What characterizes a residual herbicide?

A residual herbicide is primarily characterized by its ability to provide long-term control of weeds. This type of herbicide remains active in the soil for an extended period after application, allowing it to inhibit the germination and growth of weeds over time. This lasting effect is advantageous in maintaining weed control in various agricultural and landscaping settings, as it reduces the need for frequent reapplication.

The other choices reflect properties of herbicides that do not align with the definition of a residual herbicide. For instance, the notion of breaking down quickly in the soil contradicts the fundamental aspect of residual herbicides that highlights their longevity. Similarly, contact herbicides work primarily on the parts of the plant that they touch and do not have a prolonged effect in the soil, making them different from residual types. Furthermore, the effectiveness of herbicides that work only after plants die relates to a different mode of action than that of residual herbicides, which actively prevent weed establishment during their period of effectiveness.