What is a characteristic of post-emergent herbicides?

Post-emergent herbicides are specifically designed to target and control weeds that have already emerged from the soil. This characteristic allows them to be effective in managing established weed populations, which can be crucial for maintaining desirable crop growth and health in agricultural practices. By focusing on weeds that are currently visible and growing, post-emergent herbicides can provide immediate control over competing plants, reducing their impact on crops.

The other options refer to characteristics not associated with post-emergent herbicides. For instance, applying herbicides before plants emerge pertains to pre-emergent herbicides, while high moisture may be relevant to certain herbicide applications but is not a defining characteristic of post-emergents. Additionally, the term "non-selective" does not accurately describe all post-emergent herbicides, as some are specifically formulated to target only certain types of weeds while sparing others, particularly desirable plants in the area.