Ch. 7, pp. 2 - 6
seed | starting seeds |
Sexual propagation involves the union of the pollen
(male) with the egg (female) to produce a seed. The seed is made
up of three parts: the outer seed coat, which protects the seed;
the endosperm, which is a food reserve; and the embryo, which is
the young plant itself. When a seed is mature and put in a
favorable environment, it will germinate, or begin active growth.
In the following section, seed germination and transplanting of
seeds will be discussed.
To obtain quality plants, start with good quality seed from a
reliable dealer. Select varieties to provide the size, color, and
habit of growth desired. Choose varieties adapted to your area
which will reach maturity before an early frost. Many new
vegetable and flower varieties are hybrids, which cost a little
more than open pollinated types. However, hybrid plants usually
have more vigor, more uniformity, and better production than
non-hybrids and sometimes have specific disease resistance or
other unique cultural characteristics.
Although some seeds will keep for several years if
stored properly, it is advisable to purchase only enough seed for
the current years use. Good seed will not contain seed of
any other crop, weeds, seeds, or other debris. Printing on the
seed packet usually indicates essential information about the
variety, the year for which the seeds were packaged, and
germination percentage you may typically expect, and notes of any
chemical seed treatment. If seeds are obtained well in advance of
the actual sowing date or are stored surplus seeds, keep them in a
cool, dry place. Laminated foil packets help ensure dry storage.
Paper packets are best kept in tightly closed containers and
maintained around 40° F in a low humidity.
Some gardeners save seed from their own gardens;
however, such seed is the result of random pollination by insects
or other natural agents, and may not produce plants typical of the
parents. This is especially true of the many hybrid varieties.
(See Vegetables chapter for information on saving vegetable seed.)
Most seed companies take great care in handling seeds properly.
Generally, do not expect more than 65% to 80% of the seeds to
germinate. From those germinating, expect about 60% to 75% to
produce satisfactory, vigorous, sturdy seedlings.
There are four environmental factors which affect germination:
water, oxygen, light, and heat.
The first step in the germination process is the imbibition or
absorption of water. Even though seeds have great absorbing power
due to the nature of the seed coat, the amount of available water
in the germination medium affects the uptake of water. An
adequate, continuous supply of water is important to ensure
germination. Once the germination process has begun, a dry period
will cause the death of the embryo.
Light is known to stimulate or to inhibit germination of some
seed. The light reaction involved here is a complex process. Some
crops which have a requirement for light to assist seed
germination are ageratum, begonia, browallia, impatiens, lettuce,
and petunia. Conversely, calendula, centaurea, annual phlox,
verbena, and vinca will germinate best in the dark. Other plants
are not specific at all. Seed catalogs and seed packets often list
germination or cultural tips for individual varieties. When sowing
light-requiring seed, do as nature does, and leave them on the
soil surface. If they are covered at all, cover them lightly with
fine peat moss or fine vermiculite. These two materials, if not
applied too heavily, will permit some light to reach the seed and
will not limit germination. When starting seed in the home,
supplemental light can be provided by fluorescent fixtures
suspended 6 to12 inches above the seeds for 16 hours a day.
In all viable seed, respiration takes place. The respiration in
dormant seed is low, but some oxygen is required. The respiration
rate increases during germination, therefore, the medium in which
the seeds are placed should be loose and well-aerated. If the
oxygen supply during germination is limited or reduced,
germination can be severely retarded or inhibited.
A favorable temperature is another important requirement of
germination. It not only affects the germination percentage but
also the rate of germination. Some seeds will germinate over a
wide range of temperatures, whereas others require a narrow range.
Many seed have minimum, maximum, and optimum temperatures at which
they germinate. For example, tomato seed has a minimum germination
temperature of 50oF and a maximum temperature of 95o,
but an optimum germination temperature of about 80o.
Where germination temperatures are listed, they are usually the
optimum temperatures unless otherwise specified. Generally, 65o
to 75oF is best for most plants. This often means the
germination flats may have to be placed in special chambers or on
radiators, heating cables, or heating mats to maintain optimum
temperature. The importance of maintaining proper medium
temperature to achieve maximum germination percentages cannot be
Germination will begin when certain internal
requirements have been met. A seed must have a mature embryo,
contain a large enough endosperm to sustain the embryo during
germination, and contain sufficient hormones or auxins to initiate
Methods of Breaking Dormancy
One of the functions of dormancy is to prevent a seed from
germinating before it is surrounded by a favorable environment. In
some trees and shrubs, seed dormancy is difficult to break, even
when the environment is ideal. Various treatments are performed on
the seed to break dormancy and begin germination.
Seed scarification involves breaking, scratching, or softening
the seed coat so that water can enter and begin the germination
process. There are several methods of scarifying seeds. In acid
scarification, seeds are put in a glass container and covered with
concentrated sulfuric acid. The seeds are gently stirred and
allowed to soak from 10 minutes to several hours, depending on the
hardness of the seed coat. When the seed coat has become thin, the
seeds can be removed, washed, and planted. Another scarification
method is mechanical. Seeds are filed with a metal file, rubbed
with sandpaper, or cracked with a hammer to weaken the seed coat.
Hot water scarification involves putting the seed into hot water
(170o to 212oF). The seeds are allowed to
soak in the water, as it cools, for 12 to 24 hours and then
planted. A fourth method is one of warm, moist scarification. In
this case, seeds are stored in non-sterile, warm, damp containers
where the seed coat will be broken down by decay over several
Seeds of some fall-ripening trees and shrubs of the temperate
zone will not germinate unless chilled underground as they
overwinter. This so called "after-ripening" may be
accomplished artificially by a practice called stratification.
The following procedure is usually successful. Put sand
or vermiculite in a clay pot to about 1 inch from the top. Place
the seeds on top of the medium and cover with 1/2 inch of sand or
vermiculite. Wet the medium thoroughly and allow excess water to
drain through the hole in the pot. Place the pot containing the
moist medium and seeds in a plastic bag and seal. Place the bag in
a refrigerator. Periodically check to see that the medium is
moist, but not wet. Additional water will probably not be
necessary. After 10 to 12 weeks, remove the bag from the
refrigerator. Take the pot out and set it in a warm place in the
house. Water often enough to keep the medium moist. Soon the
seedlings should emerge. When the young plants are about 3 inches
tall, transplant them into pots to grow until time for setting
Another procedure that is usually successful uses
sphagnum moss or peat moss. Wet the moss thoroughly, then squeeze
out the excess water with your hands. Mix seed with the sphagnum
or peat and place in a plastic bag. Seal the bag and put it in a
refrigerator. Check periodically. If there is condensation on the
inside of the bag, the process will probably be successful. After
10 to 12 weeks remove the bag from the refrigerator. Plant the
seeds in pots to germinate and grow. Handle seeds carefully. Often
the small roots and shoots are emerging at the end of the
stratification period. Care must be taken not to break these off.
Temperatures in the range of 35o to 45oF
(2o to 7oC) are effective. Most
refrigerators operate in this range. Seeds of most fruit and nut
trees can be successfully germinated by these procedures. Seeds of
peaches should be removed from the hard pit. Care must be taken
when cracking the pits. Any injury to the seed itself can be an
entry path for disease organisms.