Examine the relationship between plants and light in your project. Set grow lights at various angles the same distance from potted plants to experiment with the light direction. Leave the lights in place for the duration of the project. Find out if grow lights or the sun make a plant grow faster. Watch the plants for as long as possible, preferably for several weeks to monitor the growth of the plants. Find out if plants can grow in complete darkness. Determine if the type of light determines the final height of a plant. Experiment with fluorescent lights, incandescent lights, grow lights and sunlight to see which makes a difference. Finishing a science project that experiments with light will show you how plants react to the type of light, the direction of light or the absence of light.
Determine if the pH of soil makes a difference for growing plants. Use two of the same type of plant in two pots, one with a high pH soil (alkaline) and another with a low pH soil (acidic). Use a soil testing kit to test several samples of garden soil from your yard and select samples of alkaline and acidic soils. You can also buy soils with different pH values from garden centers. Look on the bag of soil for the pH of the mixture before purchasing the soil. Note which plant grows best. Examine the effects of fertilizer on plants by growing one with it and one without. See if earth from your yard or potting soil provide a better growth medium for potted plants. A science project about the soil will show if the soil plants are growing in makes a difference in how well they grow.
Experiment with water amounts or types of liquid for your science project. For instance, water two plants with different amounts of water and record the results. Or, try using different liquids such as water and juice. Look at the water needs for different plants. See if watering a cactus and an ivy plant the same amount of water makes a difference in the plants' growth rates. Examining the water needs of plants for your science project will show you the importance of water to a plant.
Examine the plant structure of different plants. Compare the roots of plants with large taproots like carrots to tubers like potatoes. Compare the seed shape and size to the final structure of the plant. Use different shaped planters to see if they affect the final shape of the plant. See if pruning makes a difference on the final shape of a plant. Compare the plant structure of different plants to see if the structures of plants determine their function. You will note that different plants have different structures. Some plants have large, wide taproots, but others have extensive networks of slender roots. You will find that the structure of plants influences the usage of the plant. For instance, note that the spines and lack of leaves on a cactus adapts it to the desert, but the wide leaves of a tropical plant help it to live in an environment with frequent, heavy rains.