Greenhouse Conditions
Air and Soil Temperatures
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Carbon
Dioxide: A Breath
of Life
Carbon dioxide (CO2) is essential to the process of photosynthesis.
If it's scarce, plant growth slows. If your greenhouse
air is stagnant, plants can deplete the carbon dioxide
in the layer of air surrounding the leaves, even though
there may be plenty in the rest of the greenhouse. Keeping
the air moving, via vents or fans, is important for providing
necessary CO2 to plants. Some home and school greenhouse
growers have tried increasing CO2 by composting right
in the greenhouse, since the composting process produces
carbon dioxide as well as heat. Consider challenging your
students to invent other ways of providing CO2 (e.g.,
human breath or dry ice!) and experiment to examine its
effects on plant growth.
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Air
- Plant growth requires heat. Temperature determines how quickly
plants take up water and nutrients, their rate of photosynthesis,
and their growth. Maintaining a comfortable air temperature
for your plants can be a challenge. Generally, 50 to 60°F is
a minimum temperature for greenhouse plants, while 85°F is the
maximum. Plants generally do best with a 10- to 15-degree drop
between day and night temperatures.
Tropical natives or food plants that produce edible fruits (e.g., tomatoes)
can probably withstand higher temperatures. Plants native to more temperate
climates, or those with edible leaves or roots (e.g., lettuce), typically
prefer cooler temperatures.
Soil
- Soil temperature is even more important than air temperature
in your greenhouse, particularly if you're growing in beds.
When soil temperatures are below 45°F, roots grow more slowly
and are less efficient at taking up water and nutrients. Warm
soil is particularly important for germinating seeds or rooting
cuttings.
Sixty-five to 75°F is recommended for germinating
most types of seeds. Have students use a special soil
thermometer to monitor soil temperatures, and consider experimenting
with growth rates of seedlings in warmed and unwarmed soils.
You can supply bottom heat for containers by purchasing special
soil heating cables or mats. Be sure to read the safety precautions
carefully, and never try to adapt an electric blanket or heating
pad for greenhouse use!
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