I consider mulch to be an indispensable part of the summer vegetable garden.
- reduces evaporation of water from the soil
- prevents weeds (many weeds need bare soil)
- supplies nutrients to the soil (when it breaks down)
- protects the soil from damage by sun and air
- reduces disease (by preventing soil splashing on leaves)
If a genetically engineered commercial product did as much, it would be patented, hailed as a miracle of science, promoted in all of the garden magazines and sold for a hefty price. Yet mulch does all of these things and more and costs next to nothing (or nothing).
The best all around material for mulching the annual vegetable garden is straw (avoid the similar looking hay at all costs, as it is full of weed seeds and can turn your soil into a weedy nightmare). Straw comes neatly compacted into a convenient (if rather heavy) bale, is clean and easy to use and looks quite attractive on the bed. If you are growing transplants, you spread a 2 to 3 inch layer around the newly planted plants (it will settle as you water). If you are direct sowing you have to wait until the seeds have germinated and the plants are a few inches high before you can apply it.
If you don’t have access to straw, you can use grass clippings or chopped tree leaves (run these over with a lawnmower to chop them, or put them in a metal bin with a string trimmer). Compost can be used as mulch too, if you have enough of it. You can also buy the hulls of cocoa bean, cottonseed and buckwheat for use as mulch, but they are expensive when compared to straw.
The only time I don’t recommend mulch if you have lots of slugs (it provides the perfect hiding places for them) or when you want the soil to warm up rapidly (it insulates it from the warming rays of the sun). Otherwise, it’s probably the most important thing you can do for your garden.