Frost Protection - UTL Repository

PASSIVE PROTECTION METHODS
of culm rolled around trunks for young trees; and (4) roofing tiles, adobe
shelters, leaves of plants, etc., for small plants. For rows of plants, the methods
include (1) larger horizontal or inclined mats for tree rows; (2) shelters forming
a half hut with vertical wall facing the predominant wind direction; and (3) straw
layers over horticultural nurseries, where the mats and shelters use local
materials (e.g. straw, bamboo, wood, boards, hay, etc.) (Abreu, 1985).
Although the materials used for coverings generally are inexpensive, the
manpower needed to apply the materials can be cost prohibitive. Generally, this
method is only used on small plantings or on small plants that do not require a
solid frame. Sometimes, disease problems occur due to poor ventilation.
Row covers are sometimes used for protection of high value crops. Woven and
spun-bonded polypropylene plastics are typically used and the degree of
protection varies with the thickness of the material (e.g. from 1 °C for thin sheet
plastic to 5 °C for thick plastic). White plastic provides some protection and it
is sometimes used for nursery stock. It is not typically used for fruit and
vegetable crop protection. Schultz (1961) reported that 1.2 m wide, black
polyethylene sheets were used to cover grapevine rows and it increased the air
temperature next to the foliage by about 1.5 °C.
Transparent plastic covers allow sunlight to pass through during the day and
slow heat loss from the surface at night. The downward radiation from the sky
at night depends on the apparent temperature of the sky, so when covered with
plastic, the downward radiation depends mainly on the plastic cover
temperature. Since the sky is much colder than air near the ground and the plastic
will have a temperature closer to the air temperature, the downward radiation is
enhanced by covering the plants. If condensation forms underneath the plastic,
this will release latent heat, warm the plastic and provide even more protection.
Under advection frost conditions, the plastic covers can also block the wind and
provide some protection. Some characteristics for above-plant row covers are
provided in Table 6.1.
A wide variety of methods are used to cover the plants and to anchor the
plastic. To keep the plants from being touched, plastic covers are sometimes
mounted on hoops. Otherwise, the plastic can float on the canopy and rise up as
the crops grow, but disease problems are more likely. PVC greenhouses are
sometimes used to protect citrus. The plastic can be used up to three years
depending on the structural design and quality of the plastic.
A common problem is that the labour requirements for applying covers are
high and therefore the crop value must be high. Also, the plants become less
hardy against freezing and there are often problems with pollination if the covers
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