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                              TECHNICAL PAPER #42
                          UNDERSTANDING SEED HANDLING
                                FOR GERMINATION
                                  Jerry Budy
                                 Raymond Evans
                                Dr. James Young
                              Technical Reviewers
                               Dr. Charles Suggs
                                Lawrence Yarger
                                 Published By
                       1600 Wilson Boulevard, Suite 500
                         Arlington, Virginia 22209 USA
                     Tel: 703/276-1800 . Fax:703/243-1865
              Understanding Seed Handling for Germination
                          ISBN: 0-86619-255-7
              [C]1986, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
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 situations.
They are not intended to provide construction or implementation
details. People are urged to contact VITA or a similar organization
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 illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis. Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time. VITA staff included Gerald Schatz as
editor, Suzanne Brooks handling typesetting and layout, and
Margaret Crouch as project manager.
The authors of this paper, VITA Volunteer Dr. James Young and
Raymond Evans are range scientists with the U.S. Department of
Agriculture-Agricultural Research Service in Reno, Nevada. Author
Jerry Budy , is an Assistant Professor of Forestry at the University
of Nevada in Reno. The reviewers are also VITA volunteers.
Dr. Charles Suggs is a professor with the Department of Biology
and Engineering at the North Carolina State University in Raleigh.
Dr. Suggs has worked in India, Australia, Europe and South
America. Reviewer Lawrence Yarger is a horticulturalist working
with Food for the Hungry in Scottsdale, Arizona. He has worked in
Thailand and Latin America.
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries. VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations. VITA maintains an international Inquiry Service, a
specialized documentation centers, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
            by VITA Volunteers James A. Young,, Raymond A. Evans,
                           and Jerry D. Budy
Local seed production in developing countries can have important
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, etc.).
It can improve income and well-being of rural populations, enhance
self-sufficiency, and stimulate employment.
Depending on the plant and the market, seed may be produced for
direct planting, as in typical raising of cereals, or it may be
produced for germination and transplantation from seedbeds, as is
widely practiced in raising trees, commercial flowers, and some
vegetables. These applications have certain. requirements in
common, including careful harvesting, handling, and storage, and
certain seed tests are widely applicable. Care and appreciation
of the seed resource can reduce postharvest loss substantially.
This report notes general considerations in seed harvesting,
handling, and storage, and it focuses attention on methods to
enhance germination for seeds started in seedbeds. The paper is
intended to be especially useful for persons interested in setting
up a small business that produces seed for sale or for use
in a commercial nursery.
Successful germination of seeds starts with proper collection or
harvesting of the seeds. Both the timing of collection and the
handling of the freshly harvested seeds are important.
If seeds are collected too early, yields will be lowered; immature
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 flowering,
in which all the fruits on a given plant mature at close to
the same time. Unless the seed pods of determinate species are
collected shortly before maturity, there is the danger of the
pods suddenly splitting open and allowing the seed to be lost.
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 blooming
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 germinators, but
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 is
necessary to label the seed collections with some detail of the
stage of plant development and seed maturity, to record where the
seedlot was collected, and to maintain the identity of the seedlot
through germination trials.
A seed is a living organism in a resting stage. It is alive and
for germination must be kept alive. Freshly harvested seeds have
too high a moisture content for safe storage. The moisture
content of the seed must be reduced, often by artificial means,
to permit storage without loss of viability. The relative humidity
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 moisture
content that permits seed storage without using artificially
heated air for drying. In most temperate to arid environments,
it is possible to reach a satisfactory moisture content
without artificial drying. Artificial drying at high temperatures
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 with
the environment they must be stored in such a manner to allow for
free aeration. If the seed heads cannot be hung or tied on
strings, baskets or uncoated paper or mesh bags make good storage
containers for initial drying. Very shallow trays can also be
used. Never use plastic bags for storage of freshly harvested
seeds. Seed heads or mesh bags should be hung on racks if possible
and spaced apart to allow good air circulation.
Excessive moisture in freshly harvested seeds is often caused by
plant parts and other trash that accidentally contaminate the
seed collection during the harvesting period. Screening freshly
harvested seeds to remove high moisture content trash will reduce
drying time.
Freshly harvested fruits require prompt treatment to remove the
fleshy material to avoid spoilage or mummification of the fruits.
Fleshy fruits are cleaned in macerators. The macerator shreds and
dislodges the fleshy portion of the fruit so it can be separated
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 shredded
fruit floats over the lip of the container. Drying is required
before storage of the seeds.
Seeds are recovered from some fleshy fruits by allowing the
fruits to ferment. Tomatoes, cucumbers, and melons are among the
fruits that may be treated this way. After the fruit portion is
dissolved by the fermentation process the hard seeds are recovered.
