Re: HYB: cold stratification

John Bruce <jbruce1@cinci.rr.com> wrote:
  I have always heard that iris seeds need a cold period and lots of water to
germinate. Where did the "cold" part originate? 
   
  I am not as familiar with the iris litterature as amny on this list.  But I have done research on germinating seeds of wild perennials including several species and hybrids of sunflowers, curly dock (Rumex crispum), eastern gama grass (Tripsicum dactyloides, and several others.  And I have had experience starting various trees from sed for fruit and for bonsai.
  And I have done litterature searches on stratification.
  Generally, freezing isn't necessary or even good for breaking dormancy.  Stratification generally takes place most rapidly between 3 to 5 C, or just above freezing.  A warm spell before stratification is complete can reverse stratification in some species, eg. plum.
  Chet Dewald, a scientist who studied Tripsicum dactyloides would culture ripe steriled seeds on agar for a couple of weeks.  Those that started to germinate would be potted up, others were put in a refridgerater for a month and warmed for two weeks.  Again the seedlings were removed and the cycle continued.
  Some seeds would require no stratification, others required one or several cycles.  The mother plant had a significant effect on seed's need for stratification.  The requirement was partially heritible.  Selection for quick germination with no stratification was effective.  
  My experence with perennial sunflowers was similar.
  If all iris breeders selected only those seeds that germinated quicky upon ripening, we would soon have better germination, with the cost of having many lines die out.
  I'm sure we wouldn't make that tradeoff.
  But if one iris breeder is selecting under one set of conditions and another breeder selcting under another set of conditions, selection for better germination is inconcistant, and it is small wonder that some lines don't germinate well.  
  Especially when there are are several species with diverse origional climates involved.
  For now, we need to learn from each other what works, as we are doing here.  And do what works for us with our lines in our climates.
  As we compare different lines in different climates, we'll make progress, or those who come after us will, progress both in iris for the public, and for iris that are easier to start from seeds.
  As an aspiring arilbred breeder, I am amazed that the system for starting them is as well worked out as it is.
  Walter
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