Which characteristic of contact herbicides differentiates them from systemic herbicides?

The distinguishing characteristic of contact herbicides is that they only affect the parts of the plant they come into direct contact with. This is primarily because contact herbicides work by causing damage to the plant tissues that they touch, leading to cell destruction and plant death. However, these herbicides do not translocate through the plant's vascular system, meaning they cannot affect areas of the plant that do not come into direct contact with the herbicide. This limited action contrasts sharply with systemic herbicides, which are absorbed by the plant and move throughout its tissues, allowing them to target not just the areas touched but also the roots and other parts of the plant indirectly.

Understanding this key difference is essential for effective application and choosing the right type of herbicide based on the specific control needs in aquatic environments.