An herbicide is a poison that kills plants. A weedkiller.
Many plants produce their own herbicides, so they can prevent other plants from using up their water, sunlight, or nutrients. The other plants can’t grow underneath plants whose roots produce poisonous chemicals, or whose leaves fall down and leak poisons into the soil.
The leaves of bay laurel trees produce natural herbicides that prevent other plants from growing up next to them. The flavor of bay leaves is in part due to these toxic (to plants) molecules. Walnut trees also produce natural herbicides.
We make our own herbicides to control weeds. Some commercial herbicides kill almost any plant. Others are designed to only kill broad leafed plants, and not affect the grasses we use for lawns. Some of those work by acting like growth hormones that are absorbed by the leaves. Plants with broad leaves absorb so much they grow faster than they can get nutrients, and so they die. Grasses have narrower leaves, and don’t absorb as much, and only grow a little more than they normally would.
Herbicides like Roundup use a chemical called glyphosate. Glyphosate prevents a plant enzyme from producing three amino acids that are critical to the plant. Since most plants need to produce these amino acids, glyphosate is a wide-spectrum herbicide that kills most plants it is used on.
Selective herbicides are often synthetic molecules that mimic plant hormones. One selective herbicide is 2,4-D. It is inexpensive, and kills broadleaf plants while leaving grasses mostly unaffected.
2,4-D mimics the plant hormone auxin. It is absorbed through the leaves of the plant, and migrates to the fast growing parts of the plant, such as the tips of shoots. It stimulates the production in the plant of another hormone, ethylene, which can cause the leaves to fall off the plant if the dose is high enough.
2,4-Dichlorophenoxyacetic acid (2,4-D)