After being taken up, herbicides kill the weed seedling by interfering with photosynthesis, protein synthesis, enzyme systems, cell division, or in other ways. For best performance, shoot-absorbed herbicides like Treflan should be concentrated in the top 0 to 3 inches of soil. This is also true for herbicides that affect germinating weed seed. As the weed shoot passes through the herbicide zone, uptake occurs and the seedling is killed. If the herbicide is mechanically incorporated too deeply or excessive leaching occurs, poor weed control will result.
Dilution occurs if the herbicide is distributed in a larger volume of soil.
As the root emerges from the germinating weed seed, herbicide is absorbed and the weed is killed. Roots of the deeper germinating weed seed then encounter more herbicide. Herbicide Solubility Figure 1. Effectiveness of herbicide based on location in the soil. Relatively insoluble herbicides require more rainfall for activation.