Florida Hardiness Zone Maps
As described elsewhere on this site, national Hardiness Zone Maps have experienced several significant revisions over the past half century. This is clearly demonstrated by comparing the various attempts over this period to delineate between Zones 9 and 10 in Florida.
| Let's start with the 1960 USDA Hardiness Zone Map. The image on the right is from the 1965 update to the map. |  |
| This map of Florida is taken from the 1990 USDA Plant Hardiness Zone Map. Notice how Zone 9 (the brown and beige areas) has moved further south. In the map above Tampa and St. Petersburg are in Zone 10, while Orlando is in Zone 9. In the 1990 Map, Tampa and St. Petersburg are in Zone 9b along with Orlando. This movement south prompted some to speculate we were in a new ice age! |  |
| In the 1995 Florida Climate Center update to the 1990 USDA Map St. Petersburg is in Zone 10a, Tampa is just over the line in Zone 9b and Orlando is squarely in the middle of Zone 9b. |  |
Surprisingly, the 2003 AHS Draft Hardiness Zone Map looks closer to the 1960 USDA Hardiness Zone Map. Zone 9 has retreated northward. Tampa and St. Petersburg are back in Zone 10, while downtown Orlando is in Zone 9. Zone 9 does show two small heat zones that defy an easy explanation - one is southeast of Orlando and one is south of Lakeland. Neither site is in a heavily populated urban area that would give off residual heat. |
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| 2004 Arbor Day Map shows that Zone 10 continues to creep northward. |  |
The Arbor Day Map was updated in 2006. Even in two short years you can see differences. For example, Zone 10 is shown moving inland from the Tampa Bay area toward Orlando. From 1990 to 2006 the Tampa Bay area has moved from being squarely in the middle of Zone 9 (USDA 1990 Map) to now being well within Zone 10. |
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So what does this all mean? No one knows for sure. For example, depending on which map I choose, our house in Lithia, Florida near Tampa is in either Zone 9A, 9B or 10. If I look just at the current USDA map, St. Petersburg and Lithia are in Hardiness Zone 9A, yet even a casual comparison of plants that thrive in those two locations shows significant differences.
My personal opinion as to the Tampa Bay region - St. Petersburg rightfully belongs in Zone 10. If you drive around the residential neighborhoods in that city you will see tropical plants in great numbers like Bougainvillea and Jacaranda, plants that do not thrive as well across the Bay in Tampa or in Orlando. In the winter, the suburbs of Tampa and all of Orlando experience frosts, while St. Petersburg is often spared. I would put Tampa on the margin between Zones 9 and 10 and Orlando in Zone 9.
As further support, I went out to the NOAA website for the Tampa Bay Region and pulled down the annualized data for the number of days each year with temperatures less than 32 degrees. If you compare the data for Tampa and St. Petersburg you can see a distinct difference between the two cities, even though they are only 15 miles apart. From 1948 to 2005 St. Petersburg, on average, experienced freezing temperatures once every two years (.5 days/year), while Tampa was nearly five times more prone to freezing temperatures (2.3 days/year).
But what of the future....