At the National Chrysanthemum Society's annual show, held over the weekend in Northern Virginia, a steady stream of visitors discovered an alternative world of mums - blooms of rare size, color and form.

"Wow," announced one visitor. "Can I get these at Home Depot?" No, sir, you cannot.

The fall mum show has been a rite of autumn for generations - the sort of small-town subject that would have appealed to Norman Rockwell - but how much longer it can survive is an open question. Exhibition chrysanthemums are not endangered, but the demanding hobby of raising them is under threat as the elderly ranks of fanciers fade away. The blooms range from delicate, threadlike flowers suggesting spreading tentacles of coral, to enormous globes of featherlike petals.

But consider this: The show's blue-ribbon sweepstakes winner, David Eigenbrode, is 82; Robert Howell, the runner-up, is 84.

In a decade or two, the hobby "is either not going to exist or continue to limp along," said John Capobianco, whose local society in Long Island once boasted 100 members and shows of a thousand blooms. It is now down to 13 members who work hard to exhibit 100 flowers.

"We are afraid, honestly, that growing chrysanthemums for show is going to become a lost art," said Pat Stockett Johnston, a hobbyist from Southern California. "I would be embarrassed to tell you how many chapters we have lost in the past 13 years, perhaps close to 20."

Johnston, 74, became the national society's new president during the weekend show and annual gathering, held this year at the Hyatt Fairfax at Fair Lakes in Fairfax, Va.

The society has approximately 500 members, compared with 2,400 members in the early 1980s, said Galen Goss, the group's director of management services.

The anxieties of the chrysanthemum world are shared by the remaining active members of other flower and plant societies throughout the country, including those for roses, camellias, daffodils, dahlias and hostas, to name a few.

Traditionally, such local societies would attract members in their 30s who joined to get access to prized varieties, attend monthly meetings, receive growing advice from the old lions, and learn how to groom their flowers for competitive shows. Many societies formed before and after World War II, when fewer women juggled careers with families, life was slower-paced, distractions fewer and interests simpler.

Apart from the modern career and parenting pressures for millennials, many have moved into cities. They don't have the land to plant bulbs or cultivate flower beds, and much of their life involves screens.

In the new social universe of the 21st century, plant societies are trying to plug into Facebook and other social media to reach young people, but they recognize that their oldest and most knowledgeable members aren't comfortable with the likes of Instagram.

"They don't have the technical skills," said Toni McKenna, a member of the Virginia Camellia Society. "They used to show up to give a talk and bring a few blooms, and people don't want that anymore."

Participation in all manner of leagues, clubs and other civically cohesive groups has been declining for years, but what makes the flower society demise more poignant is that expert flower-growing requires a mentor-student relationship that is inherently intergenerational, said Sherry Turkle, the MIT researcher who has written extensively about technology and society.

"Young people are talking so much about nature, conservation, the Earth, and yet I fear that the easiest way to do that is to participate online," she said. Implicit in such plant societies is a connection not just to the minutiae of nature but to other people's lives.

"Young people are suffering from not having natural conversations with older people," Turkle said. "They don't know how to have empathetic conversations about the arc of human life, because they are not talking to older people."

Since 2012, a coalition of plant societies has met annually to confront membership woes, with the help of the Alexandria, Va.-based American Horticultural Society.

At this weekend's chrysanthemum show, the horticultural society's executive director, Tom Underwood, and American Dahlia Society activist Harry Rissetto made presentations on ways to revive membership.

In sum, the advice mirrors the counsel any legacy enterprise must hear these days to survive: Have a dynamic Web site full of great content, exploit social media and create the infrastructure for online transactions. "But the bottom line," Underwood said, "is the focus on people and relationships."

One tactic is to send people who don't renew their membership a letter asking them if they meant to drop it, Rissetto said. "We have a lot of older members and they simply forget."

In the past two years, two chrysanthemum clubs have been created, one in Raleigh, N.C., the second in the Bay Area of California. The former was established by Joan Matthews, a 65-year-old retired teacher who says effective recruiting can be done at garden-related events and venues. The North Carolina State Fair was one such place; another was her garden in downtown Raleigh. "When they saw how they looked in my yard, they said they wanted to grow them," she said.

Jeff MacDonald, 59, started the Bay Area chapter in 2013 by hand-writing letters to people who were lapsed members of the national society in his area. He met five of them at a restaurant and, after reviving the club, they recruited more at a garden center. Most of the current 49 members are Asian - exhibition mums have deep roots in China and Japan.

Howell, this weekend's runner-up and a longtime grower, could be found a week before the show in his Beltsville, Md., backyard with protege Polo Diaz, who, at a mere 49, is the youngest member of the Potomac Chrysanthemum Society. Diaz is a landscaper who noticed Howell's blooms while working in the neighborhood four years ago. When Diaz asked about them, the octogenarian took the younger man under his wing, giving him cuttings to pot up in the spring and grow through the summer.

Another encouraging presence at the show was the prize-winning blooms grown by middle school students from Canton, N.C., whose entries were driven to Fairfax by their (now retired) biotechnology teacher, David Curtis. The problem is that once the students hit high school, horticulture isn't offered, he said.

Capobianco, who was standing nearby, said his hope is that if teenagers are introduced to growing flowers, they will return to them later in life after the distractions of young adulthood. The question is: Will there be a local or national society for them to return to?

For a while this past weekend, such fears were put aside as growers and visitors alike savored the spectacle of the exhibition flower.

In her pep talks to her new members, Matthews tells them: "It's not just about what you get but what you give. You're giving these plants the ability to take your breath away, the ability to exist."

Breathless admiration has been the reaction since the first show in the United States. "It was in 1884 in Massachusetts," Howell said. "I wasn't there."