Herbicides kill plants. Farmers spray them to clear weeds that compete with crops. Homeowners use them on driveways and lawns. Utilities apply them under power lines. The chemicals target specific plant species or broad groups, depending on the formula.

They work in different ways. Some disrupt photosynthesis. Others mimic plant hormones, causing runaway growth that kills the plant. Glyphosate, the active ingredient in Roundup, blocks an enzyme that plants need but animals do not, which is why it is relatively low in toxicity to humans. That does not make it harmless. Runoff can harm aquatic plants and disrupt ecosystems downstream.

Resistance is a growing problem. Weeds exposed to the same herbicide year after year evolve tolerance. Over 500 weed species have developed resistance to at least one herbicide. Farmers respond by rotating chemicals, planting cover crops, and using mechanical cultivation. No single solution works forever. The arms race between weeds and herbicides is decades old and shows no sign of ending.

Common types

• Glyphosate
• Atrazine
• 2,4-D
• Dicamba
• Paraquat