Behind the manicured hedges of Coconut Grove and Coral Gables, a bit of a horticultural mystery has taken root, one tied to one of the most famous names in botany: David Fairchild.

During the heady days of botanical gardens, when plant explorers scoured the globe in search of exotic trophies to display back home, Fairchild collected two hardy mangrove trees while traveling in Indonesia. In the 1940s, he planted the trees, radiant specimens of Bruguiera gymnorrhiza with flaming, lipstick red flowers, at his house on Douglas Road, now a little jewel of a botanical garden known as The Kampong. Thirty years later, horticulturists at the more expansive and renowned botanical garden that bears his name off Old Cutler Road planted a second Asian species, Lumnitzera racemosa.

For decades, the trees flourished, showcasing the flowery beauty of exotic mangroves. But at some point, something bad happened. They escaped.

It turns out that not all mangroves — coastal trees Florida wildlife managers have taken great pains to protect — are good.

The Lumnitzera, which looks nearly identical to Florida’s native white mangrove, invaded nearby Matheson Hammock Park. Botanists suspect the tree’s buoyant seed pods floated down old mosquito ditches beyond garden borders. Eventually, the infestation covered 20 acres, with mangroves popping up in clusters inland and extending north, nearly reaching the main park road that winds along a canal leading out to Biscayne Bay and beyond. In 2010, the first year workers were sent to remove trees, they pulled out 20,000 plants.

But since then, biologists have encountered a mystery. Mangrove seeds typically don’t hide in soil, waiting for the right conditions to sprout, said Dennis Giardina, a biologist with the Florida Fish and Wildlife Conservation Commission. But now — five years after the last mature Lumnitzera tree was uprooted — thousands of seeds are still being found in Matheson Hammock, he said. The same thing happened at The Kampong. A year after workers removed the lone surviving tree and its offspring, Bruguiera saplings continue to reappear. And biologists have no idea why.

“Until this case study, we never had an example of mangroves creating a viable seed bank,” said Giardina, who is doing DNA tests on the plants to try to nail down their origin.

It’s far from the first time that exotic plants have climbed garden walls to threaten native ecosystems. Around the world, garden managers have quietly had to reconcile that stunning collections built over decades can also be the source of outbreaks.

“Unfortunately, it’s happening everywhere,” said Gary Krupnick, head of the Smithsonian Institution’s plant conservation unit. More and more, botanical gardens have turned to doing “risk assessments of collections to remove aggressive species.”

Kampong director and Florida International University associate dean Mike Maunder, who had the job of convincing his board to kill a 75-year-old “legacy” tree planted by the beloved collector, said gardens need to be on constant alert. Many, including The Kampong and Fairchild, now make promoting native plants and battling invasive species part of their mission.

“Ultimately, botanical gardens should follow a Hippocratic Oath of do no harm,” he said. “As a botanical garden manager, you’re under constant vigil.”

A Fairchild spokeswoman declined requests for interviews but provided a study documenting the garden’s horticultural history with the mangrove.

Since a 2001 summit at the Missouri Botanical Garden, gardens now widely recognize the need to treat collections not as untouchable treasures, but potential threats that can harbor infestations taking decades to come to light.

“It’s that chaos and unpredictability that we worry about,” Maunder said.

Fairchild garden’s runaway mangrove was first spotted when garden staff started a survey in 2008 after discovering the Matheson infestation, Giardina said. Horticulturists had planted 14 Lumnitzera in three locations in 1966 and 1971, according to a study by Fairchild botanist Jennifer Possley. By 2009, only one of the original trees remained. But in three locations, Possley found it had aggressively spread, growing more densely than native mangroves.

Alarmed, Giardina said the staff took a step back to dive into records tracking all the exotic mangroves ever planted by Fairchild or garden staff. According to an entry unearthed by Possley, in 1945, Fairchild hoped his mangroves would one day “brighten our coasts with their flowers.”

Seven decades later, botanists cringe at the thought — enlightened by the damaging impacts of a host of aggressive imports. From giant African land snails to the Burmese python, invaders are a constant threat in modern South Florida. Plants tend not to draw headlines, but wreak no less havoc. The melaleuca tree alone has cost the state a fortune — one University of Florida estimate put the damage at $1.7 billion in 2010 — after it was introduced in Manatee County in 1887 and later used by the U.S. Corps of Engineers to stabilize shores around Lake Okeechobee.

Mangroves pose unique challenges. Lumnitzera can easily hide in plain sight, while Bruguiera has the broadest range of any mangrove species, Giardina said. That means it can adapt to almost any location.

