What’s the Difference between Zone 8b and Zone 9b?

As I’ve said before, the scenery in California is very different from that of Seattle. I moved from zone 8b to zone 9b. Doesn’t seem like it should be that big of a difference, right?

The current USDA hardiness zones were determined by the annual lowest winter temperature, averaged over the past 30 years. (Actually, the current USDA zones are based on the average from 1976-2005.) Zone 8b has an average minimum temp of 10-15 deg F, whereas zone 9b has an average minimum temp of 25-30 deg F. Those are both below freezing, but 9b is just barely below freezing.

First and last frost dates mirror USDA hardiness zones pretty closely. The average first frost date for my area (according to the Farmer’s Almanac) is Nov 29th, and the last frost date is Feb 22nd. Palo Alto has an essentially year-round growing season. In Seattle, our first frost was Nov 16th, and our first frost was Mar 17th. That’s only a few weeks of difference.

These changes are minor, but they do have a significant impact on the plants that can survive the winter. Here in zone 9b, citrus trees and certain other tropical fruits are grown outdoors, whereas in Seattle, they would need to be taken indoors in the winter, or otherwise protected.

The Meyer lemon

Interestingly, though, some fruit trees actually need colder weather in order to produce fruit. Some apples and pears need a freeze in order to set fruit. Here, in California, we have “low chill” varieties, which don’t need to be cold in the winter in order to set fruit. “Chill hours” are the the cumulative number hours during the winter when the temperature is below 40 degrees, but above 32. Low chill varieties need fewer chill hours than normal trees, often under 500 hours (less than 20 days below 40 degrees).

Temperature aside, I think the main reason zone 9b looks so different from zone 8b is rainfall. The data below is from bestplaces.net (I can’t vouch for it’s accuracy; the average July high seems a little suspect).


In the above chart, “Rainfall” is the total number of inches of annual rain fall and “Precipitation” is the number of days with measurable rainfall. Seattle gets 50% more rain than SF, and has over twice as many rainy days. All that rain means lush green plants, something I sorely miss in the dry dusty desert of the Bay Area.