Zones, Microclimates and Chill Hours
Written by Kenneth Happel
The next piece of the puzzel needed to make a garden plan is to know the climate and even the specific microclimate that you live in.
The Basic Climate Zone
So, in the USA, there is the USDA climate zone system and in the western USA there is a second based upon the Western Garden Book published by Sunset Magazine. Many outside the western USA use the Western Garden Book system by finding which of its catagories best matches their home because the book then provides a mountain of advice specific to locating and caring for plants suitable for your zone.
The USDA Plant hardiness zones can be found at:
The USDA Plant Hardiness Zone map was developed using minimum temperature data and was most recently revised in 1990. It represents the lowest average temperatures that can be expected in that broad area based on a 12-year average (1974-1986). There are 10 different zones rated for plant hardiness and an additional zone that is essentially frost-free. The 1990 map also divided zones 2-10 in half to denote 5-degree differences in average minimum temperature rather than the old 10-degree differences.
The Western Garden Book considers minimum average temperatures but also gives equal weight to average maximum temperatures. They also divide the map more finely considering latitude, elevation, ocean influence, continental air influence, mountain ranges, and local terrain. For those that live in the Continental West Coast, Alaska or Hawaii the Sunset "Western Garden Book" The climate zone definitions can be found in the book.
For our worldwide friends of the mulch or those without the book, you can go to:
to get a description of each of the zones in the Sunset Guide. Then go to
and enter your city name, or if your town is too small use the county or provincial seat, which in my case is "San Diego." A list appeared and I selected "San Diego County 32°N 117°W" and a list of towns appeared. But not mine. At the bottom is a control that says, "Navigate." I selected, "Northwest" and, BINGO, there in "Climate Data for 33°N 117°W" were the monthly average minimum and maximum temperature, the 24-hr average temperature and the average rainfall for three measuring stations in Vista. Fit your maximum and minimum temperatures and rainfall into the catagories from the Western Garden Book or the USDA and you have a general climate guide.
Finding Detail on Your Microclimate and its Chill Hours
More detail is required if you need to answer questions like, "Can I grow an asian pear or a specific apple tree in Vista?" Many plants require cold in the winter to trigger their growth cycles. The term "Chill Hours" is used to describe these cold winter hours necessary for triggering plants.
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. Others give extra weight to early season chill hours (see Chill Hours Reconsidered for an interesting discussion about this issue).
I like and used the formula Richard Frost relates in a forum here in the mulch community: "To compute chill hours in your yard: Between Nov. 1 and Jan. 1, the # of hours below 45 °F minus the # of hours above 65 °F. Note that this date range only contains a total of 61×24 = 1464 hours."
Now, all I need is to find hourly data, and sure enough a worldwide source of detailed climate data (including hourly) can be found at the National Climatic Data Center:
Hourly temperature and percipitation data for the nearest measuring station in Vista cost $20.00 for data covering Sept. 1999 to March 2007. For those in California there is a list of chillhours for California Counties at
In my case my sunset zone is 21, my USDA zone is 9b and my chill hours are: 475-575 hrs per year.