"J. Davidson" wrote:
Does anyone know?

In general summer-blooming annual flowers, perennials such as lettuce,
trees and shrubs from temperate climates usually need stratification
which involves soaking and chilling (NOT FREEZING) seeds prior to
sowing. This simulates natural conditions where the seeds would remain
through a winter on cold, wet ground . Seeds will usually germinate
promptly and uniformly after stratification. Unstratified seeds may take
up to two years to germinate, if they do so at all.

Many tree seeds have what is called an "embryonic dormancy" and
generally speaking will not sprout until this dormancy is broken.
In the wild, "seed dormancy" is usually overcome by the seed spending
time in the ground through a winter period and having their hard seed
coat soften up a bit.* By doing so the seed is undergoing a natural form
of "cold stratification" or pretreatment. This cold moist period
triggers the seed's embryo, its growth and subsequent expansion
eventually break through the softened seed coat in its search for sun
and nutrients.

In its most basic form, when we control the cold stratification process,
the pretreatment amounts to nothing more than subjecting the seeds to
storage in a cool (not freezing) and moist environment for a period
found to be sufficient for the species in question. This period of time
is often and usually found to be somewhere between 1 through 3 months.
The crisper in a refrigerator is usually ideal.

The amount of time needed for cold treatment varies. Seed of cedar of
Lebanon, which will stay viable for six months, only need two months for
stratification. Plants like alder and chestnut need three months;
shadblow, horse chestnut and shellbark hickory need four months. A few
plants need both a warm and a cold period of stratification: cotoneaster
bearberry, dogwood yews and some of the barberries and junipers. Some
lilies, viburnums and the tree peony all need a spell of moderate
temperature followed by cold. Unless kept in the refrigerator such
seeds are planted during the summer, kept moist, and allowed their cold
dormancy during the following winter.

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