during an experiment you are required to maintain a separate group of subjects to collect data on so you will be able to make comparisons from your observations. assuming the watered plants grew, what does that mean? they grew at a quicker rate? slower rate? the same rate? compared to what? you need this control group in order to prove your observations either one way or the other such as "compared to unwatered plants, the watered plants grew at *blank* rate."
