Re: HYB: germination - chilling hours

Re: HYB: germination - chilling hours
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, could be colder, but not warmer. 

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
Soil under snow can be lower then 0C or 32F. Frozen ground will not be above 32F or 0C regardless of air temperature until ground thaws. So basically divide hours on chart by two to account for Utah Model and for soil factors of frozen ground which will not be affected by air temeratures until ground is thawed, and even then soil temperatures are almost always colder then air temperatures. I checked my records ( I keep soil and air temperatures min and max as well as air temps min and max) During most of winter soil at 4" down is 0.1C unless there is a thaw, and then it will go down quite low. This is because it does not get above surface temperature, and when ground is wet and snow cover is low, the ground will get much colder as the snow protection doesn't work.
So Linda your 1300 hours of air temp by map would likely be somewhere between 650-900 chilling hours of soil, or about 0.9 to 1.25 months equivalent to fridge chilling. 

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 chill units
1 hour >65 degrees F =-1.0 chill units

Chuck Chapman



Date: Tue, 25 Aug 2009 16:50:23 -0400
From: Linda Mann <lmann@lock-net.com>
Subject: [iris] Re: HYB: germination - chilling hours

http://www.raintreenursery.com/chill_hours.cfm
http://www.raintreenursery.com/indexpics/chill-unit-map3.gif
Does not include Canada.
<Chill hours are roughly the number of hours between the temperatures of 
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., 55oF), 
seems like the number of chill days could double.  But I've not been
able to find data for that.

Linda Mann
TN
outdoor chilling = ~1300 hrs, according to map.

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