Light has three principal characteristics that affect
plant growth: quantity, quality, and duration.
Light quantity refers to the intensity or
concentration of sunlight and varies with the season of the year.
The maximum is present in the summer and the minimum in winter.
The more sunlight a plant receives (up to a point), the better
capacity it has to produce plant food through photosynthesis. As
the sunlight quantity decreases the photosynthetic process
decreases. Light quantity can be decreased in a garden or
greenhouse by using shade-cloth or shading paint above the plants.
It can be increased by surrounding plants with white or reflective
material or supplemental lights.
Light quality refers to the color or wavelength
reaching the plant surface. Sunlight can be broken up by a prism
into respective colors of red, orange, yellow, green, blue,
indigo, and violet. On a rainy day, raindrops act as tiny prisms
and break the sunlight into these colors producing a rainbow. Red
and blue light have the greatest effect on plant growth. Green
light is least effective to plants as most plants reflect green
light and absorb very little. It is this reflected light that
makes them appear green. Blue light is primarily responsible for
vegetative growth or leaf growth. Red light when combined with
blue light, encourages flowering in plants. Fluorescent or
cool-white light is high in the blue range of light quality and is
used to encourage leafy growth. These lights are excellent for
starting seedlings. Incandescent light is high in the red or
orange range but generally produces too much heat to be a valuable
light source. Fluorescent "grow" lights have a mixture
of red and blue colors that attempts to imitate sunlight as
closely as possible. They are costly and generally not of any
greater value than regular fluorescent lights.
Light duration or photoperiod refers to the
amount of time that a plant is exposed to sunlight. When the
concept of photoperiod was first recognized it was thought that
the length of periods of light triggered flowering. The various
categories of response were named according to the light length
(i.e., short-day and long-day). It was then discovered that it is
not the length of the light period but the length of uninterrupted
dark periods that is critical to floral development. The ability
of many plants to flower is controlled by photoperiod.
Plants can be classified into three categories,
depending upon their flowering response to the duration of
darkness. These are short-day, long-day, or day-neutral plants.
Short-day, (long nights) plants form their flowers only when the
day length is less than about 12 hours in duration. Short-day
plants include many spring and fall flowering plants such as
chrysanthemum and poinsettia. Long-day, (short nights) plants form
flowers only when day lengths exceed 12 hours. They include
almost all of the summer-flowering plants, as well as many
vegetables including beet, radish, lettuce, spinach, and potato.
Day-neutral plants form flowers regardless of day length. Some
plants do not really fit into any category but may be responsive
to combinations of day lengths. The petunia will flower regardless
of day length, but flowers earlier and more profusely under long
daylight. Since chrysanthemums flower under the short-day
conditions of spring or fall the method for manipulating the plant
into experiencing short days is very simple. If long days are
predominant, a black plastic sheet is drawn over the chrysanthemum
for 12 hours daily to block out light until flower buds are
initiated. To bring a long-day plant into flower when sunlight is
not present longer than 12 hours artificial light is added until
flower buds are initiated.
Temperature affects the productivity and growth of a
plant depending upon whether the plant variety is a warm-season or
cool-season crop. If temperatures are high and day length is long,
cool-season crops such as broccoli and spinach will bolt rather
than produce the desired flower. Temperatures that are too low or
high for a warm-season crop will prevent fruit set. Temperatures
that are too high for warm-season crops such as pepper or tomato
can cause pollen to become inviable and not pollinate flowers.
Adverse temperatures also cause stunted growth and poor quality.
For example, the bitterness in lettuce is caused by high
Sometimes temperatures are used in connection with day
length to manipulate the flowering of plants. Chrysanthemums will
flower for a longer period of time if daylight temperatures are 59°F
(15°C). The Christmas cactus forms flowers as a result of
short days and low temperatures. Temperatures alone also influence
flowering. Daffodils are forced to flower by putting the bulbs in
cold storage in October at 35° to 40°F (2° to 4°C).
The cold temperatures allow the bulb to mature. The bulbs are
transferred to the greenhouse in midwinter where growth begins.
The flowers are then ready for cutting in 3 to 4 weeks.
Thermoperiod refers to daily temperature change. Plants
produce maximum growth when exposed to a day temperature that is
about 10 to 15° F. (5.5 to 8°C) higher than the night
temperature. This allows the plant to photosynthesize and respire
during an optimum daytime temperature and to curtail the rate of
respiration during a cooler night.
Temperature Effects on Plant Growth
Increases with temperature to a point.
Rapidly increases with temperature.
Increases with temperature.
May be partially triggered by temperature.
Low temperatures reduce energy use and increase sugar storage.
Warmth, after a period of low temperature, will
break dormancy and plants will resume active growth.
High temperatures cause increased respiration sometimes
above the rate of photosynthesis. This means that the products of
photosynthesis are being used more rapidly than they are being
produced. For growth to occur photosynthesis must be greater than
Low temperatures can result in poor growth.
Photosynthesis slows at low temperatures. Since photosynthesis is
slowed, growth is slowed and this results in lower yields. Not all
plants grow best in the same temperature range. For example,
snapdragons grow best when nighttime temperatures are 55°F (12°C);
the poinsettia prefers 62°F (17°C). Florist cyclamen does
well under very cool conditions while many bedding plants prefer a
higher temperature. Recently it has been found that roses can
tolerate much lower nighttime temperatures than was previously
believed. This has meant a conservation in energy for greenhouse
growers. However, in some cases a certain number of days of low
temperatures are needed by plants to grow properly. This is true
of crops growing in cold regions of the country. Peaches are a
prime example; most varieties require 700 to 1,000 hours below 45°F
(7°C) and above 32°F (0°C) before they break their
rest period and begin flowering and growth. If this cold
requirement is not met then small, misshapen leaves and fruit will
result. Many times fruit will not set. In low desert areas where
these temperatures are not experienced low chill peach trees
should be planted. Lilies need 6 weeks at 33°F (1°C or
below) before they will bloom.
Plants can be classified as either hardy or non-hardy
depending upon their ability to withstand cold temperatures.
Winter injury can occur to non-hardy plants if temperatures are
too low or if unseasonably low temperatures occur late in the
spring or early in the fall. Winter injury may also occur because
of desiccation (drying out).
Plant roots need moist soil during the winter. When the
soil is frozen the movement of water into the plant is severely
restricted. On a windy winter day broad-leaved evergreens can
become water-deficient in a few minutes, turning the leaves or
needles brown. Wide variations in winter temperatures can cause
premature bud break in some plants and consequent freezing damage.
Late spring frost damage can ruin entire crops. If temperatures
drop too low during the winter, entire trees of some species are
killed by the freezing of plant cells and tissue.