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You always answer "plastic" when at the market, and you think that plastic mulch is a lot better for the maintenance of raised beds. Many a raised bed gardening tip advises covering raised beds with plastic mulch, since one roll per raised beds will usually suffice. Plastic mulch is great for raised bed gardening where you've built on top of grass and weeds may penetrate.
On the other hand, if you have a roof raised garden bed, you may still need plastic mulch. Your raised bed gardening technique will benefit most from plastic mulch if you spread the mulch over the soil at the beginning of each planting season. This allows plants to mature faster so you can harvest more efficiently. No matter what type of mulch you use, mulch protects roots and keeps plants cool and moist in the summer.
Just be sure that your plastic doesn't leak, and buy new plastic mulch periodically--posioning crops is the opposite of good gardening technique. If you're thinking of using your billions of plastic bags for cover, maybe it's time to switch to paper.