What are cover crops and do they work everywhere?

The answer

Cover crops are plants grown mainly to protect and feed the soil rather than to be harvested. They often reduce erosion, improve infiltration and add organic material, and legume types can add nitrogen. They do not work equally everywhere: they cost time and money, can compete for water in dry climates and may be hard to fit into short growing seasons. Success depends on choosing species and timing that suit the local season.