When you choose to do an experiment with plants, you are introducing randomness into your results. Plants, like animals or people, have natural differences between them that you can't control. To limit these differences, buy plants or seeds at the same time from the same supplier. Choose plants that are similar in size and free of any visible signs of disease.
Obtain enough plants for a control group and at least one experimental group. Each group should contain the same number of plants, preferably three or more. During the experiment, treat each plant identically, except for changing the experimental variable.
Design an experiment which tests plant growth under different lighting conditions. You may elect to test groups of plants under fluorescent lights, incandescent lights, specially designed grow lights or no lights at all. Measure the differences in height as well as number of leaves or flowers.
Instead of purchasing different lights, you may also cover your plants in colored plastic to affect the color of light which reaches them and discover which colors are optimal for plant growth. Perforate the transparent bubble with small holes to allow air exchange between the plant and the environment.
Growing plants use their roots to leach nutrients from the environment. Thus, the type of soil in which a plant grows has a major effect on its size and health. Plant experimental groups in different soils, such as local dirt, commercial fertilizers or sand. You may test different concentrations of fertilizer, or compare similar products made by different companies. Ensure that each plant receives an equivalent and sufficient amount of water. Measure quantitative results as well as the qualitative appearance and health of the plants.
For a simple experiment on watering, plant a large number of plants in the same soil. Split your plants into several groups, each of which will receive different amounts of water. Carefully measure the allotted water, and deliver it at the same time each day.
You may also choose an experiment which compares different types of water. Distilled water, tap water and mineral water all contain different concentrations of essential vitamins and nutrients. You may even choose to "water" one experimental group with juice, milk or soda to observe the effects on the plants.