Horticulture Principles and Practices

and upward. Another important generalization is that within the normal temperature
range for plant growth and development, the growth rate is doubled for every 10°C
(18°F) increase in temperature. Plant metabolic rate is slowed as temperatures decrease
and accelerated as temperatures rise. Therefore, plant growth is slower in the cool season.
Temperature Stress Biochemical reactions have an optimal temperature at which
they occur. Photosynthesis declines as the temperature rises to excessive levels, with
negative consequences. High temperatures cause plants to transpire excessively. The
moisture in the soil is lost rapidly through evaporation, causing moisture stress to plants.
Dormancy Low temperatures are required for purposes other than flower induction
in plant growth and development. Horticultural plants with corms or tubers and many
flowering shrubs and fruit trees require low temperatures to break dormancy. Dormancy
is discussed in detail later in the text.
Other Temperature Effects Cool temperature is required by bulb plants such as
narcissus, tulip, and hyacinth for good flower development.
Heat Units Temperature may be used to quantify the amount of growth that occurs in
a plant because the two factors are correlated. This relationship is used by scientists to
predict the harvest dates of crops and also to determine the adaptability of plants to
various climatic zones. Plant development can be measured in heat units. A heat unit is
the number of degrees Fahrenheit by which the mean daily temperature exceeds a base
minimum growth temperature. It is calculated using the following formula:
heat unit [(daily minimum temperature daily maximum temperature)/2]
base temperature (F)
The base temperature used in the calculation of heat units varies among species.
For example, a value of 50°F (10°C) is used for corn and many fruit trees. A plant requires
a certain number of hours of warmth for a specific growth phase to occur. Dormant
buds on temperate fruit trees require winter chilling and a specific heat unit for the
buds to break. Species such as high-bush blueberry have high (long) winter chilling and
low heat unit requirements, whereas others such as pecan have low chilling and high heat
unit requirements.
Light
Light for plant growth comes primarily from the sun. The role of light in the growth and
development of horticultural plants depends on its quality, quantity, and daily duration.
When plants are grown indoors, artificial lighting is required. The most readily recognized
role of light is in photosynthesis, but it also has other important functions, such as
seed germination in some horticultural species. Solar radiation is electromagnetic in
nature. Radiant energy is described by its wavelength and frequency. The shorter the
wavelength and higher the frequency, the higher the energy transmitted. Cosmic rays
have the most energy, and radio waves have the least energy (Figure 4–4).
Because of the curvature of the earth’s surface, incoming solar radiation strikes the
earth directly at the equator but obliquely toward the pole. The rays at the poles are
spread over a wider surface and pass through more air mass and thus are more filtered
than at the equator. Hence, polar radiation has less energy (colder). The duration of the
radiation reaching the earth varies with the season, since day length is also seasonally
variable. Sunrise and sunset patterns differ from one season to another. The amount and
duration of sunlight are affected by the angle of the sun. The angles are wider in summer
than in winter. A knowledge of these seasonal changes in the sun angles is important in the
orientation of a greenhouse and other solar collectors for maximum exposure to sunlight.
Cloud cover can also reduce effective solar radiation. To increase light interception by
plants, growers may use closer spacing (increase plant density). In row crops, provided
4.2 Aboveground Environment 101