Studying plant nutrition is an effective way for younger grades to start investigating plants. Nutrition is part of what stimulates plant growth, so students can design an experiment that tests whether milk offers more nutritional support for plants than water. Do different types of water make a difference? Try testing tap, bottled, mineral or sparkling water. Determine the effects of different fertilizers on plants. Compare the difference, if any, between commercial fertilizers and homemade compost on plant growth.
Most experiments involving plant hormones are a little more sophisticated and should be researched by middle school or high school students. Determine what role hormones play in the ripening of fruit. Compare the rate of plant growth with two growth hormones while using identical plants and conditions. Does estrogen have an effect on vegetables? Explain the role of phytohormones in plants. Does soil-to-salt ratio have an effect on plant health and hormone production?
Genetics is another subject that is better left to older grades since the experiments may require multiple generations of plants. Students can repeat and verify Gregor Mendel's pea plant experiments. Students can also design an experiment to see if genetics alone determine the color and shape of radishes, or if color can be manipulated by environmental factors.
Explore the inside of a pumpkin and plant the seeds to see how they grow. What do seeds need to grow and what happens when one variable is removed? Does the amount or type of fertilizer affect a plant's growth? Investigate the best way to keep cut flowers fresh the longest. Older grades can test plant growth by adding nitrogen or carbon dioxide to a plant's soil. Would changing the color of the mulch affect growth rate or soil temperature?