Keera decided to investigate how the growth of plants is affected by the time of day it is watered. She placed an equal amount of potting soil in three identical pots and planted three bean plants in each pot. The pots were placed on the same table so that they received the same amount of light. One pot was watered at sunrise, one at noon, and the last pot at sunset. After ten days, the height of each individual plant was measured and recorded. What is the independent variable in this investigation?