Hardiness Zones

Dec 6, 2014
San Carlos, CA
I think one of the most important questions regarding climate is whether one can grow lowland or highland tropicals outside all year. Growing outside for part of the year is also important.

The regular USDA zone designation is not very useful when answering the question.

San Francisco is the largest major city in the U.S. with an all-around good climate for highland tropicals.

Miami is the largest major city on the mainland U.S. with a reasonable climate for many lowland tropicals.

San Francisco (most parts) and Miami are both USDA zone 10b.

The average high temperature in San Francisco in July is 67 F. The average low temperature in Miami in July is 77 F. There's no overlap in January, either.

The Sunset zones are somewhat useful when answering the question. San Francisco is Sunset zone 17, and (especially if frost free) areas in this zone tend to be great for highland tropicals. Miami is Sunset zone 25, and other areas in this zone are equally good for many lowland tropicals. Similarly, those climates that approximate either of these two zones are the best other bets for highland or lowland tropicals, respectively. However, it's a complicated question and it depends on the plant, and, especially in California, how close one is to the ocean and other factors.

Growing outside part of the year is also important for many. For example: D.C.'s summer climate is one that lowland tropicals are often happy with; Seattle's summer climate is one that highland tropicals often enjoy. And so on.

If this question has not been addressed in this forum in detail it should be. I find that it really confuses a lot of people. I'll try to post a separate thread, soon.
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BS Bulldozer
Apr 11, 2013
Another aspect worthy of consideration are microclimates, be they natural or man-made. From a cursory look SF seems to be the poster child for neighborhood sized microclimates, but smaller ones are created by buildings and landscaping. I used to grow a zone 8 hardy Rosemary bush each year in my zone 6 b gardens over the years. Invariably the plants seemed to survive the winter, but crapped out just as things warmed up. Until I moved into my present home. I put one in a 2'x2' square of soil in the inside corner created by the intersection of a black asphalt driveway and a black slate patio. It survived 7 winters, finally done in by last winter. I have no doubt that all that black hard surface was the key, both for heat absorption/retention, and the relative dryness caused by that patch of soil being isolated from areas where water soaks into the ground.