What type of herbicides only affect the parts of plants that are directly treated?

Contact herbicides are designed to affect only the parts of plants that are directly treated, meaning they work on a localized area where they come into contact with plant tissues. These herbicides disrupt cellular processes, leading to damage or death in the treated areas. Because they do not move through the plant's vascular system, their effect is immediate and confined to the contact sites, making them effective for controlling weeds that are directly sprayed.

In contrast, systemic herbicides are absorbed by the plant and translocated throughout its tissues, affecting the entire plant rather than just the sprayed areas. Pre-emergent herbicides prevent weed growth by creating a barrier in the soil that inhibits seed germination, and selective herbicides target specific plants while minimizing damage to desirable species. Each of these types serves a different purpose and mechanism of action compared to contact herbicides.