Which type of herbicides can kill existing weeds, requiring extreme caution when used near plants?

The correct answer is post-emergence herbicides, which are specifically designed to target and kill existing weeds. These herbicides work by being absorbed through the foliage of the plants, affecting the plant systemically. Because they are effective on actively growing weeds, there is a significant need for caution when applying them near desirable plants, as they can unintentionally cause harm or death to those plants as well.

In contrast, pre-emergence herbicides prevent seed germination, making them less of a risk for existing plants since they act before the weeds grow. Selective herbicides target specific weed species while leaving desired plants unharmed, providing some margin of safety depending on the application. Non-selective herbicides, while they can harm all plants they contact, are primarily used for clearing areas of all vegetation, not just controlling existing weeds, thus not directly implying their use near other plants in the same careful context.

Understanding the distinction between these types of herbicides is crucial for effective weed management while safeguarding cultivated plants.