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I looked up my town on this website and it says that I'm in zone 9b (25F - 30F average minimum temperature). However, the historical data shows that the temperature never dropped below 41F in the winter. I can also attest from living here for a year that it has never dropped to freezing temperatures. I live a block away from the bay, so the water should keep the weather more temperate. So why does my zone not match the historical weather?

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Just looking at weatherunderground for your location, it shows the temps for your city dipping down into the mid 30's early this January, and even colder temps last January. It may not happen often that the temps drop that low, but the hardiness zone is accounting for the worst case scenario.

That said, hardiness zones really only tell you about your macroclimate, and in a coastal region like you're in, that can be deceiving. It could very well be that since you are so close to the ocean, your yard never does see the low temps that someone living a few miles inland would. It is very possible that you could get away with planting something that should be a half a zone (or possibly even a full zone) warmer than what your hardiness zone says.

My yard is in zone 4B, but I have a spot on the south side of my house where I can get away with 5B plants most years. I say most years, because we had a terribly cold winter this year and my garden has yet to wake up - I won't be sure for another month whether the marginal plants made it through this time.

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USDA zones are reviewed from time to time, but the fact you're zoned as 9b should mean that, historically (I don't know how far back you went) it was possible for temperatures to drop that low. The point of zoning is knowing what the temperature you're likely to encounter in any given area will be, in particular, the coldest. There is, though, a problem with the USDA map in the west, where it fails to reflect accurately what the weather actually does (according to the National Gardening Association). Tucson and Seattle are both Zone 8, but coastal Seattle has far different growing conditions from Tucson.

Weather is unpredictable (more so in the UK) but even in 'settled' weather zones, changes are now taking place, many of which can't accurately be predicted. The chances of anyone's weather continuing to conform to any zoning map are growing less with each decade - climate change is bringing different weather to many areas.

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