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Re: HYB: germination - chilling hours

  • Subject: Re: HYB: germination - chilling hours
  • From: irischapman@aim.com
  • Date: Wed, 26 Aug 2009 07:02:56 -0400


This chart is neat. But it is based on model that all hours between
32-45 are equal in terms of chilling effect. Research on this has
shown that not all temperatures are equal in chilling effect. The Utah
model seems to give better data and is more consistent with actual
biological process. As mentioned, the colder it is , the slower the
biological process, and the warmer it is, the slower the accumulation
of "Chilling hours" Below is the Utah model method of calculation.
Using this model, the chilling hours in each local is much lower then
in chart. Also the Utah model gives a much better prediction in regards
to flower bud set in fruit trees. Also snow cover will reduce soil
temperature in area of soil, just below snow to around freezing area,
0C or 32F, as long as snow cover is at lest 6". Otherwise it will
fluctuate with air temperature, but be modified.

So number of chilling hours in soil will again be lower then map.

30 X 24 = 720 hours of chilling for one month, 2 months = 1440, three
months =2160, four months = 2880

I'm sticking by 2 months of fridge chilling is all that is needed.

Utah Model
1 hour below 34 degrees F =0.0 chill unit
1 hour 35-36 degrees F =0.5 chill units
1 hour 37-48 degrees F =1.0 chill units
1 hour 49-54degrees F =0.5 chill units
1 hour 55-60 degrees F =0.0 chill units
1 hour 61-65 degrees F =-0.5 ch
ill units
1 hour >65 degrees F =-1.0 chill units

Chuck Chapman

Re: HYB: germination - chilling hours

Posted by: "Linda Mann"

Tue Aug 25, 2009 1:52 pm (PDT)



Does not include Canada.

<Chill hours are roughly the number of hours between the temperatures

32-45 degrees fahrenheit. Winter hours above 60 degrees are subtracted

from the totals.>

These numbers are air temperatures for fruit trees. I'm not sure how

this relates to seeds in dirt. Might be more if in shade & under

reflective mulch, less if exposed. Map is a little hard to read, but I

think 3400 hrs (~= 4.7 months of continuous chill in a fridge) is the

most shown on this map. In northwestern Washington, extending into the

farthest northwest corner of Oregon. Looks like Salem might be around

2800 hrs or (ta da) 3.8 months.

If vernalization of seeds can occur at warmer temperatures (i.e.,

seems like the number of chill days could double. But I've not been

able to find data for that.

Linda Mann


outdoor chilling = ~1300 hrs, according to map.

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