There is some disagreement over how to calculate chill hours. Some experts refer to the total number of hours experienced between 32 and 45 degrees F (during the dormancy period). Some offset the chill hours that occur in a 24-hour period by any hours in that same period when the temperature goes above 65 degrees F. And some give extra weight to early season chill hours (see Chill Hours Reconsidered for an interesting discussion about this issue). To complicate matters, many fruit tree suppliers neglect to mention which method is being used.

Even if we agree how to calculate chill hours, we still see some disagreement. There is a shortage of good fruit tree cultivar research in Southern California. Many fruit tree growers operating in high-chill areas do not have accurate information about the minimum chill hour requirements of some cultivars. Whenever possible, the chill hours reported in our tables reflect the experiences of local nurseries and growers.

In Southern California, there is considerable variability in climatic conditions (see Southern California Plant Climates page -- coming soon!). Determining the amount of chill hours in your area can be difficult. While there are significant variations due to local microclimatic effects, an estimate can be made based on your approximate geo-climatic location as outlined below:

Less than 200 chill hours:	Most of the coastal areas but also including inland areas or south-facing slopes with good air drainage.
200 to 475 chill hours:	Most of the inland areas up to the lower foothills but also including some of the coastal valleys.
Greater than 475 chill hours:	The mountain areas and some inland valleys.