John Saia examines the mystery citrus in his Sterling garden. It has survived freezing temperatures since 2007. (Adrian Higgins/The Washington Post)

There are clues to John Saia’s passion for pushing the plant envelope in the front yard of his otherwise comfortably congruous suburban home in the Potomac Falls neighborhood of Sterling, but his wonderfully alien world deserves a closer look.

In the entrance bed you will find the hardy orange variety named Dragon Lady, a citrus that is the toughest species we have, and grows well north of Washington in spite of its exotic and fearsomely spiny appearance. Its little orange-yellow orbs are ripening as we speak. (Take one fruit and, to sweeten, add eight pounds of sugar).

Nearby, you will find the dwarf palmetto, Sabal minor, a common native palm south of Virginia but an oddity in these parts.

Next comes a eucalyptus species called the Omeo gum tree, well on its silvery way to establishment if not maturity.

A little farther down, a windmill palm named Trachycarpus fortunei grows from a foundation bed where others might raise nandina or azaleas. The palm, the books say, is safe up to the Carolinas, but here, under the flight path of Dulles International Airport, it is almost eight feet across.

Walk down by the gate and you find another eucalyptus, the snow gum.

“It’s fun coming out here when it snows,” Saia said, for a spectacle that “doesn’t make any sense.”

Let’s cut to the chase, because amid all his palms, eucalyptus, bananas and other examples of his “tropicalesque plants,” Saia is growing a mystery citrus tree outdoors that now reaches to his second floor. It is bearing 20 fruits (up from three last year) that will turn orange in November. If last year’s experience is repeated, the fruit will reach the size of a golf ball or bigger, shift to its ripe color and exhibit a sweet flavor in flesh and rind.

Surprisingly, quite a few citrus plants will grow in a Washington garden — among them the hardy orange, a hybrid called the hardy grapefruit or citrumelo, and a species named Citrus ichangensis — but Saia’s orange tree has a sweetness lacking in the others.

The citrus lover who grows plants in large pots and has a bright, cool place to keep them in winter has a rich choice to hand. Common summer patio-winter conservatory fare includes Meyer lemons, calamondin oranges, ponderosa lemons, kumquats, limes and much more. But for Saia and fellow members of the Virginia Palm Society, this enigmatic plant holds the promise of bringing an outdoor citrus experience to our region enjoyed by gardeners in states such as Florida and California.

Saia and tropical plant buddy Joseph “Boca Joe” Seamone acquired the plant from Dave Klemm, another tropical plant devotee who picked it up at the annual Southeastern Citrus Expo in 2006. Saia planted it as an eight-inch transplant the following spring.

Klemm said he bought it from a South Carolina nurseryman who received it, in turn, from a Florida citrus grower who was unloading plants raised as rootstock. Fruit trees are commonly grafted onto a rootstock to improve performance.

The nurseryman thought it might be a seedling from a rootstock developed by the Agriculture Department — a cross between the hardy orange and, possibly, a hardy mandarin named Changsha. Because its seedlings differed so much from the parent plant, its value to rootstock growers was diminished. “It was hard for them to reproduce it on a large scale,” said Klemm, who lives in Annandale. “It’s Zone 7 hardy and [Saia] claims it’s edible; hopefully I’ll get a chance this fall to sample it.”

Its fruit is not much bigger than a golfball. “If I didn’t know any better, I would have thought it was a mandarin from the grocery store,” said Seamone, a Germantown-based designer of tropical landscapes. Saia’s tree grows in the lee of the house, but in a garden that still gets the chill of the outer suburbs.

Citrus guru Tom McClendon, in his book “Hardy Citrus for the Southeast,” lists an amazing number of species and hybrids that in theory have a chance of surviving in a Washington garden, especially if sheltered. He lists six species or crosses that are hardy below 10 degrees, and another dozen than could probably take temperatures down to that threshold. These include the Ten-Degree Kumquat, which is a hybrid of Fortunella japonica, and the Juanita Tangerine (Citrus reticulata). Saia has both in pots and is working up the courage to plant them in garden beds.

Hardy citrus, palms and eucalyptus are best planted in the spring, so they can establish themselves before their first winter.

Seamone doesn’t ascribe their success to climate change, saying it is tied to the genetics of a given plant.

Fans of these outliers say their survivability is linked to how well they are grown, their locations, their age and the nature of the freezes that threaten them. My instincts tell me that they would do best in an east-facing bed sheltered from winter sun and winds, and planted in soil that drains freely, not wet clay.

With some imagination and ad­ven­ture, and the blending of hardy and tender tropical type plants (such as bananas, taros, caladiums, coleus and cannas), it is possible to create a garden that has all the hedonistic feel of a Caribbean landscape; break out the rum punch, put on the calypso music (in a neighborly way, please) and let the rush hour melt away.

“I want to be different,” Saia said. “If you drive down the typical suburb here you see Knock Out roses, some azaleas, rhododendrons, arborvitae. To me there’s no knockout factor. I know cold-hardy tropical gardening isn’t for everybody, but it’s something I have liked.”

As I left his garden, Seamone pointed out a couple of stout trees. “Live oaks, they grow fine around here.” And, yes, hanging from one was some Spanish moss.

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