TECHNICAL PAPER #42
UNDERSTANDING SEED HANDLING
Dr. James Young
Dr. Charles Suggs
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 . Fax:703/243-1865
Understanding Seed Handling for Germination
[C]1986, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
They are not intended to provide construction or
details. People are urged to contact VITA or a similar
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
almost entirely by VITA Volunteer technical experts on a
voluntary basis. Some 500 volunteers were involved in the
of the first 100 titles issued, contributing approximately
5,000 hours of their time. VITA staff included Gerald Schatz
editor, Suzanne Brooks handling typesetting and layout, and
Margaret Crouch as project manager.
The authors of this paper, VITA Volunteer Dr. James Young
Raymond Evans are range scientists with the U.S. Department
Agriculture-Agricultural Research Service in Reno, Nevada.
Jerry Budy , is an Assistant Professor of Forestry at the
of Nevada in Reno. The reviewers are also VITA volunteers.
Dr. Charles Suggs is a professor with the Department of
and Engineering at the North Carolina State University in
Dr. Suggs has worked in India, Australia, Europe and South
America. Reviewer Lawrence Yarger is a horticulturalist
with Food for the Hungry in Scottsdale, Arizona. He has
Thailand and Latin America.
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries. VITA
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
situations. VITA maintains an international Inquiry Service,
specialized documentation centers, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
UNDERSTANDING SEED COLLECTION AND HANDLING
Volunteers James A. Young,, Raymond A. Evans,
and Jerry D. Budy
Local seed production in developing countries can have
benefits. It can help to reduce dependence on seed and food
imports and so increase agricultural production. It can also
provide commodities for export (flowers, specialty plants,
It can improve income and well-being of rural populations,
self-sufficiency, and stimulate employment.
Depending on the plant and the market, seed may be produced
direct planting, as in typical raising of cereals, or it may
produced for germination and transplantation from seedbeds,
widely practiced in raising trees, commercial flowers, and
vegetables. These applications have certain. requirements in
common, including careful harvesting, handling, and storage,
certain seed tests are widely applicable. Care and
of the seed resource can reduce postharvest loss
This report notes general considerations in seed harvesting,
handling, and storage, and it focuses attention on methods
enhance germination for seeds started in seedbeds. The paper
intended to be especially useful for persons interested in
up a small business that produces seed for sale or for use
in a commercial nursery.
Successful germination of seeds starts with proper
harvesting of the seeds. Both the timing of collection and
handling of the freshly harvested seeds are important.
II. BASIC PRINCIPLES
TIMING THE COLLECTION OF SEEDS
If seeds are collected too early, yields will be lowered;
seeds can be poor germinators. If collection is delayed,
seeds may be dispersed and lost on the ground.
Many crop species have been selected for determinate-type
in which all the fruits on a given plant mature at close to
the same time. Unless the seed pods of determinate species
collected shortly before maturity, there is the danger of
pods suddenly splitting open and allowing the seed to be
Many wild plant species have indeterminate-type of flowering--flowering
continues for prolonged periods. This means that some
seeds are ripe and falling from the plant at the same time
is still occurring at other locations on the same plant. It
is difficult to avoid collecting immature seeds in this situation
or to prevent mature seed from falling from the plant.
Slightly immature seeds are not necessarily poor
they may require extensive drying before they can be stored
safely. The influence of seed maturity has to be determined
through germination trials. To conduct meaningful trials it
necessary to label the seed collections with some detail of
stage of plant development and seed maturity, to record
seedlot was collected, and to maintain the identity of the
through germination trials.
HANDLING FRESHLY HARVESTED SEEDS
A seed is a living organism in a resting stage. It is alive
for germination must be kept alive. Freshly harvested seeds
too high a moisture content for safe storage. The moisture
content of the seed must be reduced, often by artificial
to permit storage without loss of viability. The relative
of the air at a given temperature is directly related to the
moisture content of the seed. For safe storage the moisture
content of the seed should be 14 percent or less.
