Horticulture Principles and Practices

times of the year. Growers start these plants under long-day conditions and then finish
them under appropriate photoperiods. The required photoperiod is provided by covering
the plants with a black cloth between 5 P.M. and 8 A.M. The photoperiod may be prolonged
during the natural short days by artificial lighting. This extended day length keeps
the plants vegetative.
Greenhouse Effect
The earth is warmed to a limited extent by the solar radiation that strikes it directly.
As indicated earlier, the earth’s atmosphere filters out most of the short-wave radiation.
After striking the earth, some of the solar radiation is reradiated into the atmosphere
by the earth. Much of this secondary radiation is long wave (infrared). Much
(85 percent) of the solar radiation is absorbed by the water vapor in the atmosphere,
while the remainder is reflected back to Earth (Figure 4–8). The reflection back to
Earth is caused by the accumulation of gases (especially carbon dioxide, methane, and
nitrous oxide—commonly called greenhouse gases), which prevents the heat from
escaping. Consequently, there is a global rise in temperature. This phenomenon is
primarily responsible for warming (indirectly) the earth and is called the greenhouse
effect.
4.2.2 BIOTIC FACTORS
Climate influences plant diseases and insect pests. For an epidemic to occur, there must
be a susceptible host, pathogen, and favorable environment, called the disease triangle.
Disease will not occur unless all three factors are present. However, disease can occur
to varying degrees. Local weather conditions may favor the development of certain
pathogens or predispose plants to diseases by lowering their resistance. Many insects
have a short life span. Changes in the climate can adversely affect their population and
effectiveness at any stage of their life cycle. Certain insects proliferate in specific seasons
and occur in low populations in others. For example, the San Jose scale (Aspidiotus
perniciosus) is a problem in the warm temperatures of summer but not in cold climates.
The aboveground environment contains organisms that are microscopic and largely
pathogenic to plants. Plant diseases and pests are described later in this book. Other
large-bodied organisms that occur in the environment can help or harm plants. Birds help
in seed dispersal and bees and butterflies in flower pollination. On the other hand,
rodents harm plants in the field in a variety of ways. Seeds may be eaten before they have
a chance to germinate. Mature produce may be eaten before harvest; plant stems and
foliage may be eaten by herbivores such as deer and rabbits.
Sun
Shield of carbon dioxide (and other greenhouse gases)
FIGURE 4–8 The greenhouse
effect of carbon dioxide.
Trapped
heat
Burning of
fossil fuels
CO 2
added
CO 2
removed
Reflection
of light
Burning of
forests
Other
sources
Photosynthesis
Forests, other green plants
Algae
4.2 Aboveground Environment 107