What can happen if preemergence herbicides are watered in too much?

Preemergence herbicides are designed to prevent weed seeds from germinating. When they are applied, they need to remain on the surface of the soil to form a barrier that disrupts the germination process. If these herbicides are watered in too much, excessive moisture can cause them to move deeper into the soil beyond the seed germination zone. As a result, their effectiveness diminishes because they no longer maintain the necessary concentration in the topsoil where weed seeds are present. Thus, the correct answer indicates that overwatering can lead to the herbicides becoming ineffective, meaning they won’t prevent the growth of weeds as intended.

The other options focus on different aspects of herbicide behavior. For instance, a statement about them becoming more potent contradicts the principle of how these chemicals work. Similarly, while herbicides may degrade over time, excessive watering typically doesn't hasten degradation significantly in the context of preemergence applications. Lastly, while preemergence herbicides are designed to prevent weeds, they are not intended to kill existing ones, which would be a different scenario that involves post-emergence herbicides.