Growers and farmers use plant hormones, or chemicals that copy them, to control how plants grow. Auxins have two main uses. A synthetic auxin can be sprayed on lawns and cereal fields as a selective weedkiller. It makes broad-leaved weeds grow too fast and die, but it does not harm narrow-leaved plants such as grass and cereals. Rooting powder contains auxin too. Dipping the cut end of a cutting in it stimulates roots to grow, so the cutting becomes a new plant.
Gibberellins are plant hormones with several uses. A gibberellin ends seed dormancy, so it speeds up germination and makes seeds in a batch germinate together. Gibberellins also promote flower formation and fruit growth. Spraying gibberellin on unpollinated flowers makes fruit develop without fertilisation, giving seedless fruit such as seedless grapes.
Ethene is a gas that makes fruit ripen, and so it is used to speed up fruit ripening. Fruit such as bananas is picked while green and hard, which makes it easier to transport and less likely to be damaged. Just before it goes on sale it is exposed to ethene so that all of the fruit ripens at the same time.