The harsh winter killed Bo Barefoot's Mexican fan palm tree in his front yard in Glen Echo Heights, Md. The tree grew so large that Barefoot could no longer afford to protect it against the impending freeze. (Kate Patterson/For The Washington Post)

Last fall, Bo Barefoot said a quiet goodbye to his beloved Mexican fan palm and waited for the winter gods to take it.

The palm tree had grown so large in his front yard in Glen Echo Heights, Md., that he could no longer afford to protect it against the impending freeze. The gods came, and they were greedy.

In addition to killing off the 26-foot palm tree, they took a rare yucca and, possibly, clumps of Japanese bananas that had spread 12 feet wide. Since January 2014, Barefoot also has lost four tall windmill palms and a mature, two-story Bismarckia palm that together had given his suburban yard a flavor of paradise.

“The last two winters have changed everything for me,” he said, looking glumly at the brown earth of his once lush garden. “I have had these plants for years.”

Barefoot, 67, is part of an informal alliance of tropical-plant geeks in greater Washington who have spent the past 15 years or more basking in the delights of mild winters. They may have been at the edge of this shift in the gardening paradigm, but the phenomenon gardeners call “zone creep” altered the look of yards everywhere. Figs grew into large trees, camellias bloomed in the middle of winter, and many gardeners abandoned the practice of lifting cannas and dahlias, confident they would survive winters in the ground.

Then came the winter before last, when the temperature dipped to 6 degrees at Reagan National Airport and minus-2 at Dulles International Airport. This past winter, it dropped to 5 at National and minus-4 at Dulles.

Beyond the numbers, the winters featured prolonged periods of intense cold and excessive snowfall.

The first frigid winter in years left gardeners demoralized, horticulturists say, and the second one has reinforced their view that the beach party that began in the late 1990s is over. The Agriculture Department’s plant-hardiness map puts the District in the colder half of Zone 7, with winter extreme low temperatures averaging 0 to 5 degrees. But the warm years convinced many they were in a Carolinian Zone 8 (with lows bottoming out between 10 and 20), a perception borne out by their ability to grow gardenias, hardier palms, perennials from Mexico and eucalyptus trees.

“I can’t remember having a back-to-back winter this cold,” said Joe Luebke, director of horticulture and grounds at the Washington National Cathedral. The winter before last, he and his colleagues struggled to wrap blankets around signature rosemary plants in the Bishop’s Garden that had grown into mature, aromatic shrubs. They perished and were replaced with new bushes that have also died.

“We got a little complacent about shifting hardiness zones,” said Larry Hurly, of Behnke Nurseries in Beltsville, Md.

The most damage was evident last spring, though echoes of it abound in this one. Plants such as rosemaries and sages were killed outright; others, such as figs and hydrangeas, died to the ground and resprouted. Many evergreens have brown leaves that will probably regrow, including the popular and normally green-in-winter nandina.

“We talk to our customers and ask them to have patience,” said Dave Reed, vice president of Meadows Farms Nurseries in Chantilly, Va. “I’m seeing some things I thought had no hardiness issues at all.”

In his home garden in Warrenton, Va., he saw dieback in Southern magnolias and crape myrtles. Maryland gardeners, from Bethesda to Mount Airy, say they lost rose bushes, a problem rarely encountered here since the 1970s.

In some cases, shrubs that were knocked back last year have been finished off this year. At the cathedral, a bay laurel that had grown to 15 feet died back to the ground but resprouted. Now it seems finished, Luebke said. “Being near zero [degrees] in the city is for the most part unheard of, and having it on multiple mornings is really hard,” he said.

Even coastal gardens seem no match for a polar vortex. “We were lulled by several years in a row of mild winters, where the coldest was 13 degrees for a few nights,” said Michael Zajic, who has developed an impressive garden in Nassau, Del., over the past decade. “Last winter here we got down to plus-7 degrees and the winter before to minus-7.” Some of his palm trees died and won’t be replanted. “It’s too painful, expensive, but just plain sad,” he said.

The prospect of climate change, he said, doesn’t mean we’re going to be warm all the time. “It means more variable weather.”

For new gardeners worried about future winter damage, the solution is to pick plants that are suited to a zone range that Washington is safely inside. Other gardening techniques can help: watering evergreens in the fall, withholding nitrogen fertilizer after late summer and, in the case of hydrangeas, choosing newer varieties that still flower after a winter freeze. Even with those steps, hardy plants may be damaged in bad winters by de-icing salts or increased deer browsing, said Miri Talabac, Behnke Nurseries’ woody-plants buyer. “On the plus side, plants that like it colder — lilacs and spruces, for instance — are happier.”

Barefoot keeps many of his tropical plants alive by storing them in a heated greenhouse in his back yard, soon to be hauled back into his front garden of exotica. But the now-dead towering palms functioned as anchors to the floral show, he said, and lent the display an air of a botanical garden.

The Mexican fan palm — Washingtonia robusta — was housed in a tubular superstructure designed for zoo aviaries, but he would have to erect three-story scaffolding in the fall to put it up and in the spring to take it down. “I had to have three guys to help me with it,” said Barefoot, a remodeling contractor. His friends joked at what they called “the Washingtonia Monument.” The frame was sheathed in plastic and contained a heater and temperature sensor. The winter before last, he could barely keep the palm warm enough to avoid death, and although the Bismarckia was similarly protected, it died.

The windmill palms were as tall as 15 feet, and he had grown used to not wrapping them as they grew every taller each year. “That one there would cost $2,000 to replace,” he said, pointing to the largest. “But I’m not going to plant them again.”

One person rejoicing at the harsh winters is Tony Avent, whose Plant Delights Nursery in Raleigh, N.C., features a 28-acre botanical garden, where he tests plants for their performance. He plants as many as 2,000 new varieties annually, and is happy that the past two winters killed as many as a third of them.

“I’m thrilled, and all my plant people look at me as if I’ve got snakes coming out of my head,” he said. “Having these mild winters, yes, I got to enjoy the plants, but I learned nothing.”

For Avent, the weather has simply returned to a cooler cycle. “The last two winters have been dead on what it should be,” he said. “It’s just that for 15 or 20 years everybody drank the Kool-Aid.”

Avent is known for his unusual and choice varieties of hardy perennials, oddball tropical plants and those in between. The milder winters played to the garden geek’s deep desires to push the horticultural envelope. “Everybody wants the plant they can’t grow; we want what we can’t have,” he said. “That’s human nature.”

If winters return to a milder pattern, gardeners will forget the carnage, he predicted. Thirty-one years ago, a deep freeze killed all the camellias to the ground in Raleigh, he said. Within two years, homeowners were replanting them. “Humans as a group have very short memories,” he said.

In his greenhouse, Barefoot could be found last weekend rummaging for something to plant where the Mexican fan palm now stands, its trunk and silhouette imposing even in death. Amid the tree ferns, bougainvillea and birds of paradise, he caught sight of a large palm from South America named pindo, reputed to be one of the hardiest. “I have a friend from Uruguay, and she said you can see them there growing with snow on them.”