Gene Tempest -- Globe Correspondent

The most common question succulent expert Art Scarpa gets when he lectures about the plants he’s been growing for almost seven decades is: “How often should I water?”

“And,” he sighed, “there’s no answer for that.”

Scarpa’s greenhouse in the front yard of his home in Atkinson, N.H., holds one of the largest and most diverse private succulent collections in New England. Here, in 448 square feet, Scarpa grows some 4,000 individuals and more than 1,000 distinct varieties.

There’s a succulent boom going on in America, and Scarpa, 77, cofounder and past president of the Cactus and Succulent Society of Massachusetts, thinks that’s a great thing for all “succulentophiles,” young or old.

In recent years, succulent sales have surged — up 64 percent from 2012 to 2017, according to a Garden Center magazine survey. In 2014, when the USDA census was last completed, succulents represented a $40.9 million industry. This trend is usually attributed to a new wave of millennial interest in photogenic greenery. A recent search for the hashtag #succulove on Instagram turned up more than 1.6 million posts.

As houseplants, succulents are reputed to be easy — in part because of their modest water demands. Many of the adaptations that make them so beautiful are outgrowths of sophisticated water-conservation mechanisms that help them survive in deserts.

Some of Scarpa’s plants haven’t been watered in years.

Many are former best-in-show or at least blue ribbon winners at the major East Coast flower shows in Boston, Newport, and Philadelphia.

Scarpa has also judged at several such events.

In an award-winning cactus or succulent — cactuses are a succulent subfamily, and thus all cactuses are succulents but not all succulents are cactuses — Scarpa looks first for health and good proportions.

When he’s judging, he cannot touch the plants, even to turn them around.

“Sometimes,” he admitted, “some of the sins of cultivation are in the back. You judge what you can see. It’s only fair. Let’s face it — it’s a beauty contest. You’re not going to ask an actress to part her hair differently so you can see her roots.”

Scarpa grows many rare varieties from South Africa, but also takes great pride in “rescuing” fallen leaves from the floor of the garden section at Home Depot.

The plant’s birth — high or low — is of little importance to Scarpa, and lineage doesn’t matter either. If a succulent is a hybrid, he said simply: “I don’t know who the parents are.”

When I visited on a snowy 25-degree Saturday in December, Scarpa’s propane-heated hoop house was surprisingly cool. At night, inside temperatures dip to 48 degrees.

“The plants love it,” Scarpa explained. “You can’t grow these plants in a house in our conditions.”

Central heating — which you might think would be a boon to the modern apartment gardener — is the bane of the homegrown succulent. It’s too hot, too dry, too artificial.

The first plant he shows me is a sad-looking Opuntia cespitosa, or, as it’s more commonly known, a ‘prickly pear.’ This individual is shrunken and slouching drunkenly.

“Spectacular yellow flowers in June,” Scarpa said brightly. “It’s just snoozing. None of us look really great when we’re sleeping.”

To the right of the door, another contingent of cactuses — these more plump and vigorous.

All along the left side of the room are Scarpa’s many Haworthias, small plants whose unique leaf markings can drive up prices. Scarpa recently sold one for $325; in some markets, collectors will pay $2,000 per plant.

In plastic pallets, looking alien, are dozens of South African ‘Living Stones’ (Lithops). These grow flush with the ground, camouflaged among the rocks and pebbles. According to Scarpa, “they’re very shy plants.”

Most of the succulents are exotics, but some are Northeastern natives, like the prickly pear, and Scarpa himself.

He was born in East Boston in 1942, and, until the age of 18 — when he went to work as a reservation agent for the Italian Line steamship company — he lived at the top of a three-decker near the harbor.

On the ground floor, his Italian-born parents, Nazario and Innocence Scarpa, ran the Modern Variety Store. By the time he was 8, Art was working at the shop — and had already purchased his first succulent, a Bryophyllum leaf cutting from a flower store down the street.

“We called it the ‘Curtain Plant,’ ” he recalled, “because you could pin it to the curtain and it would grow.”

Soon, there were dozens of potted cactuses on the windowsill in the Scarpas’ parlor, where they presented a daily inconvenience to Innocence, who had to endure the spines every time she drew the drapes.

“Ouch!” she was routinely heard to exclaim. “Damn it. Oh Dio.”

Scarpa has lived through succulent crazes before. When he first started growing, cactuses were already internationally fashionable.

“Once upon a time, people who grew cacti were considered ‘curious,’ ” English author Vera Higgins explained in her 1956 book, Cacti for Decoration. “Now we are fast approaching the time when not to have a cactus in the home will be thought even ‘curiouser.’ ”

Like today, that first wave of #succulove was propelled by looks.

“Their rather simple forms associate well with modern trends in interior decoration,” wrote Higgins.

In working-class East Boston, though, Scarpa didn’t know anyone else who grew them. And he can’t really explain what first caught his eye.

“Something about the spines,” he said.

There was also an influential Christmas gift from his eldest brother, Alfred: the 1954 edition of The Wise Garden Encyclopedia: A Complete, Practical, and Convenient Guide to Every Detail of Gardening Written for All U.S. Climates, Soils, Seasons.

“I read it from cover to cover,” he recalled. “I was just fascinated.”

He would open the 1,380-page Wise guide, choose a listing at random, and test himself: Describe the plant, define the horticultural term.

Touring his collection, Scarpa takes special pleasure in explaining how his plants naturally behave.

“In habitat,” he mused, tenderly holding the branch of a 3-foot, 40-year-old fan aloe, Aloe plicatilis, “these get 15 to 20 feet tall.”

As a teen, Scarpa also grew palms trees from seed. At age 15, he became the youngest-ever member of the International Palm Society and traveled to Miami as a guest of the society secretary. It was the first time he’d been on an airplane.

“Finally seeing palm trees growing in the ground is something I’ll never forget,” he said.

His collection has since taken him all over the world, following the plants he loves to South Africa, Australia, South America. Traveling among them — truly taking a succulent’s view of the world — is to see human history a little differently.

Scarpa told me that nearly a decade ago he was watching the television coverage of the 2010 Chilean mining accident in the Atacama Desert — the driest nonpolar desert in the world.

“Sometimes,” Scarpa recalled, “[the television crews would] show a cactus in those shots. Copiapoa. They were in bloom — and they hadn’t had any rain the whole time. They might not be the most beautiful plants in the world, but they have a story to tell.”

Gene Tempest is a writer and historian who lives in Cambridge. Subscribe to the Globe’s free real estate newsletter — our weekly digest on buying, selling, and design — at pages.email.bostonglobe.com/AddressSignUp. Follow us on Facebook, Instagram, and Twitter @globehomes.