Thanks for writing. Covering your soil with black plastic will cause the soil to heat up. This can be a good or bad thing, depending on what you're growing and what your temperatures are like. (It is possible to heat up the soil too much for many plants and kill their roots.) Also: One problem with plastic is that it doesn't allow moisture or air to penetrate the soil as well --- so your plants may not do as well as they would without the plastic.
---Justin, Senior Garden Editor, BHG.com "