what would be the control?

Question

Purpose:
1) To create a self designed lab
2) To determine whether the sunlight could harm the plant growth
- The higher the amount of Sunlight (dishwasher), the lower the rate of plant growth

Answer 1

Just to add to the previous answer, you might want to have more than one "dishwasher addition" group -- at least two groups, that is "low" and "high" treatment groups. That will give you a better handle on the effects of Sunlight on plant growth. Then, in addition, you would have a third group that you treat in exactly the same way *but* you don't add any Sunlight detergent (= Control group).

Ideally, you would have (for example) 30 plants, and you would randomly assign 10 plants to the Low group, 10 plants to the High group, and 10 plants to the Control group. You need to replicate each group, because of variation you will likely see among plants regardless of the treatment you apply. Then, you would grow them all together, intermingled (but labeled) -- it would be no good to grow all of the Low plants on one bench, the High plants on another bench, etc.

Answer 2

To have a control, you'd need a group of plants that you give some dose of "sunlight" to, and another group that you don't. The group that doesn't get it is your control.