The seeds of species collected from marshes and wetlands often
require special handling. The technique used depends on the
species involved. Often it is necessary to keep the seeds in a
cool, wet environment, or actually stored in water, to avoid loss
of viability.
Seed Cleaning
Generally, the faster that seeds are cleaned and placed in storage
after they reach moisture equilibrium, the less chance there
is of predation from birds or small mammals or contamination from
Avoid rough handling of seeds during cleaning. Remember that the
seeds are alive, and the embryo can be very fragile. Never use a
hammer mill in seed processing unless you have first determined
by careful testing that seed-viability is not being adversely
affected by the process.
Proper seed cleaning makes subsequent handling of the seeds in
the germination process much simpler. If the seedlot contains
trash, weed seeds, empty or obviously immature seeds, much time
will be wasted sorting the material to find germinable seeds.
Seed Storage
To avoid problems with storage insects start with clean, insect-free
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 conditions
so that the seeds remain alive but ungerminated. Remember
that the amount of water that the storage atmosphere will hold as
a vapor is directly related to temperature. The warmer the air,
the more moisture it will hold. When the temperature drops relative
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 possible
to avoid insect contamination. Some seeds can be stored easily
in small lots, but suffer losses in viability when larger quantities
of seeds are stored together. Some seeds have short
storage lives, and seed stocks of these species must be renewed
There are two common determinations that are made from seed
tests: viability and germinability. Viability simply means that
the seed is alive. It does not indicate that the seed will
germinate. Viability tests may be as simple as cutting a seed
with a knife blade to determine if an embryo is present. More
complex viability tests involve the use of a tetrazolium TZ
test. After the proper sectioning and preparation of the seed,
this chemical helps certain enzymes remove the hydrogen from the
seed during the respiration process in viable seeds. Essentially,
respiring or living tissue in the seeds is shown by a red color
That the seeds contain living tissue does not necessarily mean
the embryo will germinate. For seeds of the major crop species,
standards have been developed that relate the tetrazolium reaction
to potential germination. These standards have not been
developed for the seeds of most wildland species.
Germinability is a much more meaningful factor for individuals
interested in propagating plants from seeds. To obtain an estimate
of germinability, the seeds must be subjected to a germination
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 international
organizations for seed testing. Unfortunately, for the
seeds of most wildland species, no standard germination tests
exist. The AOSA has draft standards for about 100 wildland
species. Until these standards are accepted and/or developed for
the seeds of important wildland species, germination figures as
given on their seed tags are meaningless.
The seeds of many species will not germinate immediately after
they are harvested. They must pass through a period of dormancy
before germinating. This dormancy requirement varies with the
species and allows for certain physiological changes to occur
within the seed that make it capable of germination. This is
referred to as after-ripening and has been attributed to immature
embryos requiring post-harvest time to mature.
A variant of this type of dormancy is called temperature-dependent
after-ripening. In this type, seeds will not germinate at
one incubation temperature (usually moderate to high incubation
temperature) but will germinate at other temperatures (usually
cold incubation temperatures). Other variations include responses
to light, stratification, alternating temperatures,
leaching of growth inhibitors, and other conditions. As a practical
matter, the after-ripening requirement means the farmer has
to wait to obtain germination with the seeds of certain species.
If seeds do not germinate soon or after a reasonable after-ripening
period, the first germination factor to check is whether
the seeds take up water. This check can be made by pressing the
seed with a thumbnail or by cutting. If the interior of the seed
appears chalky and hard, water has not been imbibed through the
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 termed
"hard seeds."
To break the hard seed coats some form of scarification is required
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 be
accomplished by filing a notch in the coat or clipping so as not
to injure the embryo. Smaller seeds can be mechanically scarified
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 with
rotating drums lined with an abrasive material in which the seeds
are tumbled. Hammer mills may be used (with care), and the clearance
between the concave bars in threshing machines can be set
to just crack the seeds of legumes to obtain increased germinability.
Any mechanical scarification that increases germinability
results in decreased viability. In other words, you pay a price:
the mechanical process that gets some seeds to germinate, fatally
injures other seeds. Great care must be taken not to injure
seeds excessively with these treatments.
Thermal scarification is obtained by dropping seeds into boiling
water and then allowing the water to cool. Such treatment may
have many other influences, such as thermal shock to the embryo
or leaching soluble inhibitors. In areas that have freezing
winter temperatures, thermal cracking of seed coats can also be
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 tricky
treatment, with many side effects. The duration of treatment has
to be determined for individual seedlots. Heating from the acid
reaction along with rinse water and the resulting hydrolysis of
the seed tissue may induce germination rather than simply increasing
the intake of water as intended.
