What is a key feature of systemic herbicides?

Systemic herbicides are designed to be absorbed by plants and then translocated throughout their tissues, which allows them to target the entire plant system, including the roots, stems, and leaves. This movement through the plant is what makes them effective for controlling a wide range of weeds and unwanted vegetation. When applied, systemic herbicides are taken up by the plant's vascular system, allowing them to interfere with the plant's normal growth processes, ultimately leading to the death of the plant.

The other options do not accurately reflect the characteristics of systemic herbicides. For instance, while some herbicides may require immediate application after certain conditions, systemic herbicides do not have this limitation. Additionally, while they can be effective after a plant has died, their primary action is during the plant's active biological processes. Lastly, systemic herbicides do not only affect the surface of the plant; instead, they penetrate and act throughout the entire plant structure, which is a defining feature that distinguishes them from contact herbicides that only affect the parts they touch.