NEW YORK — The difference between a vacant lot and a community garden comes down to a single thing: a sunflower.

Helianthus annuus is not just a plant but a kind of logo, said Deborah Greig, 30, an urban agriculture coordinator at East New York Farms. She seems to have a knack for brand management. Once you’re shoveling manure, advertising is a logical next step.

“They’re a really iconic way to make people notice that you’re trying to make a change in the community,” Greig said. “There’s not a lot of green space in East New York.”

The half-acre plot she helps manage, the United Community Centers Youth Farm, lies almost in the shadow of a commuter train stop. The main mission here is to help the neighborhood grow its own nourishing food. But a perimeter of brilliant sunflowers, towering over the cyclone fence, seems to function like commercial signage, she said.

The sturdy sunflower does not shrink from a little “hard labor, too,” Greig added. “I think they do some catching of the trash that blows in.”

At the South Bronx community garden La Finca del Sur, she said, growers are experimenting with sunflowers in a soil-treatment practice called phytoremediation. In field tests last summer, the plant’s deep taproots seemed to pull heavy metal contaminants like mercury and lead from the garden’s polluted soil.

When the sunflower isn’t doing dirty work, it dresses up nicely. This year, Greig and her friend Molly Culver, 31, will include sunflowers in designs for a half-dozen weddings as part of a side business called Molly Oliver Flowers. Many of these blooms will come from the Youth Farm at the High School for Public Service in Brooklyn, where Culver is a manager.

“We’re planting around 150, or more, every other week through the end of June,” Culver said. (The farm supplies an on-site market and a cut-flower Community Supported Agriculture operation.) All told, Culver will be raising more than 1,000 sunflowers in the concrete heart of Brooklyn.

As it turns out, this is a perfectly natural place for a sunflower to be. When Greig and Culver — and I — stocked up on sunflower seeds this winter, we were joining a gardening tradition that goes back some 5,000 years. Helianthus annuus is the quintessential American flower. And its path to the garden each year is a kind of tall, shaggy tale, rather like the sunflower stalk itself.

Deep roots

Sunflowers are likely the second-oldest domesticated seed crop in eastern North America. (Squash came first.) Archaeological digs in the river valleys of Appalachia (sites with names like Cloudsplitter and Napoleon Hollow) have identified the burned hulls of cultivated sunflower seeds from 4860 BP (or before present). Other sites in Mexico may be older still.

New York’s pre-colonial dwellers, the Lenape, likely planted sunflowers at the edges of the maize fields alongside their camps.

From the accounts and detailed drawings of European explorers along the Eastern Seaboard, these sunflowers were of the common cultivated variety, macrocarpus. The plants reached seven or eight feet tall and formed a single large head. (Wild sunflowers grow with a branched habit and form numerous smaller heads.)

Writing in 1951, the eminent U.S. sunflower taxonomist Charles Heiser Jr. concluded that oil from the crushed seeds “was used chiefly to anoint the hair.” This practice could still be observed in the Iroquoian tribes of Ontario in the 1940s.

On other occasions, the seeds “were roasted over a fire, then pounded and cooked with roasted white corn, sweetened with maple sugar and used in somewhat the same way we use lard.”

The sunflower packets in the seed rack at the hardware store did not come directly from America’s pre-colonial stock. In the 16th century, the plant began a grand tour of Europe: The Flemish botanist Rembert Dodoens recorded an accurate drawing as early as 1568. In the centuries of European breeding that followed, the sunflower probably learned how to curtsy before royalty and wear a powdered wig and dance a gavotte.

It is mostly the repatriated sunflower that we grow today. (Heiser reports that the plant made its homecoming debut in C.W. Dorr’s seed catalog in 1880.)

The search for the modern sunflower might lead you to Tom Heaton’s breeding nursery in Woodland, Calif., on the outskirts of Sacramento. Some of the most popular ornamental cultivars came into existence here, and in 100 acres of seed-production fields spread across the valley.

The sunflowers go by names like the Joker (which has a motley ruffle like a fool’s collar), Moulin Rouge (burgundy), Moonshadow (pale) and Kong (which Heaton has grown to 19 feet). And there’s the ProCut line, which might be the hardest-working sunflower in the floral trade.

A recent weekday morning found Heaton, 61, puttering around his seedlings. “We typically plant 4,000 or 5,000 different kinds of sunflowers that we breed with different heights, different petal types, different colors and other characteristics,” he said.

Planting seeds

Any 4-year-old can get a sunflower to grow. Little hands do well with big seeds. But I’m not 4 and I’ve failed at the task plenty of times.

So a few weeks ago, I dropped by Red Barn Farm in the St. Croix River Valley of Wisconsin, where Debbie and Jim Barron grow 5,000 sunflowers a year. Or you could say that Debbie grows 5,000 sunflowers a year and Jim grows 120 varieties of tomatoes.

“He was always against the flowers,” said Debbie Barron, 53.

“I thought when the economy went bad, people would want to eat, not buy flowers,” replied Jim Barron, 58.

Many of their flowers are pollenless by design. But leave the mammoth, open-pollinated varieties in the yard to set seed and you’ve got a 12-foot-tall bird feeder. Alternately, tie a paper bag on, like a muzzle, to catch the crop.

These are the familiar seeds with the bootlegger’s gray-and-white pinstripe down the sides. The marking, it seems, is purely ornamental.

As Heaton said: “A black sunflower seed hides a lot of blemishes in handling. I think it’s just product presentation: It’s what the customer likes.”

Native people would have found a different shell when they harvested wild seeds to roast, grind into flour and then prepare as “cakes or mush,” Heiser writes.

Ultimately, he admits to being baffled by the sunflower’s most enduring mystery. The “hulls of the wild sunflower are very fibrous and probably not very palatable,” he muses. “Although their removal would be a difficult and tedious task.”

It’s not hard to imagine the first American gardener, standing at the edge of a meadow while pondering a palmful of sunflower seeds: What could go wrong if I swallowed the shell?