Always try to control the temperature of the acid-treated seeds
in a water bath, rinse a small amount of acid and seed in a large
volume of water, and use a neutralizing solution after the treatment.
A seed's after-ripening time cannot be shortened, but the germination
of seeds following the after-ripening period may be stimulated
by any of a variety of methods.
Seeds that imbibe water but fail to germinate are good candidates
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 duration
of stratification requirements can range from a few days to
many months. For prolonged stratification, a substrate must be
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 seeds
overnight in water and then placing the damp seeds in plastic
bags that are sealed for the duration of the stratification.
Seeds of other species require specific stratification temperatures.
Their seeds are very difficult to germinate without prolonged
The most influential factor in enhancing germinations of seeds is
the supply of nitrogen, usually in the form of potassium nitrate
In the field or nursery bed, lush growth in spring or after the
rains may be associated with the availability of nitrogen in the
seedbed. Farmers or nursery operators should have their soil or
growing medium tested for nitrogen content if possible. Nitrogen
fertilizer can be added if necessary.
Gibberellic Acid
Scientists don't know exactly how gibberellic acid, a growth
regulator, works in seed germination, but they do know that very
low concentrations of it can greatly enhance germination. Concentrations
of from 1 to 250 parts per million (ppm) are commonly
used to improve germination. Combinations of gibberellic acid and
potassium nitrate are often more effective than either material
alone. These materials can be obtained from chemical suppliers.
The potassium nitrate is more easily obtained than gibberellin.
Good measuring equipment is needed for preparing the minute
concentrations of gibberellic acid. A solution with a concentration
of 1 ppm of gibberellic acid consists of 0.001 grams of
gibberellic acid dissolved in 1,000 milliliters (ml) of water.
Gibberellic acid is sold as a 10-percent active-ingredient preparation,
which makes the weighing simpler. One alternative is
to prepare higher concentrations than needed and dilute to the
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, however,
for gibberellic acid is relatively expensive and breaks
down very rapidly at warm temperatures.
Hydrogen Peroxide
Germination of the seeds of several species, especially members
of the rose family, is enhanced by soaking the seeds in hydrogen
peroxide solutions. Dramatic germination enhancement has been
obtained with seeds of bitterbrush (Purshia tridentata) and curl-leaf
mountain mahogany (Cercocarpus ledifolius). A wide range of
concentrations from 1 to 30 percent is effective. Generally, the
higher the concentration, the shorter the soaking time, but the
greater the risk of damaging the seed. Hydrogen peroxide is a
very reactive chemical. Concentrations greater than 3 percent
are particularly dangerous to handle. Hydrogen peroxide, however,
has an advantage in that it is generally available and inexpensive.
Other Chemicals
Many other chemicals have been used to enhance germination.
These include various sulphydryl and ethylene-producing compounds.
Many seeds are sensitive to light during germination. Both light
intensity (candlepower) and light quality (color or wave length)
can influence germination. The light intensity requirement varies
with the type of seed from a few foot candles, such as that from
moonlight, to strong daylight. Germination is enhanced or inhibited
by the color or wave lengths of light. Orange to red wave
lengths (660-700 nanometers) stimulate germination while far red
or infra-red (700 or more nanometers) inhibits germination. The
impact of light rays on seed is also affected by other factors
such as the age of the seed, temperature, and chemicals present
in the germination medium. Cool-white fluorescent light enhances
germination, and incandescent light should be avoided. Seeds that
require light for germination have to be placed virtually on the
surface of the seedbed. The seeds should be pressed into the
seedbed for optimum moisture transfer.
Seeds must absorb moisture from the germination medium faster
than they lose it to the atmosphere. In a well-firmed seedbed,
optimum germination conditions can occur with proper water management.
Planting small seeds on the surface of a firmed seedbed
and covering them lightly with fine vermiculite can produce an
ideal germination environment. Moisture loss can be reduced by
shading the seedbed with large leaves or, if excess temperatures
are not generated, by covering with plastic film. These should be
removed after germination occurs to give the plants light or, in
the case of clear plastic, to prevent temperature build up.
Seeds with low germination percentages can be established satisfactorily
if a sufficient number of seeds are planted in a well-prepared
Seed production can contribute substantially to local and national
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 illustrated
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 reduce
postharvest losses and to realize the greater value from seed
crops. Germination of seeds can be stimulated by special treatments,
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 seed are
identified to make them cost effective.
                      Bibliography/Suggested Reading List
Bradenburg, N.R. Bibliography of harvesting and processing forage
     seed, 1949-1964. U.S. Department of Agriculture, Agricultural
     Research Service, ARS 42-135, Washington: U.S.
     Department of Agriculture, 1968.
