Temperature - Agroecology: The Ecology of Sustainable Food Systems - page 69

FIGURE 5.8 Hoop houses pro-
tecting frost-sensitive crops. The
hoop-house coverings, acting as a
nonliving canopy, are put in place
at the end of the day to trap heat
and reduce nighttime heat loss; in
the morning they are removed to
allow light to reach the crop. Frost
is still visible on the ground just
outside the shadow of the center
hoop house.

Greenhouses, on the other hand, are more often used
to conserve or trap heat. Light energy penetrates the
glass or plastic cover on a greenhouse, and inside it is
absorbed and reradiated as long-wave heat energy. The
reradiated energy then becomes trapped inside the
greenhouse. During extended cold or cloudy periods,
growers can heat the interiors of their greenhouses from
many different sources. Recirculating hot water is often
used to heat the floors of greenhouses, or at least provide
heat on benches in the houses for germination or early
plant development.
At certain times of the year or in particular climate
zones, excess heat can be trapped in a greenhouse, requir-
ing venting and air cooling. Another way of reducing
greenhouse temperatures is to block some of the incoming
solar radiation with shade cloth or other materials. Sophis-
ticated greenhouse management now employs computer
technology and automation to achieve remarkable levels
of microclimate control.

atmospheric moisture itself provides some protection
for the plants.
In low-lying areas subject to cold air drainage at
night, farmers have long employed relatively simple
means of raising the temperature to few degrees
necessary to avoid frost damage. One technique is
smudging, in which some kind of fuel — diesel fuel,
garbage, old tires, or plant material — is burned to
generate heat-trapping smoke or to create enough air
turbulence to keep cold air from settling in depressions
during a calm night. Recent concerns about health haz-
ards and air pollution have reduced the use of smudg-
ing, however, and prompted farmers to use large fans
to keep the air moving in frost-prone areas. Obviously,
such techniques work only under certain conditions
and when a few degrees of temperature difference will
matter.

T EMPERATURE AND S USTAINABILITY

Methods of Preventing Frost Damage

Temperature is a factor of considerable agroecological
importance. Part of dealing with this factor is under-
standing local climatic and weather patterns and how
larger-scale patterns may affect them. Another part is
knowing how to control and modify the microclimate.
Farmers have employed techniques for modifying the
microclimate for a long time, and modern scientific
knowledge has provided many new ones. Yet, agriculture
still faces the challenge of finding more and better ways
to design agroecosystems that modify microclimate
themselves rather than relying on costly and often non-
renewable external inputs.

In more temperate regions of the world, especially at
higher elevations and latitudes, frost damage early or
late in the growing season may be a constant danger.
Mulching and row covers are important ways of pro-
viding some frost protection, but other means exist as
well (Figure 5.9).
Raising soil moisture with irrigation when frost is
expected may help raise temperatures close to the
ground because evaporation of the moisture transfers
heat from the soil to the evaporated water vapor,
which then surrounds the crop plants. The increased