Horticulture Principles and Practices

100 Chapter 4 Plant Growth Environment
frost damage is not likely. Sometimes, certain plants in the locality may act as
indicator plants to guide growers as to when it is safe to plant crops outside.
For example, when the American dogwood blooms, it is safe to plant outside.
2. Protect plants.
a. Hot caps. Hot caps are dome-shaped, moisture-resistant paper caps used to
cover plants individually in the field. As plants grow taller, the tops can be
torn off to give more room for plant growth. When the threat of frost is over,
the caps are removed. Because of its high cost, this protective measure is
cost effective only in the production of high-premium crops such as tomato,
summer squash, and pepper.
b. Sprinkler application. Growers may provide additional heat for frost
protection by sprinkling water on plants. The principle behind this strategy
is that water releases latent heat for fusion when it changes state from liquid
to solid (ice). This heat energy is enough to protect some plants against frost
damage. The amount of heat generated is even greater if warmer water
(above 0°C or 32°F) is applied. Further protection of the buds is derived
from insulation provided by ice as it encases the young bud. Sprinkling is
necessary throughout the duration of the frost period. However, too much
sprinkling may lead to excessive ice formation on plants that may cause
limbs to break under the weight of the ice. The soil may also be in danger
of flooding under conditions of continuous sprinkling.
c. Plastic mulching. Spreading polyethylene sheets over the seedbed
provides warmth for germination and seedling growth. Opaque sheets
minimize weed problems, since weeds receive partial light while
germinated seeds receive full light.
d. Row covers. Polyethylene sheets or other fabric may be used to cover
rows of crops for protection from frost. These materials may be laid directly
on plants in certain cases or they may be supported with wire hoops. Row
covers also protect plants against certain insect pests. Plants under the cover
are warmed and experience early and increased growth.
e. Polyethylene tunnels. Polyethylene tunnels are a kind of row cover, but
the top is designed to be opened (if necessary) during the daytime when
temperatures are too high.
f. Wind machines. Wind machines are like giant fans erected in crop fields
to be used for mixing up the colder bottom (near the soil) layer of air with
the warmer top layer of air. The air is colder near the soil surface because
the soil radiates heat into the atmosphere at night. The condition in which
colder air underlies warmer air is called air inversion. The temperature
differential (between the colder and warmer air) may be small such that
normal daytime conditions can eliminate the temperature inversion.
However, when the temperature near the surface of the soil is very low, it
may be necessary to use mechanical processes such as a strong draft from
a fan to mix the air.
g. Heaters. Orchards may be heated by using portable burners (gas burners).
This practice is not common because of the high cost of fuel and the
pollution that results from its combustion.
Temperature has a diurnal pattern; that is, it varies between the daytime and nighttime,
rising in the day and falling at night. In some horticultural operations, especially
under controlled environments in the greenhouse, success depends on maintaining a
certain nighttime temperature.
As a general rule, planting dates for horticultural crops are delayed as one moves
northward. This generalization is embodied in the Hopkins bioclimatic law, which states
that crop production activities (such as planting and harvesting) and specific morphological
developments are delayed four days for every one degree of latitude, five degrees
of longitude, and 122 meters (400 feet) of altitude as one moves northward, eastward,