One science project about plants for a third grade student is to test whether a cut flower lasts longer in warm or cold water by adding food coloring and tracking the amount of color the flowers display over time. Another idea is to test the affect of various chemical compounds, such as motor oil, orange juice or baking soda, on the growth of a plant.
One plant-based project studies the affect that light has on plant growth. This project uses at least two of the same kind of plant or sprout. Students subject the plants to various colors or degrees of light exposure, such as placing a plant in direct sunlight, then comparing it to one inside and away from sunlight, or setting a plant under a colored lamp or artificial light source and tracking its growth.
Third graders can check whether caffeine or various vitamins affect plant growth or determine how the saltiness of water affects a plant's development. Another project determines whether the size of a fruit affects the amount of seeds inside of the fruit, while a similar project tests whether the seed size affects the height or size of a plant. Students can also test whether fertilizers promote plant growth or extend the life a dehydrated plant.