall to make the herbicide immediately available to the germinating seed followed by bright intense sunlight and cold temperatures.
Pursuit is not frequently used in peas due to the extremely long plantback restrictions that prevent rotating to other vegetables for several years after use. When Pursuit is used on early planted peas, use the lowest recommend rate.
There are ways to reduce the risk of herbicide injury in early planted peas.
First, use the lowest recommended rate for the texture and organic matter content of the soil.
Second, incorporate preplant herbicides shallowly and thoroughly to eliminate “hot spots”, and try to not incorporate the herbicide as deep or deeper than the seed will will planted. This is especially important if Treflan is used. Consider seeding the crop, then incorporating the herbicide very shallowly with a rotary hoe or rolling basket cultivators. Incorporation only a quarter of an inch deep or even less will protect a susceptible herbicide from breakdown by sunlight, and the herbicide will have good activity against small seeded annual grasses and annual broadleaf weeds. The much larger seeded and deeper planted peas will be less exposed to the herbicide.
Finally, consider delaying your herbicide application for several days after planting, even until you see the nub of a root protruding from the seed. We have all been made aware of the importance of planting into freshly worked soil, applying preemergence herbicides immediately after planting, and activating those herbicides with overhead irrigation if rainfall does not occur within a day or two when growing conditions are favorable. This ensures that the herbicide will be moved into the soil and have moisture available to enhance uptake by germinating weeds, but weeds germinate more slowly when it is cold too, or they wait patiently for optimum growing temperatures before germinating.
Delaying preemergence herbicide applications a few days in late February or early March will not reduce weed control and can improve crop safety! Check the field daily. Peas can emerge quickly in response to a warm sunny day!