In the humid tropics it may be very difficult to obtain a
content that permits seed storage without using artificially
heated air for drying. In most temperate to arid
it is possible to reach a satisfactory moisture content
without artificial drying. Artificial drying at high
or drying in direct sunlight are not desirable and can be
especially harmful to seed viability.
For freshly harvested seeds to reach a moisture equilibrium
the environment they must be stored in such a manner to
free aeration. If the seed heads cannot be hung or tied on
strings, baskets or uncoated paper or mesh bags make good
containers for initial drying. Very shallow trays can also be
used. Never use plastic bags for storage of freshly
seeds. Seed heads or mesh bags should be hung on racks if
and spaced apart to allow good air circulation.
Excessive moisture in freshly harvested seeds is often
plant parts and other trash that accidentally contaminate
seed collection during the harvesting period. Screening
harvested seeds to remove high moisture content trash will
Freshly harvested fruits require prompt treatment to remove
fleshy material to avoid spoilage or mummification of the
Fleshy fruits are cleaned in macerators. The macerator
dislodges the fleshy portion of the fruit so it can be
from the seed. Separation is usually done by flotation: The
macerated fruit seed mixture is dumped into a container into
which water is running; the heavy seeds sink, and the
fruit floats over the lip of the container. Drying is
before storage of the seeds.
Seeds are recovered from some fleshy fruits by allowing the
fruits to ferment. Tomatoes, cucumbers, and melons are among
fruits that may be treated this way. After the fruit portion
dissolved by the fermentation process the hard seeds are
The seeds of species collected from marshes and wetlands
require special handling. The technique used depends on the
species involved. Often it is necessary to keep the seeds in
cool, wet environment, or actually stored in water, to avoid
Generally, the faster that seeds are cleaned and placed in
after they reach moisture equilibrium, the less chance there
is of predation from birds or small mammals or contamination
Avoid rough handling of seeds during cleaning. Remember that
seeds are alive, and the embryo can be very fragile. Never
hammer mill in seed processing unless you have first
by careful testing that seed-viability is not being
affected by the process.
Proper seed cleaning makes subsequent handling of the seeds
the germination process much simpler. If the seedlot
trash, weed seeds, empty or obviously immature seeds, much
will be wasted sorting the material to find germinable
To avoid problems with storage insects start with clean,
storage conditions. Do not introduce pests with the seeds to
be stored. Most seed storage insects are of tropical origin.
Cool storage conditions such as in the shade of the house or
underground lessen the chances of insect problems.
The key to seed storage is maintaining proper moisture
so that the seeds remain alive but ungerminated. Remember
that the amount of water that the storage atmosphere will
a vapor is directly related to temperature. The warmer the
the more moisture it will hold. When the temperature drops
humidity will increase. Droplets of water may then condense
and form in storage containers.
Storage in paper or mesh bags in a cool, dry location is satisfactory
for most seeds. Once the seeds have reached moisture
equilibrium, storage in glass jars or plastic boxes is
to avoid insect contamination. Some seeds can be stored
in small lots, but suffer losses in viability when larger
of seeds are stored together. Some seeds have short
storage lives, and seed stocks of these species must be
There are two common determinations that are made from seed
tests: viability and germinability. Viability simply means
the seed is alive. It does not indicate that the seed will
germinate. Viability tests may be as simple as cutting a
with a knife blade to determine if an embryo is present.
complex viability tests involve the use of a tetrazolium TZ
test. After the proper sectioning and preparation of the
this chemical helps certain enzymes remove the hydrogen from
seed during the respiration process in viable seeds.
respiring or living tissue in the seeds is shown by a red
That the seeds contain living tissue does not necessarily
the embryo will germinate. For seeds of the major crop
standards have been developed that relate the tetrazolium
to potential germination. These standards have not been
developed for the seeds of most wildland species.
