It was 3:37 a.m. on Atlantic Avenue in Brooklyn when Lewis Miller let out a sigh of relief.

“Right here is my happy place,” the 46-year-old florist and guerrilla artist said. After zhushing a coral peony and throwing in a few gerbera daisies, he stood back to consider the framing of his six-by-four-foot orange-hued flower heart: black pavement, white crosswalk lines, a “No Turns” sign, the marquee of Barclays Center casting a quote from Dr. Martin Luther King Jr. — “The time is always right to do what is right” — into the early-morning dark.

“We’re good,” he said. “Let’s go.”

The heart was one of four “flower flashes” — Lewis Miller Design’s signature — that New Yorkers would wake up to on June 16. Though he has surreptitiously placed these elaborate arrangements for years, Mr. Miller’s pandemic-era flashes, around a hospital lamppost or in a midtown garbage can, have been met with particular enthusiasm. Social media viewers from around the world have sent him hundreds of heartfelt letters and fan art. Bette Midler raves about his work on Instagram.

“During good times, flowers are awesome, we all know that,” Mr. Miller said. “But now more than ever we need flowers in the city. Who isn’t looking for a little joy?”

“This is the most beautiful thing I’ve seen in a long time,” said an observer who had ventured over from 4th Avenue, a rickety cane in each arm.

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Credit…Nina Westervelt for The New York Times

Irini Arakas Greenbaum, whose job includes scouting locations for Mr. Miller (“I’m always on the hunt for the Kate Moss of garbage cans,” she said), offered him a free spirit rose.

“Nah,” he said. “I’m so super pretty already.” She insisted. “OK,” he said. “I’m gonna see a homeless girl and give it to her. Spread the love!” Mr. Miller told the man to stay safe. Then he jumped into a large white van carrying some 12,000 flowers in the back.

“It’s like driving around a hundred wedding cakes,” said Manny Mejia from behind the wheel. Despite a few potholes, the daisy mums and stardust roses emerged unscathed at the second installation site, in Fort Greene. Mr. Miller zip-tied the heart onto a green C train entrance under the eye of “Comandante Biggie,” a mural of the Notorious B.I.G. flanked by white doves.

As Tawana Schlegel, a florist with the company, softened the heart’s curves with lilies placed in messy perfection, Mr. Miller noticed a Cellino & Barnes ad above the subway entrance. “Is that even a real phone number?” he asked no one in particular about all those eights, while sweeping up fallen petals and a bonus used Q-tip. Before bolting he grabbed a mister of Crowning Glory from the van to give the arrangement a spritz, because like so many New Yorkers, lilies need extra hydration.

Credit…Nina Westervelt for The New York Times
Credit…Nina Westervelt for The New York Times
Credit…Nina Westervelt for The New York Times

Crossing the illuminated Manhattan Bridge to the third site, in SoHo, Mr. Miller pondered the future. “What’s our city going to look like in three months?” he said. Almost all of this year’s gigs were canceled, and early 2021 events were already being postponed. Though Mr. Miller has paid for past flashes himself, he accepted 1,200 roses donated from a fan with a farm in Ecuador for this one, as well as some funding from L.E.A.F., an organization that puts on flower festivals.

“I’m not opposed to taking money,” he said, noting his installations for Equinox, Old Navy and one businessman who requested a custom flash for his wife as a lunch-break surprise. “But for these there needs to be integrity or my joy is dead.”

By 4:47 a.m. on Spring Street, the deep hum of garbage trucks was serenading Mr. Miller’s crew as they placed a purple heart against a blood-red wall of graffiti: “We may be alone but together we’ll conquer.” Mr. Miller rounded out the design with rhododendron while Ms. Schlegel threw in an extra allium, the onion-family flower that could double as a Willy Wonka lollipop.

“We always joke about how a good flash is both confident and cavalier, but the true secret sauce is the city,” Mr. Miller said. “I’ve seen street art everywhere from Nashville to L.A., and it’s just not the same. There are certain things that just work best in New York.”

Credit…Nina Westervelt for The New York Times
Credit…Nina Westervelt for The New York Times
Credit…Nina Westervelt for The New York Times

But street art doesn’t always cooperate. Dismayed by a dark patch of wall not providing adequate color contrast, Ms. Arakas Greenbaum pulled Mr. Miller aside to discuss options. Move the heart? White spray paint? Mr. Miller came up with another solution involving what some consider to be the floral equivalent to a vending machine hamburger.

“Carnations have gotten a bad rap,” he said, after adding a few white and purple-tipped ones he had on hand. “They’re beautiful flowers that smell like nutmeg and have a high petal count.” (If any stem snobs are wondering, Mr. Miller would take a carnation any day over a moth orchid or even, he whispered, the “overrated” calla lily.)

The cobblestone plaza on Gansevoort Street was the final stop, empty at 5:21 a.m. The team lay down giant cardboard stencils of Milton Glaser’s “I ♥️ NY” logo on the street and replaced them with bold blooms. Mr. Miller poked and prodded the red heart, yanking out a rose here, situating a caladium leaf there. Ms. Arakas Greenbaum climbed to a fifth-floor walk-up’s fire escape to get the aerial view as four pigeons wandered by.

“I wish it looked like St. Mark’s Square,” Mr. Miller said, sprinkling his breakfast granola bar over the work. By 6:27 a.m., the morning flocks, avian and human, were milling.