What is necessary for pre-emergence herbicides to be effective?

Pre-emergence herbicides are designed to prevent weed seeds from germinating, and for them to be effective, they need to be activated in the soil. This activation commonly requires moisture, which can come from irrigation or rainfall. When the herbicide is applied, it forms a chemical barrier in the upper layers of the soil. If it remains dry, the herbicide will not dissolve and will not provide the necessary control over germinating weeds, leading to ineffective weed suppression.

In contrast, applying herbicides during the winter, in dry conditions, or requiring direct sunlight does not align with the mechanisms by which pre-emergence herbicides function. Therefore, ensuring that the herbicide is watered in or encounters sufficient rainfall helps to initiate its activity and establish the protective barrier against emerging weeds.