several times with cold water to remove the acid. Stir the seeds carefully during rinsing. Decant the wa-
ter, spread the seeds uniformly on old newspaper, and allow to dry at room temperature before sowing.
Properly treated seeds are firm as little water is absorbed. The length of time for acid treatment, if
unknown, must be determined empirically to prevent seed injury. Although suitable for a small amount
of seed, acid treatment may not be practical for large quantities because of the hazards of working with
concentrated acid. Treatment with nitric acid or with other chemicals, including potassium or sodium hy-
droxide, sodium hypochlorite, hydrogen peroxide, alcohol, acetone, and various growth-regulating sub-
stances, may be effective for some seeds [2].
- Hot Water
Soaking in hot water is a treatment commonly used with hard-seeded species. Soaking softens, and some-
times ruptures the seed coat and leaches naturally occurring substances that may inhibit germination.
Pour about five times the volume of hot water (75–100°C) over the seeds and allow them to soak in
the gradually cooling water for 6 to 24 hr. The amount of swelling of the seeds will indicate the degree of
water uptake. Occasionally, seeds are boiled in water for 2–5 min; however, this procedure is apparently
injurious to seeds of most species. Also, seeds may be soaked in running water or by exposing them to
frequent changes of water to leach inhibitors.
Soaking in water may not yield as consistently good results as acid treatment [8,39]. However, soak-
ing is easier to do, is not hazardous, and needs no special equipment. Although damp or wet seeds are
more difficult to sow, they should normally be sown immediately because drying may make the treatment
ineffective.
- Other Considerations
In temperate climates, abrasion of seeds may occur from soil particles as the soil is alternately frozen and
thawed. In warm climates, seeds are ruptured by swelling. Also, organic acids in the soil or substances
and enzymes excreted by soil microorganisms soften or degrade the seed coat to some degree.
Removal of the fleshy seed coating or passage through the intestine of animals is required to over-
come dormancy of some species [8,11]. Dry heat may cause increased germination of some hard-seeded
species by rupturing the seed coat or by denaturing seed coat inhibitors [11].
Commercial seed companies routinely treat seeds with chemical disinfectants and/or hot water to
prevent infection by surface-borne fungi and bacteria. Hot water appears to be a good disinfectant.
B. Stratification
- Cold
The term stratificationformerly applied to storing alternate layers of seeds with moist sand and subject-
ing them to the cold or, more generally, freezing temperatures [43]. Nowadays, seeds are sown or mixed
in the substrate rather than in layers, although the term is still used. The major requirements for cold strat-
ification (often referred to as moist chilling or cold treatment) are adequate moisture, aeration, low tem-
perature, and time of exposure. During stratification, the levels of growth-promoting substances in the
seeds increase and those of growth-inhibiting substance decrease [44].
In temperate climates, seeds of many species of plants sown directly in seed beds, or in flats kept out-
doors, undergo natural cold stratification during the winter and are ready to germinate in the spring. Seeds
sown in flats or containers, or simply mixed with moist medium, may at any time be “stratified artifi-
cially” in refrigerators or coolers. For small lots, a plastic bag may be used. A moist medium such as sand,
peat moss, vermiculate, or a combination of these ingredients is mixed with seed and placed in the bag,
which is then sealed and placed in a refrigerator. The plastic allows gaseous exchange but keeps in mois-
ture. Once stratification has begun, seeds should not be allowed to dry because drying may reverse the
process. Therefore, the medium should be inspected periodically during the process. Seeds may also be
chilled or frozen without being mixed in a substrate.
Freezing is unessential but is sometimes recommended for certain herbaceous seeds. In general, best
results occur with temperatures from just above freezing to 5°C. Although seeds may require higher strat-
ification temperatures in milder climates, from a practical viewpoint, lower temperatures will help to pre-
vent germination while seeds are being stratified. During stratification, root radicles may emerge from the
110 CHONG ET AL.