What is the first step an herbicide must take to effectively kill weeds?

For an herbicide to effectively kill weeds, it must first be absorbed into the plant. This initial step is crucial because absorption allows the active ingredients in the herbicide to enter the plant's system, where they can then disrupt various physiological processes, leading to the plant's demise.

Once absorbed, the herbicide can move throughout the plant, targeting specific functions or pathways necessary for growth and survival. Without this absorption, the herbicide would remain on the surface of the plant or in the environment without achieving its intended effect.

The other steps, such as blocking photosynthesis or translocating to the roots, are secondary processes that rely on successful absorption first. Application during a rainstorm may influence absorption and effectiveness but is not a fundamental prerequisite for herbicide action. Therefore, the first key step is indeed absorption into the plant.