Stratification

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Most woody plants produce seed that will not germinate immediately after
harvest. Normally this is because of one of three reasons:
– Seed is immature and needs more time to develop;
– A mechanical barrier is keeping water from reaching the seed;
– A physiological block is inhibiting germination.
Immature seed needs time to complete development and does not require
special treatment. The water barrier and/or physiological block require
special treatments to prepare the seed for germination. One such
treatment is stratification. Stratification is a process whereby seed is
given the moisture and temperature conditions normally found in its
natural environment. Seeds that are shed in early fall often require a
warm, moist stratification period before the seed will germinate. Those
that drop later in the fall may respond to cool, moist conditions. In
Kansas, the most common stratification needed is the cool, moist type.
The amount of time required for stratification varies with the plant
species. For example, apple requires 75 days, red oak needs 30 to 45
days and sugar maple should have 60 to 90 days. All three of these
species require cool, moist conditions. If unsure of the amount of time
required for a specific species, 3 to 4 months usually is sufficient.
For cool stratification, temperatures just above freezing are best, with
a range between 35 and 45 degrees considered ideal. Temperatures higher
and lower than this are less effective. The minimum temperature at which
stratification occurs is reported to be 23 degrees, and the maximum is
62 degrees.
Stratification should be done in a medium that is moist but not soggy.
If there is too little moisture, the seed coat does not take up the
water needed. Too much reduces the amount of oxygen available to the
seed. If peat moss is used, a ratio of 1 or 1 1/4 parts water to 1 part
air-dried peat moss by weight is recommended. When wetting peat moss,
use warm water, which is absorbed more quickly than cold.
Small amounts of seed can be stratified by placing the seed in moist
peat moss inside a plastic bag and placing the bag in the refrigerator.
Small seeds can be placed between two sheets of cheesecloth so they are
not lost in the medium. Larger amounts of seed can be placed in a
plastic container or wooden box. Place layers of seed between layers of
moist sand or a mixture of sand and peat moss. Bury the container
outside so the top is even with the soil surface, and cover with leaves
or straw. Alternatively, the container may be placed in an unheated
garage or root cellar. (Ward Upham)

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