For people living in California, the University of California
provides chilling accumulation models with data for many locations
throughout California:
http://fruitsandnuts.ucdavis.edu/Weather_Services/…
This information is provided for fruit and nut growers but is very
useful for growers of other species requiring vernalization and/or
stratification. Other universities likely provide similar
information. This data is mostly limited by the number of
locations. The amount of winter chilling varies greatly in the Bay
Area between years and especially by location. For example, If you
look at the data for the "Oakland Foothills" which is the closest
station in Alameda County most similar to the University of
California Botanical Garden, there have only been about 450 to 900
cumulative chilling hours per year since 2004. However, if you live
30 miles away in Pleasanton (further inland), there have been about
800 to 1200 cumulative chilling hours per year since 2004. With such
huge annual chilling differences, successful flowering of many
species requiring "more" vernalization will undoubtedly vary
significantly from year to year in marginally cold locations like Berkeley.

From a cumulative chilling perspective, this past winter has been
one of the coldest in the Bay Area in years. So far, I have seen
this reflected in both earlier, more uniform flowering times for
bulbs that generally require more vernalization (Tulips) and
outstanding germination rates for species (Calochortus, Fritillaria)
planted late.

Nathan

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