Auxins were the first class of plant hormones to be discovered. Their main function is to help plants grow and auxin stimulates plant cells to elongate .

Auxins are one specific group of hormones that are used:

• as weed killers.
• as rooting powders.
• for promoting growth in tissue culture.

Selective weedkillers kill some plants, but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or weeds that compete with crops such as wheat. The selective weedkiller contains a growth hormone that causes the weeds to grow too quickly and die. Because most weeds have broader leaves than grass or wheat, the weedkiller is absorbed in larger quantities by the weeds.

Selective weedkillers kill plants that some species of animals rely on as a food source. This can result in a reduction of biodiversity.