HYB: cold stratification

I have always heard that iris seeds need a cold period and lots of water to
germinate. Where did the "cold" part originate?  Going on the assumption that
this was gospel, I have almost always planted my seeds in mid to late December
and set them outdoors to let nature take its' course. Here in southwest Ohio
that means lots of chilly temps followed by lots of cold temps followed by
more chilly temps and copious spring rain. The reward has been an average of
60-65% germination, with the bulk of germinants appearing from very late April
until mid-May. Germination slows or stops when temps start hitting 75 or so
daytime. I veered from this methodology one time, because of seeds I did not
have until the middle of March. I tied them in the cut-off leg of some old
pantyhose and hung them in the toilet tank for about 3-1/2 weeks. They swelled
up quite nicely and I planted them in pots as usual and set them outside in
the middle of April with the rest of my wintered-over seeds. They germinated
with the rest and were above 90% germination. They got no cold, but did get
plenty of water.

So where does the wisdom about cold come in? Is it because of what passes for
"cold" varies from area to area? We do not generally consider temps above
freezing here cold. My brother-in-law lives in San Jose and thinks 60 degrees
is cold. He has a heavy jacket here when the rest of us ar running about in
shirt sleeves. I am not familiar with temperatures for the continental USA,
but it strikes me that the folks in Southern California, at Sutton's, up at
Superstition, or down where Walter is in Mississippi see much of what we
consider "chilly", yet they hybridize.

I can tell you that freezing probably reduces germination here in the midwest,
because pots of seeds do freeze and then stand in water sometimes, causing
seeds to rot. Late freezes probably also nail their share of
germinated-but-not-yet-sprouted seeds. Anyway, the toilet tank method is a
real pain so I doubt I change anything with how we do seeds here. There are
always too many seedlings to transplant and care for anyway.

John Bruce
SW OH, zone 5b

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