The control is the plant that receives only pure water. It is referred to as the control, not control variable.

The dependent variable is the height of each of the sunflower plants after the two week period.

The independent variable, the one you are controlling, is the amount of salt water that each plant is receiving.

You are trying to determine how the height of the plant depends on the amount of salt water it received.

In general, the independent variable is the one you are changing. The dependent variable change in some manner as a result of the changes you made in the independent variable.

The sunflower plants are one of a number of constants in the experiment. You would want each plant to receive the same amount of light, using the same seeds, the same soil mixture, etc.