Chan, F.J., R.W. Harris, and A.T. Leiser. Direct Seeding of Woody
     Plants in the Landscape. Division of Agricultural Science,
     University of California, Leaflet No. 2577, Davis:
     University of California, 1977.
Colby, M.K., and G.D. Lewis. Economics of Containerized Conifer
     Seedlings. U.S. Department of Agriculture, Forest Service,
     Fort Collins, Colorado, Wasington: U.S. Department of
     Agriculture, 1973.
Copeland, L.O. Principles of Seed Science and Technology. Minneapolis:
     Burgess PUblishing Company.
Emery, D. Seed Propagation of Native California Plants. Santa
     Barbara Botanical Garden Leaflet Vol. 1 (1964) No. 10, pp.
Grabe, D.F., ed. Tetrazolium Testing Handbook.   Contribution No.
     29 to the Handbook on Seed Testing. Boise: Association of
     Official Seed Analysis, 1970.
Harmond, J.E., N.R. Brandenburg, and L.J. Klein. Mechanical Seed
     Cleaning and Handling. U.S. Department of Agriculture
     Agriculture Handbook No. 354, Washington: U.S. Department of
     Agriculture, 1968.
Harmond, N.E., and L.M. Klein. A Versatile Plot Thresher. U.S.
     Department of Agriculture, Agricultural Research Service
     Note ARS 42-4-1, Washington: U.S. Department of Agriculture,
Harmond, N.E., J.E. Smith, Jr., and J.K. Park. "Harvesting the
     Seeds of Grasses and Legumes." In U.S. Department of
     Agriculture Seeds, the Yearbook of Agriculture, Washington:
     U.S. Government Printing Office, 1961; pp. 181-188.
Harrington, J.F. "Problems of Seed Storage," in W. Heydecker,
     ed., Seed Ecology, University Park and London: Pennsylvania
     State University Press, 1973.
Hartman, J.T. and D.E. Kester. "Sexual Propagation," in Plant
     Propagation--Principles and Practice. Englewood Cliffs,
     Prentice Hall, 1968; pp. 53-188.
Harry, E.M., and J.W. Collier, and M.J. Norris. "A Simple
     Harvester for Perennial Grass Seeds", Agronomy and Range
     Science, University of California, Davis. Agronomy Notes,
     July-August 1969, pp. 24-27.
Heydecker, W., ed.  Seed Ecology. University Park and London:
     Pennsylvania State University Press, 1973.
Larson, J.E. Revegetation Equipment Catalog. Missoula: U.S.
     Department of Agriculture, Forest Service, 1980.
McKenzie, D.W. Survey of High-Production Grass Seed Collectors.
     Project Record. U.S. Department of Agriculture, Forest
     Service, Equipment Development Center, San Dimas,
     California, 1977.
Maquire, J.D. and A. Overland. Laboratory Germination of Seeds
     of Weedy and Native Plants. Washington Agricultural Experiment
     Station Circular No. 349;  1959.
Mitrakos, K., and W. Shropshire, Jr., eds. Phytochromer. London
     and New York, Academic Press, 1971.
Nord, E.C. "Bitterbrush Seed Harvesting: When, Where, and How,"
     Journal of Range Management, Vol. 16 (1963), pp. 258-261.
Peterson, B.O. "Bitterbrush (Purshia tridentata) Seed Dormancy
     Broken with Thiourea, Journal of Range. Management, vol. 10
     (1957), pp. 41-42.
Schneegas, E.R., and J. Graham. "Bitterbrush Seed Collecting by
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     (1967), pp. 99-102.
Schopmeyer, C.S., ed. "Seeds of Woody Plants in the United
     States," U.S.: Department of Agriculture, Agriculture Handbook
     No. 450. 1974; 878 p.
Storey, C.L., R.D. Speirs, and L.S. Henderson. "Insect Control in
     Farm Stored Grain". U.S. Department of Agriculture Farmers'
     Bulletin No. 2269, 1979.
Tinus, R.W., W.I. Stein and W.E. Balmer, eds. Proceedings of
     the North American Containerized Forest Trees Seedlings Symposium,
     Denver Colorado, August 26-29, 1974. Great Plains
     Agricultural Council, Publication No. 68. Washington: U.S.
     Government Printing Office, 1974.
U.S. Department of Agriculture. Woody Plant Seed Manual. Miscellaneous
     Publication No. 654, 1948.
     Stored Grain Insects. U. S. Department of Agriculture,
     Agriculture Handbook No. 500, 1979.
Young, J.A., R.A. Evans, B.L. Kay, R.E. Owen, and Jerry Budy.
     Collecting, Processing, and Germinating Seeds of Western
     Wildland Plants. ARM-W-3. Science and Education Administration,
     U.S. Department of Agriculture, Oakland, Cal., 1981.