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The science of climate zone maps

Source:  Copyright 2007, Newsday
Date:  February 7, 2007
Original URL: Status DEAD


Climate zone maps were invented to help gardeners determine which plants would thrive in their region. Unfortunately, those maps can confuse the heck out of Long Islanders.

A glance at the USDA Hardiness Zone Map places Long Island in zone 7a. But the National Gardening Association, another reputable source, has an online map that places Long Island and Queens squarely in 6b. Often, plants that are suitable for 6b also are zoned for 7a. But what if one wants to plant Spider lily (Hymenocallis liriosome), which is suited only to zones 7-11? If you subscribe to the NGA map, you have a bit of a conundrum.

Most of Long Island belongs in zone 7a, says Kim Kaplan, spokeswoman for the Agricultural Research Service, the in- house research agency of the U.S. Department of Agriculture. But Suffolk County has two zones. An oval of 6b runs through the center of the county because it's more protected from ocean winds, Kaplan says. That oval is surrounded by a zone of 7a.

Vincent Simeone, director of Planting Fields Arboretum State Historic Park in Oyster Bay, concurs. "Most of Long Island is in 7a, no doubt about it," he says.

Currently, there isn't a way to pinpoint which communities fall into Suffolk's 6b oval. The official USDA plant hardiness zone map was created in 1990, before the dawning of the Internet age. "It is literally a hand scan of our 4-foot-by-4-foot map covering all of North America," Kaplan says. "County lines are designated on the map, but towns are not."

The USDA is working on a new and improved digital map that will be interactive. "Users will be able to click on a town and get specific information for their exact location," Kaplan says.

The current USDA map is based on minimum winter temperature averages from 1974 to 1987. Maps that place Long Island in different zones might take different years into account. Some might rely on weighted averages; others simple averages. "We're asking the map to predict the future based on the past," Kaplan says. "This is our best guideline."

The National Garden Association Web site maintains the map has its shortcomings: "In the eastern half of the country, the USDA map doesn't account for the beneficial effect of a snow cover over perennial plants, the regularity or absence of freeze-thaw cycles, or soil drainage during cold periods."

John Gonter, business development manager for the NGA, attributes the difference to the subjective nature of hardiness zones. "Our ZIP code finder is a manual replication. Somebody looking at the map used their judgment and made the decision that Long Island is in zone 6b," he says.

The National Arbor Day Foundation, meanwhile, has recently released a new zone map to reflect rising temperatures it attributes to global warming. The foundation based its new map on information collected from 5,000 National Climatic Data Center cooperative stations across the continental United States over the past 15 years.

The research shows that significant portions of many states, such as Illinois, Indiana and Ohio, have shifted one full hardiness zone. Some areas of the country have warmed two full zones.

Long Island, however, was unaffected and remains in zone 7 on the new map.

To fall into zone 6b, an area must have registered an average winter low temperature between zero and 5 below. Zone 7a connotes a region with lows from 5 to zero degrees. But that doesn't necessitate a span of 5 to 10 degrees. It could mean a difference of as little as 2 degrees. Technically, if your backyard has an average of 1 degree, you fall into 7a; if your garden is a minus 1, then you're in 6b.

To confuse things even further, let's examine a phenomenon known as The Microclimate. Long Island is surrounded by water. That mitigates to the warm because water holds heat but also exposes us to cold winds that come along from the east without obstruction.

Your yard has many different microclimates in it. Even if your property is in that central Suffolk 6b oval, all or part of your yard could be a 7a. Shelter created by trees, a garage or a fence can place one particular section of the same lot in an entirely different zone, regardless of what county you live in.

"There might be a sheltered area where the wind doesn't blow across quite as much as on your neighbor's property," Kaplan says. That could explain why a particular plant thrives next door but languishes for you.

In the city, concrete and blacktop hold a lot of heat. Green spaces tend to be colder. So if your home backs up to a park, it might fall into a cooler zone.

When will the new-and-improved, clickable USDA map be available? "In the near future," Kaplan says. When could that be? Kaplan says she hopes within a year but is noncommittal, saying, "we can't put science on a schedule."

"Scientists are working on it now. Then, it will go through a technical review, and finally graphic artists will create a Web site around it."

In the meantime, gardeners would do well to take into account all the resources available to them, and to familiarize themselves with their own microclimates.

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