The hypothesis is a statement that requires confirmation. Your hypothesis is incorrect, it should be something like "Plants grow the best when they are watered with one cup of water a week," or "One cup of water is the minimum requirement of water for a plant."
The control group is the group of plants that gets exactly one cup of water every week. The health of this group is compared against other groups of plants that are given different amounts of water every week. Some may be given more water and others less.
If water is the independent variable you have to make sure that only the amount of water is changed for the different groups of plants. No other variable like the soil they are being grown in, the amount of sunlight they are exposed to, the temperature they are grown at, etc. can be changed.