Real Dictionary

mulch
variable noun: A mulch is a layer of something such as old leaves, small pieces of wood, or manure which you put on the soil round plants in order to protect them and help them to grow.
verb: To mulch plants means to put a mulch round them to protect them and help them to grow.

mulch in American English
leaves, straw, peat moss, etc., spread on the ground around plants to prevent evaporation of water from soil, freezing of roots, etc.
to apply mulch to
noun: a covering, as of straw, compost, or plastic sheeting, spread on the ground around plants to prevent excessive evaporation or erosion, enrich the soil, inhibit weed growth, etc
transitive verb: to cover with mulch

mulch in British English
noun: half-rotten vegetable matter, peat, etc, used to prevent soil erosion or enrich the soil
verb: to cover (the surface of land) with mulch