New USDA map puts Palm Beach County in warmer zone for plants

A new map that tracks three decades of weather conditions has placed all of Palm Beach County in the mildest temperature zone in the continental United States except for the Florida Keys.

The plant hardiness zone map, based on 30 years of temperatures, was unveiled last month by the U.S. Department of Agriculture.

Many areas of the country are designated as warmer by about 5 degrees, assuring gardeners that it's OK to install plants that, based on the 1990 map, might not have survived.

"For the gardening enthusiast, this is a big deal," said Jennifer Nelis, spokeswoman for the Florida Nursery, Growers and Landscape Association in Orlando.

The map zones are based on the average lowest temperature of the year in each area. Much of northern and western Palm Beach County was previously in zone 9b, with average lowest temperatures of 25 to 30 degrees, while a sliver of the coast was 10b, with average lows of 35 to 40. Other areas were 10a, with lows of 30 to 35.

But now most of the county is in zone 10a. Zone 9b has dropped off our map. Wellington, for example, is in 10a. Near the coast, cities are in 10b. The Florida Keys are in zone 11b.

The map, a $500,000 project of the USDA and Oregon State University, is just one tool that helps the nation's 80 million gardeners choose plants that will thrive. Consumers can enter a ZIP code and check their zone on the map at planthardiness.ars.usda.gov. Zoom in and major highways appear.

But to anyone who thinks the map is proof of global warming, USDA Agricultural Research Service spokeswoman Kim Kaplan said that supposition is just plain wrong. It's only a more sophisticated plant map designed to provide horticultural guidance, not proof of lasting temperature change, she said.

The map tracks weather, not climate, and looks only at the average yearly lows, not overall temperature changes, Kaplan said. It doesn't reflect the coldest it has ever been or ever will be at a specific location.

"It is not a good instrument for trying to prove global warming. It does not cover a long enough period," Kaplan said. "Climatologists like to talk in terms of a minimum of 50 to 100 years. We only have 30 years here."

The temperature data in the new map are similar to that in the 1960 version, which also trended warmer. The 1990 map, however, covered a cold period that some people suggested was the start of a new ice age, Kaplan said.

The latest map is based on average annual winter low temperatures between 1976 and 2005 at 8,000 weather stations. It also considered for the first time such factors as changes in elevation and proximity to large bodies of water.

The slight shift in temperature zones for Palm Beach County might not be significant for everyone .

For example, gardeners who have visions of growing ultra-tropicals, plants that can't tolerate temperatures below 40 degrees, are still in for a disappointment.

Allen Sistrunk, director of Mounts Botanical Garden in West Palm Beach, said it's difficult to generalize temperatures when it comes to Palm Beach County.

"I have never seen a locale with as many micro-climates as Palm Beach County, not just the temperatures, but the soil and water. Everything but the sunlight varies dramatically in a matter of a few miles."

David Bache, sales director at Boynton Botanicals, said he thinks the fact that zone 9b has been obliterated from the county is misleading.

"People who live in the western part of Palm Beach County know from historical experience that there can be a significant difference in temperature," Bache said.

Another problem is that the zones are based solely on the lowest temperatures of the year and don't consider high temperatures and humidity. A plant that might grow in zone 10 in California where it's dry won't make it in humid Florida's zone 10, Bache said.

Whatever the zone, some people float into "zone denial" by purchasing a plant that's not recommended for their area, Nelis said.

"People still like to push the zone envelopes," she said. "They like to have something different from their neighbors."

Palm Beach County environmental horticulture extension agent Laura Sanagorski said, "This comes down to the 'right plant for the right place' principle. Look at the site and choose a plant that will thrive in that site."

Many plants are sold with tags indicating their zones, and that can make it easier for people who aren't familiar with what grows here to decide which plant to buy.

Local nursery owners know what works here.

"We were subtropiclal," West Palm Beach Garden Club President Sandy Arlund said. "And we're still subtropical."