BOYLSTON -- The mercury is nudging the nether side of zero, and the wind howls through the parking lot and whistles across acres of formal gardens lying under a glaze of snow.
It's a perfect day to visit Tower Hill Botanic Garden. The reason is Tower Hill's Orangerie, a garden under glass filled with citrus trees, flowering shrubs and vines, and an assortment of spring flowers forced into early bloom. In winter the brick-and-glass building gleams like the Holy Grail on the far side of a wind-scoured snowfield.
At this time of year, the entrance to the orangerie is through the visitors center in an adjoining building. A stroll down an enclosed gallery lined with potted plants and fountains hints at the delights to come.
The orangerie at Tower Hill is descended from the heated garden structures invented by the Romans and perfected by the English in the 18th century, when passionate collectors struggled to nurse their tender exotic plants, including citrus trees, through the winter. In early orangeries, the roof was solid and only the south-facing wall was made of glass. This state-of-the-art version, which opened in 1999, combines the stout masonry and single-glassed wall of early orangeries with a vented glass roof that allows plants to breathe.
Designed for subtropical species, the orangerie heats up inside to as high as 70 degrees during the day and cools to 45 degrees at night. (Tropical plants require a constant 65 degrees.) It is, in the words of John Trexler, Tower Hill's executive director, ''the perfect envelope for plants."
Even before a view opens into this perfect envelope, the sound of trickling water announces the entrance to an exotic realm, and the fragrance of orange blossom beckons from the corridor. Visually, the 4,000-square-foot space is a study in texture, color, and form, spiked with exuberant accents. Three aisles create orderly pathways on the ground. Spires of blue-tinged cypress flank the doorways, squat palms spread out in fan-shaped silhouettes, and the banana leaves throw jungle shadows. All of these overarch about a dozen scattered citrus trees.
The tangerine, orange, lime, and citron trees scattered thoughout this lush interior sport both fruit and flowers from January through March, cued by evolution to squeeze in all this reproduction during the shortest days of the year. Below them, hundreds of perennials and annuals fill in the spaces and create lush borders along the stone floor. Water burbles from several fountains on the wall and floor, tucked away to surprise passersby.
Visitors trickle in, tucking gloves in pockets. The sun has on its own managed to coax the temperature up to about 60 degrees -- light sweater weather -- despite the arctic chill outdoors. After swiveling their heads to take in the soaring space, most people amble down the aisles, ogling plants that would be felled by the first hard frost in a New England garden. There are so many half-hidden delights that people tend to stop short, drawn to a flash of color or a phantasmagorical shape.
Trumpet-shaped bells of brugmansia dangle at eye level, terra cotta pots bristle with paperwhite narcissus, and a deep magenta rose climbs the north wall. Nearby, a pale, mysterious fruit the size and shape of an ostrich egg dangles from a vine. A label identifies it as Passiflora edulis ''McCain": a yellow passionfruit. In the aisle next to the glass wall, an eye-catching flower with a center like an exploding violet star turns out to be in the same family. Fat buds clustered among glossy deep-green leaves prove to be camellia. Bougainvillea, a flowering vine with rose-tinged blossoms, twines near a feathery cypress. Ferns of all descriptions abound.
Beyond the glass, garden structures -- pergolas, pavilions, balustrades -- cast intriguing shadows on the snow, and the views to the Wachusett Reservoir are crystalline. In the other three seasons, the entire property, with its 132 acres of formal gardens, orchards, and woods, is a paradise of a different kind. Even in winter, on more temperate days visitors explore the frozen outdoor gardens, tracking wildlife and spotting winter birds. Others strap on cross-country skis and head out on the three miles of trails looping through the property.
But when icy winds frost the nosehairs, the Latin motto chiseled over the orangerie's exterior entrance, borrowed from a Moorish garden in Shalimar, India, reads like the answer to a prayer: ''If there be heaven on earth, this is it -- joy everlasting."
Jane Roy Brown is a freelance writer in western Massachusetts.