Whenever you want your plant to stop growing vertically you can "top" the plant by cutting off the apical meristem, which is the main growing point before manipulation by cutting. This will trigger what is known as apical dominance. Apical dominance is a natural response made by the plant. When the main growing point, apical meristem, is removed the plant triggers other lateral buds to become dominant and that is where the new growth will take place. you can also tie down the plant and watch as the plant bushes out and reaches for the light.