Whole Foods Market will grow produce on roof

Lynnfield store expects a yield of at least 10,000 pounds a year

The soon-to-open Whole Foods Market in Lynnfield will offer its customers something the company says no other major grocery chain has offered before: “rooftop produce,” picked from a field atop the store.

Tomatoes, carrots, kale, chard, marjoram, basil, tarragon, and more will be sown in more than 300 tons of soil contained in a rooftop planter installed over the past several weeks. The space — about 17,000 square feet, considerably more than a third of an acre — is expected to yield 10,000 to 11,000 pounds of produce a year.

“Definitely, it’s a farm,” said Robert Donnelly, senior executive coordinator of development at Whole Foods, a company based in Austin, Texas.

Whole Foods and its contractors say the commercial roof garden is an experiment that, if it succeeds, could encourage other grocers to do the same, boosting efforts to expand rooftop gardening. Such gardens not only insulate buildings, lowering heating and cooling costs, but also decrease storm-water runoff, which can overwhelm sewer systems and carry pollutants into waterways.

And they yield fruits and vegetables that do not need to be trucked or flown, cutting transportation costs and emissions, including of greenhouse gases. The rooftop produce — a tiny fraction of Whole Foods’ inventory — will be sold in the Lynnfield store or used in its prepared foods.

A green roof, however, is not cheap. It can cost up to 60 percent more than a traditional roof, according to the Sustainable Cities Institute, a program of the National League of Cities.

“The cost-benefit analysis does not add up, but at what point is the marketing value worth the investment?” asked Mark Winterer, director of operations at Recover Green Roofs, which oversaw the roof garden’s design and construction. “We really don’t know, and we need clients like Whole Foods to try it.”

Whole Foods began thinking about the project three or four years ago, Donnelly said, and at first planned to build a basic green roof — essentially, a lawn atop the store. Then the company came across Green City Growers and Recover Green Roofs, two Somerville companies that partnered on a 4,000-square-foot garden above the Ledge Kitchen & Drinks restaurant in Dorchester. (That garden provides about 75 percent of the veggies and herbs served at the Ledge.)

Whole Foods’ plans quickly became more ambitious as company officials realized the 45,000-square-foot roof (nearly an acre) provided plenty of space for farming.

The challenges were many. First, Recover Green Roofshad to consider how much weight the completed garden would add to the roof and reinforce the building’s support structure with steel joints.

It had to ensure that water used to grow the plants would not seep through the roof. And it needed to keep farmers safe, protecting them from falling from the roof.

Ultimately, the company designed a 130-foot-square box held together with stainless steel edging. It is lined with several layers, including a waterproof membrane and another layer of protection to keep roots from growing into and damaging the roof. The system uses an egg-crate-like container that holds lightweight, porous rock that absorbs water and helps anchor root systems.

Then Recover Green Roofs had to get more than 300 tons of growing material —a dirt-like mix, made up of shale, sand, compost, and a charcoal-like substance — to the roof, roughly 20 feet high. Earlier this month, a crane lifted the growing material, three tons. It was then spread into rows about 10 inches deep.

Green City Growers, the urban farming company that will maintain the garden, began planting last week, using seedlings from Red Fire Farm in Granby for tomatoes, eggplant, onions, swiss chard, collard greens, and a variety of herbs.

Jessie Banhazl, chief executive of Green City Growers, said the challenge for her team is to make the most efficient use of the space. To do that, they are planting not only on top of the dirt mounds, but on the sides, and squeezing plants as close together as they can without crowding them.

“We’ve gotten it down to a pretty good science,” she said.

The first rooftop crops are expected to be ready by the store’s opening in the fall, and will sell for about the same amount as similar items in the store.

“It will be defined as ‘rooftop produce,’ which will differentiate it from the [regular] organic,” Banhazl said. “Hyper local, that’s what we call it.”

-Erin Ailworth