UF/IFAS Okeechobee County Extension Service

458 Highway 98 North

Okeechobee, FL 34972-2578

Phone: (863) 763-6469

E- mail:  dfculbert@ifas.ufl.edu

July 5, 2007

Feature Article - for release the week of  July 8, 2007

Dan Culbert - Extension Horticulture Agent 

 Gardening in the Zone

Popular media are having a field day with stories on global climate change.  Is it really getting warmer or is it just a roll of the climatic dice?  With recent drought and hot humid temperatures, many home owners may feel the effects of heat on their Florida Yards.  Some may be ask, "Can it really be too hot for some of our plants?" 

Heat is just one thing to consider when choose the right plant for the right place.  The traditional USDA Hardiness zone map now has a mate that talks about the upper end of the thermometer.  The American Horticultural Society’s heat-zone map was introduced ten years ago.  Today’s column will outline some climatic maps and how we can use their zones to find plants that work best in our area.

Cold hardiness

Most gardeners have heard of the hardiness zone map developed by the USDA.  It separates the country into zones of average minimum temperatures in a given area.  For a look at the map, visit the National Arboretum Web site, http://www.usna.usda.gov/Hardzone/index.html.

Zones can help gardeners choose plants based on how well they will survive cold temperatures. The currently accepted map was revised in 1990 to reflect changes in temperature averages.  Zones were subdivided into "a" and "b" zones. 

Most of the area north of Lake Okeechobee is designated as zone 9b.  This means the areas would expect to see average temperatures dip only to 30 degrees F.  A small portion of the north-western area of our county is in the 9a zone, where average temperatures are expected to reach down to 20 to 25 F each winter.

Many plants have been rated for their cold hardiness based on these designations.  For our area, knowing a tender palm is rated for Zone 11 means that it runs the risk of being damaged by frost if planted in Zone 9.  Is this important?  Just ask some of our nursery growers that planted a number of Foxtail palms last fall, only to have them turn brown when frost hit last winter.

So are we warming up?  The American Horticultural Society has a 2003 draft map that takes a close look at minimum temperatures from 1987 to 2001; it moves much of Okeechobee County into Zone 10.  The 2006 National Arbor Day Foundation has moved the county in a similar transition into a warmer microclimate, based on the last 15 years of temperature data.

It’s More than Minimum

USDA hardiness zones provide little information on how a given plant will cope with other environmental factors.  An attempt at providing a planting zones map began in the late 1980s by Drs. Mark Shulman and Arthur T. DeGaetano of Cook College - Rutgers University.  They devised a Plant Hardiness map and came up with 26 zones in North America. Their divisions were based on humidity and temperatures.

While these zones may be useful in describing our climate, there was no rush to accept this system and determine which zone supports the thousands of ornamentals that are grown across the country.

Gardeners and landscapers have also used the Sunset Garden Climatic Zones.  These zones are based on the length of growing season, the timing and amount of rainfall, winter lows, summer highs, and humidity.  Sunset Zone Map fits the zones to the plants, not the plant to the zone.  Their maps place us in their Zone 26, Central and Interior Florida.  The National Gardening Association has a similar zone map.

How Hot will it Get?

Of particular interest to our part of the country is the AHS heat zone map. It was first published in 1997 and is used in much the same way as the hardiness map – it can help with plant selection.

The country is divided into 12 zones based on the average number of days of temperature at or above 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. The AHS map shows how many heat days are expected at a given location. While many plants will survive outside of their heat zone, they may not thrive out of zone.

Central Florida is divided into two zones by the AHS map. Okeechobee, Highlands and Glades Counties are primarily in heat zone 10-11, where plants receive 180 to 210 heat-days a year. 

The AHS specific heat-zone ratings have begun to appear in plant reference books.  Ratings are typically found as pairs of number ranges.  The first set of 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 of plant ratings taken from the book,  Heat-Zone Gardening:

• Lily of the Nile Agapanthus: 8-11, 12-1.  This beautiful blue flowering perennial grows from a bulb.  I found it in growing in Costa Rican landscapes, but would bring the pot inside in a Carolina winter.
• Columbine Aquilegia: 4-7, 9-3.   This northern wildflower may take the heat of our area, but needs the colder temperatures of the Florida panhandle to live as a wildflower.
• Lilyturf Liriope: 5-11, 8-3.  According to the ratings this frequently used Central Florida groundcover should not thrive here – but it does!
• Golden Rain Tree Koelreuteria:  5-9, 8-5.  The ratings say this would not be well adapted, but this weedy tree grows all over our area and appears to be thriving locally.  
• Myrtle Myrica: 2-9, 8-1.  The southern Bayberry or Waxmyrtle is a common native shrub in our area, and despite the ratings is well suited to our hot summers.

As seen in some of these examples, the process rating the heat zones for plants is evolving.  Nearly 500 ornamentals were initially rated by the AHS, and thousands more have since been added.  It will take a number of years before plants with a hardiness zone rating will have a corresponding heat zone equivalent.  Advice on correct ratings is also being requested.

Remember also that factors such as water, light, soil and oxygen can also affect plant growth.  And micro-climate differences also make ratings less that accurate.   But, the next time you look at a plant tag, look for our hardiness zone (9-10) and the heat zone (11-10) for the right plant for our area.

I’ve placed more information on our Okeechobee web page, http://okeechobee.ifas.ufl.edu.  If you need additional information on heat zones, please email us at okeechobee@ifas.ufl.edu, visit the AHS website, or call us at 863-763-6469.  Local residents can stop by our office at 458 Hwy 98 North in Okeechobee, and visit our Okeechobee County Master Gardeners from 1 to 3 PM on Tuesday afternoons. GO GATORS!

 

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: 07/05/2007.  This page is maintained by Dan Culbert