For more than 40 years, the zone system has been the building block of home gardening. Books, magazines, nurseries, seed catalogs and many plant identification tags describe plants by using growing zones. Gardeners rely on those numbers to help them choose plants that will thrive in their gardens year after year.
The Tampa Bay area is in Zone 9, but that could change early next year when the U.S. Department of Agriculture completes its analysis of 30 years of weather data. It's likely that our area will be classified as a warmer, more tropical zone, which means we can plant more sensitive tropicals, such as the coconut tree (Cocos nucifera L.), with confidence. That also means more cold-hardy plants, currently rated up to Zone 9, technically would be out of our zone. Those include milkweed (Asclepias), butterfly bush (Buddleia) and Carolina jasmine (Gelsemium), which are commonly planted here with success.
As a master gardener for Pinellas County and a gardening enthusiast since my childhood morning glory and zinnia days, I've planted gardens from zones 5 to 8 (that's Maine to Georgia), but it's Zone 9 (St. Petersburg) that I call home. As a new garden writer for the St. Petersburg Times, I will focus on gardening topics of local interest _ from plants to products to people. We have a thriving gardening community with plenty of resources and advice. I welcome your gardening questions and if I can't answer them, I'll find an expert who can.
Let's start by getting into the zones. The tried-and-true U.S. Department of Agriculture plant hardiness zones are based on a plant's ability to survive cold temperatures. Then there are the new American Horticultural Society's plant heat zones, based on a plant's ability to survive the heat. It's not rocket science, but it does add a new, somewhat confusing dimension to gardening.
The USDA's first map was published in 1960 and updated in 1990 to help gardeners determine which plants would "winter over," based on the lowest average temperatures recorded. Zone 9 was subdivided into two sections that represent a 5-degree variation.
According to the USDA, the lowest temperatures in Pinellas County, the western portion of Hillsborough County and most of Manatee County are 30 to 25 degrees. In most of Citrus, Hernando and Pasco counties, most of Hillsborough County and the northeastern part of Manatee County, lows can drop to 25 to 20 degrees. The variation in temperature is the result of our area's microclimates. St. Petersburg and Tampa are warmer because buildings and concrete radiate heat. The same is true for waterfront communities, primarily the gulf beaches and locales on Tampa Bay.
In theory, if our winter temperatures stay at or above those temperatures, our plants _ those that are rated up to Zone 9 _ should come through winter just fine (assuming you are giving your plants proper care). If temperatures drop below those ranges for an extended time, some plants will be damaged and some will die.
At the other extreme, too much heat can zap plants. That's why the AHS developed the heat zone map. It assigns zones based on the average number of days each year with temperatures over 86 degrees. Most of our area falls into Zone 10, with 150 to 180 days each year over 86 _ the "magic" temperature at which plants begin to suffer damage from heat, according to the AHS.
Garden centers, books and catalogs already are combining these ratings into a four-zone coding system. So, when selecting a plant, you would check whether it would survive the winter (plant hardiness zone) and summer (heat zone).
For example, society garlic (Tulbaghia), which is a low-growing evergreen with clusters of white or violet flowers), is cold-hardy from zones 9 to 11 and heat-tolerant in zones 12-4, making it a good choice for our area.
Experts at the Pinellas County Extension Service in Largo have adopted a wait and see attitude on the zones and haven't completely embraced the heat zone concept, says horticulturist Michael Pettay.
His advice? Don't run out and buy plants such as the tropical almond tree (Terminalia catappa L.), which grows well from Sarasota to the south. Likewise, don't dig up plants that are doing well just because they might not meet new zone requirements.
If you're a new gardener or a recent transplant to Florida, remember your zone when selecting plants. A common mistake is putting in a garden like the one you had up north with perennials such as delphinium and forget-me-not (Myosotis), which are rated to Zone 8.
"The poor things (plants) might survive a season or two, but then they will die," Pettay warns. You also should not waste your money on tulip, daffodil or crocus bulbs this fall. Those plants require chilling temperatures to flower and thrive, and it's much too warm in Zone 9. Those are best enjoyed as indoor potted plants available seasonally.
One last note: Every garden is different, and yours probably has its own microclimates. There may be warm, protected spots next to your house or fence and in a courtyard where sensitive tropicals will thrive. Or you may have beds facing north that could be the perfect home for plants that require cooler temperatures.
Yvonne Swanson is a freelance writer in St. Petersburg and a master gardener for Pinellas County. If you have a garden questions, e-mail floridiansptimes.com (put Garden in the message window); or write: Yvonne Swanson, Garden Writer/Floridian, St. Petersburg Times, P.O. Box 1121, St. Petersburg, FL 33731.
TO LEARN MORE:
To learn more about the AHS heat zone map, log on to www.ahs.org or read Heat-Zone Gardening, published by Time-Life Books.