Three stories above a busy stretch of North Halsted Street, Chicago, in the United States, on a rooftop with views of downtown skyscrapers, crickets chirp and prairie flowers dance in the breeze.

Waist-high grasses set the tone in a sprawling 2,500sq ft (232sq m) native garden dotted with bright blooms: golden brown-eyed Susan, purple hoary vervain, yellow stiff goldenrod, snowy white turtlehead, lime-tinged spotted beebalm.

In all, 85 species native not just to Illinois but to Cook County are growing in shallow, gravelly soil so dry it powders in your hand.

Butterflies – including monarchs and swallowtails – feed here, as do bees, wasps and lemon-yellow finches.

This native garden atop the Center on Halsted community centre in Lakeview is many things to its architect, Robert Sullivan, a retired environmental scientist: an exciting challenge, a place of healing, a way to get his message out.

At a time when studies show big decreases in insect pollinators – including the beloved monarch butterfly – Sullivan wants to show Chicagoans that they can help bring back these unsung heroes of the food chain.

“My crusade is to create good pollinator habitat for native pollinators in this city,” said Sullivan, who also gives away seeds and moderates the 6,100-member Facebook page Northern Illinois Native Plant Gardeners.

Interest in pollinator gardens shows signs of growth, with the Wisconsin-based native plant nonprofit Wild Ones reporting a rise in national membership, from 4,300 people in January 2021 to 8,100 in January 2023.

Susan manages the herb and vegetable garden.

Sullivan embraces bestselling author Doug Tallamy’s vision of a national network of relatively small native gardens – in yards and balconies and, yes, even on roofs – that create critical habitat for pollinators and other animals.

Sullivan, 64, of Western Springs, works on the rooftop garden with his wife, Susan, a retired elementary school teacher who grows hundreds of pounds of organic vegetables for senior citizens and a culinary training programme.

Sullivan has seen 110 species of insects in the garden, including fireflies, grasshoppers and seven different kinds of dragonflies.

“It’s fantastic,” said Joey McDonald, Center on Halsted’s manager of events and volunteers. “It’s an oasis in the city.”

Natalia Jones, a meditation teacher who lives across the street from the Center on Halsted, said she used to watch the Sullivans working in their garden during the Covid pandemic.

She ran into the Sullivans on the street, started volunteering at the rooftop garden, and even grew coneflowers and evening primrose on her sixth-floor balcony, where she has seen monarch butterflies and goldfinches.

“Bob is incredible,” Jones said. “His passion for the topic has really inspired me. Literally, it’s changed how I approach the neighbourhood. I make a lot more time to see what’s around me and get excited about it.”

During a recent walk through his rooftop garden, home to thousands of native plants, Sullivan showed how to crush the little teardrop-shaped capsules on the ends of foxglove beardtongue stems and obtain seeds.

A pepper is seen in the herb and vegetable garden.

Seeds aren’t that expensive, he said, but he likes to give them away as a way to encourage native gardens.

Sullivan and his wife, who gardens on a 500sq ft (46.4sq m) section of the roof, were drawn to this site after the suicide of their daughter, Sloane, who was transgender. Sloane, a computer programmer, died in 2017 at the age of 22.

“She was an incredible programmer. She was really, honest-to-God, brilliant, and an artist, and she composed music. She designed this,” Sullivan said, pointing to the medallion composed of interlocking ovals on his custom T-shirt.

Sloane’s family, friends and employer donated to Center on Halsted in her memory, and the Sullivans went to see where her signature was going to be etched into a memorial window. That was when Sullivan looked out onto the centre’s roof and saw some plants.

“That’s what I want to do here,” Sullivan told his wife.

He volunteered for the centre’s “green team”, and got permission to prune a rooftop tree.

“Next thing we know we’re in charge of all the green space here and it has just changed my life in such a wonderful way,” Sullivan said.

“It’s a very healing thing for us, and we feel close to Sloane here,” Susan said.

Today, a grassy, 2,000sq ft (185.8sq m) native plant section and an additional 500sq ft (46.4sq m) native plant area that Sullivan calls “the bakery” due to the blazing sun, house a remarkable group of survivors: plants that tolerate city pollution, high winds and hip-high snowdrifts, all while rooted in soil just 16 inches (40cm) deep or less.

Some areas of the roof have just six inches (15cm) of soil.

Of the 120 species of native plants that Sullivan has tried here, 85 survive.

“These are the toughest of the tough.”

The purple coneflowers are a foot shorter than they would be under gentler conditions. Butterfly weed does well, but only in the “ridge” section where for reasons not entirely clear to Sullivan – perhaps an old roof repair – the soil is about 16 inches (40cm) deep.

From a distance, the garden’s big field evokes a native grassland rich with nectar and pollen. Up close, you can see a low-lying underlayer of delicate succulents – tiny and tightly clumped in a mosaic of mint green, olive, burgundy and pink.

The garden isn’t open to visitors because the roof can’t be fully fenced due to historic preservation constraints, but the culinary students who use Susan’s organic produce visit, and the Sullivans hold weekend tours, which they announce on the garden’s Facebook page, Center on Halsted Garden Space.

The rooftop offers different colours and textures, depending on the month. At the end of May, lanceleaf coreopsis and foxglove beardtongue bloom all at once.

Later this month, Sullivan expects purple, blue, pink and white asters and yellow goldenrod.

During a late August visit, the last golden coreopsis blooms complemented the purples of hoary vervain and rough blazing star. Lush grasses swayed in the breeze. A fat bumblebee fed on a candy-coloured blossom.

Framed by a cloudless blue sky, Sullivan spread his arms and laughed out loud.

“If you’re into flowers, this is like heaven,” he said. – Chicago Tribune/Tribune News Service