What would you try to do next?
Since a plant in the sunlight was the same height as one in the shade right next to it, I could conclude that the amount of sunlight a plant is exposed to does not affect its height. This conclusion would lead me to consider other possible explanations for the difference in plant growth by thinking what other variables could potentially affect plant growth, observing the yard, and determining where those variables could lie. Since plant height varied in different areas of the yard, it's possible that the soil composition is different in those areas and is affecting plant growth. I would design an experiment using that as the independent variable.