What characterizes a contact herbicide?

A contact herbicide is characterized by its mode of action, which involves killing plants upon direct contact with the plant tissue. This means that when the herbicide is applied, it must come into physical contact with the leaves, stems, or other parts of the plant for it to be effective. Unlike systemic herbicides, which are absorbed and translocated throughout the plant, contact herbicides do not move within the plant after application. This property makes them suitable for targeting visible portions of plants, and they often work quickly.

The other options describe characteristics that do not align with the definition of contact herbicides. For instance, absorption by plant roots pertains to systemic herbicides, which are taken up by the root system and distributed throughout the plant. Working only in saturated soil could misrepresent the ideal application conditions for certain herbicides but does not define a contact herbicide. Additionally, saying that it affects only aquatic plants is incorrect, as contact herbicides can be used on a variety of environments, including terrestrial settings, and are not limited to aquatic applications.