Sunset Climate Zones

Everyone is familiar with the USDA hardiness zones. But have you heard of the Sunset Climate Zones? Am I the only one who hadn’t heard of this?? How did it take me this long to find out???

The Sunset Climate Zones concept was created by Sunset Magazine. It originally applied to only the western United States, but has since been expanded to cover the entire US. The Sunset Climate Zones, similar to the USDA Hardiness Zones, helps gardeners determine which plants will do well and which plants will do poorly in their area. Unlike the USDA Hardiness Zones, which uses only coldest average temperature to designate zones, Sunset Climate Zones uses hottest temperatures, humidity, rainfall, and length of growing season in addition to coldest temperature to designate different zones. There are 45 Sunset Climate Zones (1-45) as compared to to 26 USDA hardiness zones (1-13, with an “a” and a “b” for each zone). Obviously, the additional features make Sunset Climate Zones more precise than USDA Hardiness Zones. Sounds great, right?

Seattle and Portland, OR are both USDA Hardiness Zone 8b, but are Sunset Zone 5 and 6 respectively. Palo Alto is USDA Hardiness Zone 9b, and is Sunset Zone 15. Practically speaking, what do those numbers mean?

Well….. that part I’m still learning.

Seattle and Portland are the same USDA hardiness zone, but different Sunset Climate Zones. Portland, being more inland than Seattle, had slightly higher highs in the summer and slightly lower lows in the winter, which is good for certain plants that need a bit more chill to have a good fruit set, and like hotter summers.

Okay. Fine. I don’t think there’s all that much difference between Sunset Zone 5 and Sunset Zone 6 to be honest. Microclimates (whether the plants are located in a valley or on a sunny slope or next to a house) probably could be equally influential in happiness of a plant than whether it is planted in zone 5 or zone 6.

I find Sunset Zones more helpful for the Bay Area, however. Palo Alto is zone 9b, but so is Orlando, Florida, and Orlando is very different from Palo Alto. Orlando, FL is hot and humid in the summer with storms; Palo Alto is dry and mild with rarely even a drop of rain for months on end. While a Floridian could probably grow sweet potatoes in zone 9b, we would struggle to get the heat that sweet potatoes like. Accordingly, Palo Alto, in Sunset zone 15, and Central Florida is Sunset Zone 26. That’s more like it.

A little bit about my current Sunset Zone 15 in Palo Alto: Zone 15 is apparently “influenced by marine air approximately 85 percent of the time and by inland air 15 percent of the time.” Plants that do well in zone 15 like moister air, cooler summers and mild (but not tooo mild) winters.

I think, for the Bay Area in particular, Sunset Zones are much more helpful than USDA Hardiness Zones. Here, I have come to learn, lack of heat in the summer can be just a limiting in the garden as cold winter temperatures are. I’d be curious to hear others thoughts on their Sunset or USDA Zones. Do you find the Sunset Zones useful in your area?

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 (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.