ported by the crop itself and rising as it grows [87]. Sometimes such plastic covering serves essentially
for weed and soil moisture control. Often, however, some degree of temperature elevation is sought, and
air and soil temperatures are commonly included in research reports. The elevation of temperature under
plastic film will depend on both the climate and the type of plastic [88]. In sunny climates, temperature
rise may be sufficient to provide effective disinfestation of pathogenic fungi [89].
- Sunshading
Another form of large-scale microclimate control is by shading. A practice that started as “slat houses”
for orchids and “cloth houses” for high-quality tobacco has developed into very considerable industries,
usually growing ornamentals. A high proportion are foliage plants, grown under coarse-woven plastic
material developed to give certain “percentages” of shade. Obviously, any modification of insolation (ir-
radiance) also modifies temperature. It is remarkable that although research reports commonly pay con-
siderable attention to the expression of the exact degree of shade [90], temperature differences are often
not mentioned. It can be very helpful to include temperature as a variable, as demonstrated in a study of
disease intensity under different levels of shade [91]. Research workers in this field are urged to routinely
measure and report the temperature variations that inevitably accompany any modification of irradiance.
Shade conditions can be expected not only to lower daytime temperatures but also to raise night tem-
peratures, particularly under cold night–clear sky conditions, in which ground-to-sky radiation can cause
a very rapid, possibly harmful, drop in temperature near the ground. Similarly, the use of spectral filters
can be expected to modify not only light quality but also temperature.
- Freeze Protection
The first, most obvious, and least expensive protection against freeze injury is to select a planting site
where injurious freezing is unlikely to occur. Because this is often not possible, freeze protection mea-
sures may be necessary. Burning fossil fuels should be regarded as a last resort—the fuels themselves are
very expensive, and their use is often environmentally questionable. Only too often, freeze protection
methods are ineffective because of ignorance of the following basic thermodynamic and meteorological
principles.
- Cold air will roll down a slope until arrested by some physical barrier, which then forms a “frost
pocket.” - Hot air rises vertically. It cannot be made to move up a slope.
- Radiated heat travels in all directions uniformly but only in line-of-sight (straight) lines. Thus,
to be warmed by irradiation from a heat source (such as an orchard heater), a plant must be able
to “see” the heat source. Because radiated heat, like all forms of radiation, is subject to the in-
verse square law (i.e., intensity decreases proportionately to the square of the distance traveled),
radiation warming decreases sharply with distance from the heat source. - The total heat content of a mass of air depends not only on its temperature (sensible heat) but
also on its latent heat, the two together approximating its total energy content or enthalpy. Thus,
total heat content can be very much greater for moist air than for dry air at the same temperature.
Putting this in a different way: air masses at the same atmospheric pressure and conditions of
15°F (9.4°C) and 100% RH, 20°F (6.7°C) and 40% RH, and 25°F (3.9°C) or 0% RH all
have the same heat content of approximately 5.5 Btu per pound of dry air (ca. 3 kg cal kg^1 )
[92]. - The latent heat of evaporation is approximately 7.5 times as great as the latent heat of freezing.
Thus, when spraying irrigation water for freeze protection (a common practice for Florida straw-
berries and various other crops), it is essential to freeze at least 7.5 times as much water as is evap-
orated [93]. In calm or near-calm weather, this is no problem. Continuing to spray after the onset
of a brisk breeze, however, can be disastrous. An ingenious application of this principle is to use
such evaporative cooling to delay the blooming of fruit trees until the danger of a blossom freeze
is over. The blooming of apple trees was delayed by as long as 17 days by use of thermostatically
controlled sprinkling whenever prebloom temperatures exceeded 7°C (44.6°F) [94]. - Smoke from burning oil or other fuel does notform a protective shield. It used to be believed
(particularly in California) that “smudge pots” could create a low cloud that reflected heat back
24 GRIERSON