Heat-zone gardening

With the continuing drought and hot temperatures, many home owners will probably notice the effects of heat on their Florida Yards.  Some homeowners may be asking, can it really be too hot for some of our plants? I recently found a way to answer this question.

The American Horticultural Society has developed a new "tool" that can help gardeners choose the right plant for the right place. Their new heat-zone map was introduced to more than 800 Extension agents and volunteers at the recent 2001 International Master Gardener Conference in Orlando. The presentation was made by AHS President Emeritus Dr. Marc Cathey is the focus of today’s column.

Cold hardiness

Many gardeners have heard of the hardiness zone map developed by the USDA. It separates the country into zones of minimum temperature averages in a given area.

It will helps gardeners choose plants for the location based on how well they will survive cold temperatures. For our area, knowing a tender palm is rated for Zone 11 means that it runs a good risk of being damaged by frost if planted in Zone 9, which is our area’s rating. USDA Hardiness Zone 9 means that, on the average, our minimum winter temperature will not go below 25 degrees Fahrenheit.

The map was revised in 1990 to reflect changes in temperature averages. Zones were subdivided into "A" and "B" zones, so 9b areas along the Indian river and Atlantic ocean would expect to see average temperatures dip only to 30 degrees.  For a peek at the map visit the National Arboretum Web site. 

Rutgers Plant Hardiness Map

However, USDA Hardiness zones provide little information on how a given plant will cope with other environmental factors. A first attempt at providing additional information on planting zones was began in the late 1980s by Drs. Mark Shulman and  Arthur T. DeGaetano of Cook College - Rutgers University. They devised a Plant Hardiness Map of 26 zones in the North American continent. Their divisions were based on an area’s frost-free days, average annual precipitation, plus January temperature minimums and July maximums.

The Rutgers Plant Hardiness Map shows three zones in Florida.  Most of the Treasure Coast falls into their zone 26, which covers the peninsula from Cape Canaveral and Tampa southward, with a dip in the central part of the state extending down to Lake Okeechobee.   These areas approximate many of the University of Florida references that describe three zones for our plants; north, central and south.

On the average, the Rutgers map states that this area has an average temperatures from 26 to 99 degrees F., 49.2 inches of rainfall, and no expected frosts. While these zones may be useful in describing our climate, there has been no rush to accept this system and determine which zone supports the thousands of ornamentals that are grown across the country.

AHS Heat Zone Map

In 1997 the American Horticultural Society published the heat zone map. It may be used in much the same way that you do the hardiness map. The county is divided into 12 zones based on the average number of days the temperature reached or exceeded 86 degrees F.  This temperature is chosen because plants begin to become damaged when they get hotter than this level. Any day above 86 is called a heat-day, and the AHS map sows how may heat days are expected at a given location.

Indian River County is divided into two zones by this map. The dividing line is approximately equal where to Interstate 95 bisects the county. East of this line is heat zone 9, which can expect between 120 and 150 days of plant damaging temperatures. West of the highway is zone 10, where they should plan on baking for 150 to 180 days.  Coastal breezes and the moderating effects of the ocean are responsible for this difference. (Note: most of Okeechobee County lies within Heat Zone 11, with 180-210 days above 86 degrees F.)

While the AHS map has been out for five years, the other essential part of the equation is now being addressed: specific heat-zone ratings have begun to appear in some newer plant reference books.  I recently picked up a new reference at a local bookstore, the AHS Great Plant Guide, which lists more than 300 plants and gives the ratings for the old USDA and the new AHS zones.

Ratings are typically found as pairs of number ranges. The first set if numbers (in increasing order) refers to the USDA Hardiness Zone, while the second set (in decreasing order) gives the heat zone rating.  Here are a few examples: 

• Red Maple Acer rubrum 3-9 9-1 This native tree is on the edge of it’s range in our area; planting it any farther south will mean it does not get enough cold temperatures for proper dormancy and it would not take the heat.
• Butterfly bush Buddlea alternifolia 6-9 10-4. This great butterfly magnet is barely getting enough cold dormancy when grown here, but likes the heat.  That’s part of why they fail to perform in many of the local areas.
• Empress tree Paulownia tomentosa 5-8 8-4. This tree has been marketed in some aggressive advertising campaigns as an "instant tree" that produces lots of flowers. It is not going to get enough cold temperatures in zone 9, and if planted here, would wilt in our heat.

The Great Guide lists plants for all over the country and emphasizes flowering plants. It will take several years to evaluate our local plants and fit them into this system.  The AHS staff was anxious to mingle with out Master Gardener as they hope to use their knowledge and expertise in developing Heat Zone ratings for plants all over the country.

If you need additional information on heat zones, computer users can visit the AHS Web site.  We can provide others with a look at the map at our office, and visit our Master Gardeners to find if your landscape plants have been rated by this new plant selection system. 

Reference:  Shulman, Mark D., and Arthur T. DeGaetano, "The Interpolation of Gridded Climatic Classification Schemes," August 1989, 14:3, 28.

Trade names, where used, are given for the purpose of providing specific information.  They do not constitute an endorsement or guarantee of products named, nor does it imply criticism of products not named. The Florida Cooperative Extension Service - Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information, and other services to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. Florida Cooperative Extension Service / IFAS /University of Florida. Larry A. Arrington,  Dean.  Last update: 10/25/2004 .  This page is maintained by Dan Culbert