Greenhouse Zinnia Race

By Eve Pranis, June 23, 2008

"If we want students to become better scientists, problem-solvers, and thinkers, we have to give them opportunities to design investigations to answer intriguing questions," explains teacher Diane Hren from McLean, VA. A school greenhouse provided a context for her students to do just that. With an eye toward engaging students in exploring how different factors affect plant growth, she presented her eighth grade class with a problem and challenge. Student teams of two to four were first charged with using a variety of resources (parents, plant specialists, the Internet, seed packets) to learn about what prompts plant growth, then designing an investigation to test the impact of specific environmental factors on zinnia plants.

Each team of researchers received a packet of zinnias and was challenged to grow the tallest, healthiest plant in seven weeks. "Through discussion, we decided that we would classify a 'healthy' plant as one that had a rich green color, no signs of disease, and no insect pests," explains Diane. To make the competitive challenge a bit spicier and more compelling to students, she offered extra credit "exam" points for each inch of plant growth. Each team had the same size and type of pots, number of seeds, sterile soil, and growing environment (the greenhouse), but other variables were up for grabs.

Students chose to investigate such variables as fertilizer quantity or frequency, watering strategies, drainage, use of extra lights, and so on. "Although students were expected to follow a scientific process, including using repetition and controls, I left the challenge fairly open-ended to allow for some creativity," says Diane. Some groups chose to germinate all of their seeds at once, while others kept some in reserve in case the first batch failed. Several groups decided to play with germination strategies, such as using warming pads, to give their plants a head start. "Students were required to keep track of their process so if something failed, they could devise a new plan," explains Diane. "There was a lot of trial and error as students learned, sometimes the hard way, how science really works. And, they had to solve problems along the way, such as deciding how to rid some plants of aphids."

As the competition progressed, each group was required to regularly measure each plant from the base of the stem to the top of the uppermost growth point, then find an average for each group of plants. As the project wound down, students used computerized spreadsheets to create tables and graphs to display their data. "As each group presented its findings and described the strategies it used, a great informal conversation developed about the scientific process and what affects plant growth," says Diane. One group of boys with 19-inch-tall plants revealed that they had thinned the plants to one per pot and kept increasing the pot size. This sparked a discussion of competition in plant and human life. Some students discovered that too much of a good thing (fertilizer) was harmful, and others found that it's hard to over-water in a sunny greenhouse. "One group of students had no plants left at the end, but they presented their findings in a very scientific way," explains Diane.

What did the kids gain from this experience? "There's no question that students understand more thoroughly when they've had an opportunity to investigate questions firsthand," says Diane. "And a warm, blooming greenhouse makes a wonderful context." The plants, by the way, were all salvaged and taken home or given to teachers.

School Greenhouse Guide
If you currently have a school greenhouse or are considering building or buying one, stay tuned to our Kidsgardening Web site. Our comprehensive School Greenhouse Guide, visit for more information, offers advice on planning for school greenhouses, maintaining healthy conditions, and integrating greenhouse growing projects into classrooms and curricula.

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