Most garden coverups operate on the same basic principle. Radiant (light) energy from the sun can pass through transparent and semi-transparent materials. When the light arrives inside a closed space, it is absorbed by the surfaces within, then radiated again as thermal (heat) energy. The heat energy is less able to pass through the materials, so heat is trapped inside. Anyone who has entered a car parked in a sunny location knows what trapped heat feels like! This energy warms the air, which supports plant growth. But you don't need to reveal this secret to your students. Why not let them grapple with it themselves? Consider the following investigations.

The Heat Within
As a simple but powerful exploration of the greenhouse phenomenon, invite students to place a thermometer inside a clear, covered glass jar in the sun. Place a second thermometer next to the jar. After half an hour, they should compare the two temperatures. What can they infer about light and heat? Encourage students to generate questions based on this experience and then set up simple investigations to try to answer them. For instance, they might try punching air holes in one jar, leaving another sealed, and putting a thermometer in each. Or they could fill three jars with water and wrap one in white paper, one in black paper, and one in aluminum foil before leaving them in the sun, and then predict what will happen to the water temperatures. For each mini-investigation, ask students to try to explain what they think is happening.

Creative Coverings
Once students have explored the relationship between heat and light under different conditions, have small groups use their findings to help them design unique protectors for seedlings or mature plants. (This could mean creating structures that provide shade.) Be sure that students consider plant needs as they create and monitor their setups. For instance, How can we keep the temperature in an acceptable range for plants? How can we ensure that plants get enough water? The class can then compare and critique designs and then predict and monitor plant growth and environmental conditions (moisture and soil and air temperatures, for instance) under each setup. (For comparison, consider also growing some plants in the open air.)

Where in the World?
Many of what we consider to be cool-season crops, such as broccoli, lettuce, peas, and radishes originated in relatively cool regions of Europe and Asia. Crops that prefer hot weather — peanuts, peppers, tomatoes, and squash, for instance — tend to hail from more tropical regions of South and Cental America, Mexico, or Africa. Invite students to select some vegetable garden favorites and find out, via seed packets or catalogs, whether each plant is considered a cool-season or warm-season crop. Then challenge them to discover where in the world each plant originated and learn about the climates in those regions. Do they notice any patterns between where plants naturally occurred and the conditions they prefer in our gardens? Ask, How does knowing a plant's origins help us determine the degree of protection it needs from weather extremes?

Predicting Frost
After reviewing the previous activity, it may come as no surprise that many crops adapted to thrive in cooler weather can withstand some degree of frost, while their tropical garden mates wilt and die at first nip. Students can check with gardening references, area gardeners, or the local Cooperative Extension office to find out the average last spring frost or first fall frost in your area. Regardless of the weather station's prediction on a given day, the microclimate in each garden is unique. Invite your young sleuths to try to use some of the information below to try to predict when a frost might occur. Based on what they've learned about which crops are frost hardy or tender, they can make decisions about what plants to cover and how to do so. (The more tender the plant, the more protection it will require.)

When the sky is clear and there is low humidity, temperatures may drop enough to cause frost. (Without clouds to act as a blanket, the earth loses heat to the atmosphere.) To more accurately predict frost, students can find out the dew point from the weather forecast. When air cools to the saturation point (can no longer hold water vapor), the vapor condenses and forms dew. If the air temperature is below the dew point and below freezing, frost occurs.

For an extra challenge, have students try to find out what about a plant's cell structure or chemistry enables it to resist being damaged by frost.

Tracking Seasonal Sun
Because of the Earth's rotation, the angle of the sun as it hits your garden will change with the seasons. In summer, the sun is high overhead and shines for a longer period of time with a more direct angle. In the spring, fall, and particularly in the winter, the sun is lower in the sky. Garden structures such as cold frames are typically built with angled lids to capture the most sunlight during those three seasons.

You can challenge older students to explore the sun's seasonal movements by determining the average height of the sun at noon in each of the school year seasons. Establish a viewing spot where observers can stand each time. Let the students decide how to record the path of the sun; for example, "it is three index cards above the tree at noon in September and two index cards above the tree in November." If you have a large window on a south-facing side of the school, students can mark the path of the sun for a day using adhesive dots, every two hours or so. This will establish an arched path across the window. Repeat this once each season using red dots for a "fall sun," another color for a "winter sun," and so on, then compare where the paths fall. It is important to establish a viewing spot that is used each time by observers of roughly the same height so the spots remain fairly accurate. Warn students never to look directly at the sun!