Germinability is a much more meaningful factor for
interested in propagating plants from seeds. To obtain an
of germinability, the seeds must be subjected to a
test. The Association of Official Seed Analysts (AOSA), in
Boise, Idaho, prescribes rules for testing seeds of specific
plants in the United States. There are corresponding
organizations for seed testing. Unfortunately, for the
seeds of most wildland species, no standard germination
exist. The AOSA has draft standards for about 100 wildland
species. Until these standards are accepted and/or developed
the seeds of important wildland species, germination figures
given on their seed tags are meaningless.
The seeds of many species will not germinate immediately
they are harvested. They must pass through a period of
before germinating. This dormancy requirement varies with
species and allows for certain physiological changes to
within the seed that make it capable of germination. This is
referred to as after-ripening and has been attributed to
embryos requiring post-harvest time to mature.
A variant of this type of dormancy is called
after-ripening. In this type, seeds will not germinate at
one incubation temperature (usually moderate to high
temperature) but will germinate at other temperatures
cold incubation temperatures). Other variations include
to light, stratification, alternating temperatures,
leaching of growth inhibitors, and other conditions. As a
matter, the after-ripening requirement means the farmer has
to wait to obtain germination with the seeds of certain
HARD COAT SEEDS
If seeds do not germinate soon or after a reasonable
period, the first germination factor to check is whether
the seeds take up water. This check can be made by pressing
seed with a thumbnail or by cutting. If the interior of the
appears chalky and hard, water has not been imbibed through
seed coat. Seeds that have imbibed water should be soft and
easily squashed with the thumb. Seeds with coats that do not
freely allow the passage of either water or oxygen are
To break the hard seed coats some form of scarification is
to make the seed coat permeable to water. This scarification
can be accomplished with mechanical, thermal, or chemical
treatments. If the seeds are large enough, scarification may
accomplished by filing a notch in the coat or clipping so as
to injure the embryo. Smaller seeds can be mechanically
by mechanically abrading them in some manner. This may be as
simple as rubbing the seeds between sheets of sandpaper.
Mechanical scarifiers have been developed, such as those
rotating drums lined with an abrasive material in which the
are tumbled. Hammer mills may be used (with care), and the
between the concave bars in threshing machines can be set
to just crack the seeds of legumes to obtain increased
Any mechanical scarification that increases germinability
results in decreased viability. In other words, you pay a
the mechanical process that gets some seeds to germinate,
injures other seeds. Great care must be taken not to injure
seeds excessively with these treatments.
Thermal scarification is obtained by dropping seeds into
water and then allowing the water to cool. Such treatment
have many other influences, such as thermal shock to the
or leaching soluble inhibitors. In areas that have freezing
winter temperatures, thermal cracking of seed coats can also
obtained by fall seeding at shallow depths.
One chemical method of scarification is to use concentrated
sulfuric acid to remove hard seed coats. This is a very
treatment, with many side effects. The duration of treatment
to be determined for individual seedlots. Heating from the
reaction along with rinse water and the resulting hydrolysis
the seed tissue may induce germination rather than simply
the intake of water as intended.
Always try to control the temperature of the acid-treated
in a water bath, rinse a small amount of acid and seed in a
volume of water, and use a neutralizing solution after the
STIMULATING THE GERMINATION OF SEEDS
A seed's after-ripening time cannot be shortened, but the
of seeds following the after-ripening period may be
by any of a variety of methods.
Seeds that imbibe water but fail to germinate are good
for stratification--placing of seeds in a wet environment at
temperatures that normally are not conducive to germination.
Such treatments are termed cool-moist stratification. The
of stratification requirements can range from a few days to
many months. For prolonged stratification, a substrate must
furnished to retain moisture. Peat is often used, but other
common materials include sand and vermiculite.
Naked stratification has proven effective for seeds of some
species of conifers. This is accomplished by soaking the
overnight in water and then placing the damp seeds in
bags that are sealed for the duration of the stratification.
