Chicago ? British artists Heather Ackroyd and Dan Harvey know where the grass is greener, and fences don’t have anything to do with it.

As any homeowner who ever found yellow lines on the lawn after leaving the garden hose in one place for too long has discovered, grass needs sunlight to stay green. That simple fact about chlorophyll led Ackroyd and Harvey, both 44, to create a new art form using living grass as a photographic medium.

One of their latest works, commissioned by the Chicago Department of Cultural Affairs, went on display recently at the Chicago Cultural Center. Titled “Supernatural (After Piero di Cosimo),” the vertical lawn, measuring 13.5 feet by 32 feet, bears a detailed reproduction of the Florentine artist’s 1505 painting, “Satyr Mourning Over a Nymph.”

“Think of the grass blades as pixels — or, better yet, think of each cell in the grass blade as a pixel,” said Harvey during a close inspection of the work. “We have found we can transpose the whole tonal scale of black-and-white photography into the yellow and green range of living grass.”

To create the work, Ackroyd and Harvey covered a moveable partition with burlap, which they then coated with nutrient-rich clay. They then planted the clay densely with grass seed and kept it watered. When the grass sprouted, they placed it in a darkened room in front of a slide projector, which shone a black-and-white photographic negative of the painting onto it for 12 hours at a time.

Like the silver nitrate grains darkening in photographic print paper, the chlorophyll molecules in the grass turned deep green when hit with intense light, but faded to yellow when left in darkness.

“It took us eight days to produce the image, but see how much detail it has — the satyr’s hair, the wildflowers in the foreground, the dogs off there in the background?” Harvey said.

Ackroyd and Harvey, who have been collaborating since 1989, discovered grass photography accidentally while doing a piece of installation art in an Italian hill town in 1990. One part of the installation was a wall covered with living grass, and another part was a ladder.

“We noticed that sunlight shining into the room was leaving a very faint shadow image of the ladder in the grass,” Ackroyd said. “We wondered if we could reproduce the effect, and we started experimenting.”

Shoppers and workers on State Street in downtown Chicago walk past a grass artwork called Specific

For the first few years, the grass images they produced were temporary, and began to fade the moment they went on display.

“They would start off looking like photographs like this one,” Harvey said. “But soon they would look more like faded tapestries, and in a few more days they would vanish completely. A picture like this one would last maybe a week if it were living grass like our early ones. The fading actually became part of the artistic process, showing that everything in nature is evanescent.”

But since patrons usually want the art they sponsor to be a bit more permanent, Ackroyd and Harvey looked to science to find a way to make the pictures last longer. They found the answer at the Institute of Grassland and Environmental Research in Aberystwyth, Wales, where scientists had developed a new strain of grass called “staygreen.”

“It’s a hybrid of rye grass and fescue, and it keeps its green even after it dies,” said Harvey. “We’re in the process of killing off this grass right now. … Once it’s dead, the image won’t change appreciably for many years.”

But if the artists have achieved a certain permanence in their medium, their message is still one of change and decay.

“I think our work tends to keep coming around to certain themes, such as … change and the persistence of memory,” said Ackroyd. “It’s one reason we chose this painting to reproduce. We think it’s inspired by the Roman poet Ovid’s ‘Metamorphoses,’ which is all about change and regeneration.”