Horticulture Principles and Practices

Light
Light is required by many weed species and some small seeds such as lettuce (Lactuca
sativa) before germination can occur. Soil tillage exposes buried seeds to light. Small
seeds should be sown in loose soil and to a shallow depth to expose them to light.
The light requirement to stimulate germination must be of a certain quality
(wavelength). Exposure of lettuce to red light (about 660 nanometers) induces germination,
but far red light (730 nanometers) inhibits it. It was discovered that if seed received
red light after exposure to far red light, germination occurred. In fact, germination
occurred as long as the last treatment before sowing was red light.
9.10.2 CHEMICAL
Acid Treatment
Seed dormancy due to a hard seed covering may be overcome by soaking the seed in concentrated
sulfuric acid for a period ranging from a few minutes to several hours. Soaking in
acid is done in a nonmetallic container. Periodic stirring helps to uniformly expose the seed
to the acid. Concentrated acid is very injurious and must be handled with great caution: gloves
should be worn. It is best to test a small sample first to find the best duration of soaking before
treating the whole lot. Acid-treated seed should be washed thoroughly under running water
for about ten to fifteen minutes before sowing. Sowing should occur without delay.
Leaching with Water
Leaching may be used where dormancy is due to chemical factors (such as coumarin).
Hot-water leaching is done at 67–88°C. It is more effective than cold-water leaching.
However, if the water is too hot, the seed may be injured. As previously mentioned, the
seed of some fleshy fruits, such as strawberry and tomato, will not germinate in the fruit
because of the presence of chemical germination inhibitors. Some desert plant seeds germinate
only after a heavy downpour of rain that is able to wash away the inhibitors.
By germinating only after a heavy rain, the seedlings are assured adequate moisture for
survival and development until their roots are developed enough to absorb moisture.
Okra seed may be soaked in water for twelve to twenty-four hours to leach away a
water-soluble inhibitor. Soaked seed should be planted immediately after the treatment
without allowing it to dry. If prolonged soaking is needed, periodically refreshing
the water is necessary.
Primed Seed
Seed soaked in a specific
solution to initiate
physiological processes for
quicker germination.
Primed Seeds
Primed or enhanced seeds are seeds that have been soaked in a solution (e.g., ethyl alcohol
or potassium chloride [KCl]) to give the germination process a head start by activating
the enzymes and hormones. Seeds must be primed carefully to ensure success.
Primed seeds germinate early and uniformly.
298 Chapter 9 Sexual Propagation
9.11 SEED TREATMENT
Unlike soilless mixes that are sterile, field soil harbors a wide variety of pathogens that
are harmful to seeds. Some seeds benefit from seed treatment in which appropriate pesticides
are applied to seeds to protect against soilborne diseases prevalent in that soil.
Seeds may be coated with a fungicide such as calcium hypochlorite or cuprous oxide
before planting. Also, seeds may be dipped in 10 percent household bleach or sodium
hypochlorite for five minutes for the same effect. The soil may be sterilized or treated
against pathogens that cause damping-off in seedlings. Greenhouse soil and soilless
mixes are easier to sterilize than field soil.
Direct seeding is extremely difficult when seeds are very tiny. To overcome this
problem, tiny seeds may be pelleted, by coating with clay, for example. Bedding plant