Palmer amaranth has a tendency to emerge throughout the summer; and it grows very rapidly during the heat of June and July when many other species are suffering from the temperatures. Thus, it is very important to include effective herbicides that provide residual weed control. It is also important to apply these products at a time when they will provide the best weed control for the crop.
Most residual herbicides for corn and soybeans will provide about 4 weeks of effective control, so the longer before planting they are applied, the sooner after planting they will start to break down. Therefore, if a product provides 4 weeks of control and it is applied 2 weeks before planting, the Palmer amaranth is likely to start emerging 2 weeks after planting. However, if that same product is applied at planting, it should provide control until 4 weeks after planting.