Contact herbicides are most effective against which category of weeds?

Contact herbicides are designed to kill only the parts of the plant that they come into contact with. This makes them particularly effective against annual weeds, which complete their life cycle within a year and typically grow quickly. Annual weeds are often in the early stages of growth when these herbicides are applied, allowing for maximum effectiveness as the herbicide can readily penetrate their foliage and disrupt cellular processes.

In contrast to annual weeds, perennial weeds have extensive root systems, which can allow them to recover even if the above-ground portion is damaged by a contact herbicide. Biennial weeds take longer to grow and often do not present suitable targets for contact herbicides until their second year when they are more developed. Invasive weeds may vary in type, including perennials or annuals, but their effective control often requires systemic herbicides that can move through the entire plant system, rather than just targeting the surface.

Thus, the effectiveness of contact herbicides aligns closely with annual weeds, making them the most appropriate answer.