On 11/16/10 8:24 AM, gardengal wrote [in part]:

Chilling hours are determined by the cumulative hours between 45F and
32F occuring typically between November and March (end of February) or
during the normal period of dormancy.

There are varying definitions of "chilling hours" , "chilling units",
"chilling degree hours", or "chilling accumulation". See, for example,
http://aggie-horticulture.tamu.edu/stonefruit/chillacc.htm by Texas
A&M and http://fruitsandnuts.ucdavis.edu/chillcalc/index1.htm by UC
Davis.

Furthermore, I saw a mail-order catelogue for bare-root fruit trees that
gave different definitions according to the kind of fruit (e.g., apples
vs peaches vs cherries). That is, for some fruits, "chilling hours"
involve total number of hours between 40F and 32F while, for other
fruits, it's the number of hours between 50F and 32F.

I have also seen a definition that multiplies the number of minutes by
the number of degrees below 45F (but not lower than 32F) at that time
and then divides by 60. This gives more chilling hours for an hour at
32F than for two hours at 40F. The Utah definition uses temperature
bands to simplify such computations.

Because sap contains sugars and other dissolved substances, it doesn't
freeze at 32F. Thus, some definitions have the computation extend down
to 25F or even 20F; and some definitions do not involve any minimum
temperature. Since different plants have different concentrations of
dissolved substances in their sap, this could explain why the definition
of "chilling hours" might vary for different plants.

--
David E. Ross
Climate: California Mediterranean
Sunset Zone: 21 -- interior Santa Monica Mountains with some ocean
influence (USDA 10a, very close to Sunset Zone 19)
Gardening diary at http://www.rossde.com/garden/diary