Final answer:
The factor affecting the tomato plants' growth is the independent variable, which is the amount of light received by the plants. The control variable is important to ensure that any changes in the dependent variable are solely due to the independent variable.
Step-by-step explanation:
The factor in your observations that affected the tomato plants' growth is the independent variable. In this case, the independent variable is the amount of light received by the plants, with the north side receiving more light due to the house's shadow on the west side. The dependent variable is the plant growth, which is affected by the independent variable. The control variable is also important, as it is the variable that is kept constant to ensure that any changes in the dependent variable are solely due to the independent variable. In this experiment, the control variable could be factors such as the amount of water, plant food, soil, and type of soil.