Mulch

 The benefits of Bark Mulch: 

Maintaining a minimum 2 - 3" depth of mulch in all planting beds results in water conservation, plant health, deters weeds, conditions soil and provides aesthetic beauty.

Here is a simple mulch calculator:
cubic yards (27 sq.ft) 
1" depth - 1 cubic yard covers 330 sq. ft.
2" depth - 1 cubic yard covers 155 sq. ft
3" depth - 1 cubic yard covers 105 sq. ft