Helen Atthowe of http://www.veganicpermaculture.com talks about using winter annuals as a lving mulch. In this case, field penny cress – Thlaspi arvense.
It grows in the late fall or early spring and then, conveniently dies for the summer growing season, thus leaving behind a light mulch that makes it difficult for weeds to get a start.
This is a case of choosing your weeds. If you choose to have no weeds, then you will get weeds that you don’t want. The penny cress is an extremely polite weed: it does not compete for water or nutrients throughout the typical growing season, and it helps to retard other weeds.