Weatherwise Gardeners

By Eve Pranis

Weatherwise School Gardens

"My seventh graders had created a blueprint for their dream school garden, but realized they'd have to revise their plans once they thought about the amount of sun required by their proposed plants," reports Lostant, IL, teacher Jean Smith.

Jean's students had learned enough about climate to know that the sun would shift positions throughout the year, and realized they hadn't factored that into their initial plan. In March, the students investigated where the school building's shadow hit the garden site, then used what they knew of the sun's movement to predict where the shade would fall in the summer. Each week, the students drew an additional line on their original blueprint, showing how the building shadow was progressing. "We used our predictions of where the shade would hit to rethink which plants we put where in the garden, based on how much sun each required," explains Jean.

School gardeners have lots of opportunities to think about the influence of climate and weather. Although your students might like to grow bananas in their school garden, for instance, chances are that your growing season and climate conditions wouldn't permit it. How does our climate affect what we can grow and when we can grow it? How can we find a site that offers at least six hours of sun a day? When is the danger of frost past in the spring? Can we extend the season so we can garden earlier in the spring? As your class plans to grow plants indoors and out, seize the opportunity to examine how gardeners notice and work with the climate and modify conditions for plants.

Zoning In

Many seed catalogs, packets, and gardening resources depict charts or maps indicating conditions -- first and last frost dates, average minimum temperatures, or amount of sunshine or rainfall -- in different climatic zones throughout the country. A commonly used map type assigns zone numbers indicating which perennial plants can adapt to each area. If the winter temperature in your area dips below a certain temperature, for instance, certain tender perennials will simply not survive. Excessive summer heat can also spell the end of certain plants.

Invite students to use gardening references or Internet resources to examine different climatic zones throughout the U.S. Brainstorm which geographic factors might influence the climate and, in turn, zone designations. Some of the factors influencing climate are proximity to equator (latitude), oceans, mountains, and so on. Ask, Do the temperatures in your schoolyard or garden reflect those in your zone? Have students regularly measure high and low temperatures in these locations, then compare these with temperatures in your local paper. Is your garden (or other spot) warmer or cooler? What factors do you think might affect this? How does this affect what you might be able to grow?

Plant Cover-ups: The Greenhouse Effect

Unlike many individuals, gardeners are people who actually do something about the weather. That's because we like to extend mother nature's growing season and we enjoy growing plants that are not naturally adapted to our climates. (For instance, tomatoes, a favorite garden crop, are actually native to tropical regions of South America.) Many gardeners use strategies for extending the season and protecting plants from frost and cold temperatures. Cold frames, plastic milk jugs, and plastic or nylon row covers keep sun-warmed air around plants and soil, prevent frost from settling on plant leaves, provide wind protection, increase humidity, and may protect plants from insects.

All these plant protectors 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. This energy warms the air, encouraging plant growth.

As a simple but powerful exploration of this 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, compare the two temperatures. Once students have explored this concept, invite them to try designing plant protectors for their seedlings. Have them compare different types of designs, predicting and then monitoring differences in temperature and plant growth rates. (Although there are many commercial products available, homemade gadgets can provide a good design challenge for students.) Be sure that students also consider other plant needs as they create and monitor their setups. For instance, Is the temperature too high under the plant protector? How can we provide ventilation? How can we ensure that plants get enough water?

Predicting Frost

Concerns about frost damaging precious plants run high in the spring and fall for many school gardeners. Challenge students to check with gardening references, area gardeners, the Cooperative Extension Service, or other local sources to find out the typical 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 use what they've learned about factors that influence weather to try to predict when a frost might occur.

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.

If your students are concerned about frost, but don't have enough sheets or other materials to protect the whole garden, encourage them to discuss which plants might need the most protection, and why. (Typically, crops like tomatoes and peppers that originated in warmer climates are the most sensitive to frost. Crops adapted to cooler conditions, such as spinach, lettuce, and broccoli, can handle modest frosts without damage.)

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