On Christmas Island, a 52-square-mile summit of an underwater mountain in the Indian Ocean, they grow more than 135 feet tall in a freshwater spring. They can also grow in a forest understory, just waiting for a chance to take over — a scary prospect in hurricane-prone Florida, where mangrove forests face yearly threats. If either mangrove were to get a toehold, botanists worry they could easily take over.

“The danger is exotics do their job better,” said Ian Simpkins, chief horticulturist at Vizcaya Museum and Gardens.

While Giardina says officials think they have identified and removed all the mangroves around Matheson, Fairchild records show the garden held a plant sale in the 1970s and sold 14 Lumnitzeras. No one knows where they ended up.

“I’m just waiting for that email or phone call,” he said.

And because they don’t quite know how long seeds can survive, hiding unseen, scientists worry changes in weather patterns or the landscape could occur to fuel a sudden outbreak.

“There’s this time lag that’s been recorded for many plants,” Krupnick said. “For 20 years, they’re doing great, and then all of a sudden 20 years later there’s an escape.”

Most gardens are able to precisely document escapes based on well-kept records of plantings, yet anticipating such behavior is something they have only begun documenting in the last decade or so, sharing information on what plants are likely to break out, he said.

“You’ve got to be really careful with discovering new species. If no one has ever planted it, you want to do trial tests before you introduce it outside,” Krupnick said. “You have to do a lot of research to understand what makes a species become naturalized.”

Since finding the runaway mangroves in Matheson, Fairchild volunteers and staff have returned every two years to hunt for saplings. In 2012, they found about 17,000. Two years later, the haul dropped to about 7,500. This year, according to Possley, a seven-man crew pulled up 1,380 plants over 12 days.

Meanwhile, staff members at The Kampong have wrestled with how to handle the Bruguiera. Last year, Maunder asked Giardina and National Park Service invasive species coordinator Tony Pernas to come to the garden and examine the surviving tree. The pair found 87 saplings. The decision was made to kill the tree, not easy medicine to swallow.

“David Fairchild himself planted the tree, and besides, it was a beautiful specimen,” Giardina said. “So Mike faced a lot of opposition.”

But six months after it was removed, Giardina said surveys turned up a dozen saplings. Then in May, Giardina received an email from John Ricisak, chief of Miami-Dade County’s coastal resources division. Ricisak had been called out to a ritzy house on Poinciana Avenue where a homeowner had illegally cut a stand of mangroves to get a better view of Biscayne Bay. While surveying the damage, Ricisak came across a stand of Bruguiera, less than a half-mile from The Kampong. When Giardina and Pernas showed up, they found a mature, flowering tree, seedlings and saplings.

They suspect the trees are related, although a pinkish white flower bloomed on the Poinciana Avenue tree.

“We do know that The Kampong specimen tree had been producing propagules (seed pods) and dropping them in the water for at least a half-century,” Giardina said in a July presentation to scientists tracking invasive species across the Everglades.

Even while the invasive mangroves appear confined to a small corner of Miami, the scientists worry about what would happen if they get a foothold in South Florida, which holds the largest mangrove forest in the U.S.

Hawaii offers a perfect case study. Before 1900, the archipelago had no mangroves. Then in 1902, American Sugar Co. planted a Florida mangrove in its fields to stop soil erosion. They worked so well that in 1922, 14,000 more mangroves were imported from the Philippines. Today, Hawaii treats mangroves as an invasive species that chokes out native plants and targets them for removal. With that in mind, Giardina and Pernas are closely watching the shores around Miami, mapping patterns of currents that could carry seeds up the coast and to Key Biscayne and Virginia Key.

A thornier issue, as with many exotic plants and animals, involves politics and money. Mangroves — there are 70 species in all — are widely used for aquariums and, like tegus, tortoises and tropical fish, can easily be bought online. An Austrian website recommends Bruguiera for beginners and boasts “implementation of Bruguiera gymnorrhiza into your tank is generally absolutely easy.”

“I can’t tell you how many times I’ve seen aquarium gravel on the bank of a canal,” Giardina said. “All it takes is one enterprising individual to promote it commercially.”

While most states, including Florida, have “noxious weeds” lists that regulate dangerous plants, once a plant grabs the attention of the nursery or pet trade, getting it placed on the list is tough.

“You can still go to these big-box stores and buy plants that are a problem,” Krupnick said. “It would be a lot better if people planted native species. It would attract the native pollinators, and there’s a huge health issue with them going on in this country. Native bees and butterflies need native plants.”

The scientists say their best shot at controlling the invasive mangroves is now, before they become widespread. And that means taking a tough stand on sales to protect South Florida’s four native species.

“In this case,” Giardina said, “a mangrove is guilty until proven innocent.”