Mulch used in the vegetable garden can be beneficial in several ways as long as a few basic rules are observed.
Mulches preserve moisture. This is especially important in our area of low humidity and rapid evaporation, quickly drained and rocky soil and high water costs. Deep mulch – at least four inches – will discourage weeds from sprouting between rows.
Normally, any organic material may be used. However, because of our cool days and cold nights, the selection of mulch is more critical. A very light colored material, such as straw or newspaper will reflect sunlight sway form the soil and may actually prevent the soil from warming. Cool weather plants such as peas may not mind having cool, moist roots but warm weather crops may grow more slowly under the circumstance. If you use a light-colored mulch, be sure the soil has warmed thoroughly before you apply it to the garden.
Source: UI Extension