Alcatraz Island Hardiness zone

The chilly weather this week has many Californians bundled up inside. The temperature on Alcatraz is moderated by the San Francisco Bay and we are very fortunate to be frost free; but the dampness, wind, and fog can still make any time of the year feel cold. San Francisco is in United States Department of Agriculture (USDA) Hardiness zone 10 and zone 17 to residents of western U.S.A. who refer to the Sunset climate zones.

The USDA revised the plant hardiness zone map in 1965 and again in 1990. The 11 zones aim to show the lowest temperatures that can be expected. Plants are rated for which minimum zone they can survive in.

Sunset Magazine created a 24 zone climate map for the thirteen Western states. These zones are much more detailed and precise as “they factor in not only winter minimum temperatures, but also summer highs, lengths of growing seasons, humidity, and rainfall patterns to provide a more accurate picture of what will grow there.”(

Micro climates exist even on a small island like Alcatraz. This week, with the wind coming from the north, there are a few gardens that volunteers and staff gravitate towards on chilly days. The southwest facing inmate gardens that usually get the brunt of the summer winds and fog are the perfect gardens to bask in during the winter sunshine. The low winter sun casts a chilly shadow on the east side of the island for most of the day.

West side Toolshed Terrace gardens in sunshine. Photo by Shelagh Fritz

East side of the island in mid-day shade. Photo by Shelagh Fritz

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