Plastic fabric/mulch in raised 4x4 beds????

I grow some veggies ( tomatoes, cukes, eggplant, greenbeans, peppers, etc) in raised 4x4 beds. They can ten to dry out quickly and I am wondering if covering the surface around the plants with plastic fabric would help hold moisture and stop weeds....if I attached the outer edges to the box so the water would no run out. Would it be black (too hot?) or clear (weeds would still grow)?

Any suggestions would be appreciated.

Tammylp: Gardeners Supply has these raised beds they sell and directions for what can be planted in square foot gardening and they also show where mulch covers are used; so, you might want to take a peek at what they are doing. I would think that a mulch cover would cut down on the weeds and hold in moisture too.
joy