Seeds of other species require specific stratification
Their seeds are very difficult to germinate without
The most influential factor in enhancing germinations of
the supply of nitrogen, usually in the form of potassium
In the field or nursery bed, lush growth in spring or after
rains may be associated with the availability of nitrogen in
seedbed. Farmers or nursery operators should have their soil
growing medium tested for nitrogen content if possible.
fertilizer can be added if necessary.
Scientists don't know exactly how gibberellic acid, a growth
regulator, works in seed germination, but they do know that
low concentrations of it can greatly enhance germination.
of from 1 to 250 parts per million (ppm) are commonly
used to improve germination. Combinations of gibberellic
potassium nitrate are often more effective than either
alone. These materials can be obtained from chemical
The potassium nitrate is more easily obtained than
Good measuring equipment is needed for preparing the minute
concentrations of gibberellic acid. A solution with a
of 1 ppm of gibberellic acid consists of 0.001 grams of
gibberellic acid dissolved in 1,000 milliliters (ml) of
Gibberellic acid is sold as a 10-percent active-ingredient
which makes the weighing simpler. One alternative is
to prepare higher concentrations than needed and dilute to
desired concentration. For example, 1,000 ppm would be 1g in
1,000ml. It is best not to mix too large a batch at once,
for gibberellic acid is relatively expensive and breaks
down very rapidly at warm temperatures.
Germination of the seeds of several species, especially
of the rose family, is enhanced by soaking the seeds in
peroxide solutions. Dramatic germination enhancement has
obtained with seeds of bitterbrush (Purshia tridentata) and
mountain mahogany (Cercocarpus ledifolius). A wide range of
concentrations from 1 to 30 percent is effective. Generally,
higher the concentration, the shorter the soaking time, but
greater the risk of damaging the seed. Hydrogen peroxide is
very reactive chemical. Concentrations greater than 3
are particularly dangerous to handle. Hydrogen peroxide,
has an advantage in that it is generally available and
Many other chemicals have been used to enhance germination.
These include various sulphydryl and ethylene-producing
Many seeds are sensitive to light during germination. Both
intensity (candlepower) and light quality (color or wave
can influence germination. The light intensity requirement
with the type of seed from a few foot candles, such as that
moonlight, to strong daylight. Germination is enhanced or
by the color or wave lengths of light. Orange to red wave
lengths (660-700 nanometers) stimulate germination while far
or infra-red (700 or more nanometers) inhibits germination.
impact of light rays on seed is also affected by other
such as the age of the seed, temperature, and chemicals
in the germination medium. Cool-white fluorescent light
germination, and incandescent light should be avoided. Seeds
require light for germination have to be placed virtually on
surface of the seedbed. The seeds should be pressed into the
seedbed for optimum moisture transfer.
Seeds must absorb moisture from the germination medium
than they lose it to the atmosphere. In a well-firmed
optimum germination conditions can occur with proper water
Planting small seeds on the surface of a firmed seedbed
and covering them lightly with fine vermiculite can produce
ideal germination environment. Moisture loss can be reduced
shading the seedbed with large leaves or, if excess
are not generated, by covering with plastic film. These
removed after germination occurs to give the plants light
the case of clear plastic, to prevent temperature build up.
Seeds with low germination percentages can be established
if a sufficient number of seeds are planted in a
Seed production can contribute substantially to local and
rural economies. It depends more on care than on investment,
and the equipment required may be improvised easily. Simple
seed dryers and storage facilities, for examples, are
in numerous publications worldwide. Like any seed-production
industry, seed handling for germination and transplantation
requires proper timing and care in harvest and storage, to
postharvest losses and to realize the greater value from
crops. Germination of seeds can be stimulated by special
some of which use chemicals that may be relatively expensive
but are used in very small quantitites. These techniques
are well worth considering if sufficient markets for the
identified to make them cost effective.
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Colorado, Wasington: U.S. Department of
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Emery, D. Seed Propagation of Native California Plants.
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