By DOUGLAS MARTIN

A nervous thread runs through the life of David P. Murbach. "It's on my mind every single day of the year," he said.

Though it might vary a foot or two from year to year, his obsession is forever green, 80 feet tall and weighs seven and a half tons. It is the Christmas tree at Rockefeller Center, and his always urgent mission, as the complex's chief gardener, is to find the next one.

"When I'm at the nursery shopping for Easter lilies, I'm looking for a tree," he said. "When I'm going to the beach in the summer, I'm looking for a tree."

This year, the 42-year-old horticulturalist landed a dandy, a 70-year-old Norway spruce from just across the Connecticut border. Worried that it might eventually fall on their house, its proud owners mailed in a snapshot, something that often happens but almost never works out. Last year, Mr. Murbach rented a helicopter to spot his arboreal prize, in New Jersey. He looks for height, fullness and a nice triangular shape. As always, a sapling is planted in the felled giant's place.

Tonight at 6 o'clock, if only for the split second it takes to flip a switch, Mr. Murbach will be able to relax at last: 25,000 lights aligned on five miles of wire will be illuminated, and it will officially be Christmastime in New York City.

Gardeners like Mr. Murbach have always been part of Rockefeller Center, which the American Institute of Architects Guide to New York City calls "the greatest urban complex of the 20th century." There used to be more greenery than there is now: old pictures show the Rockettes playing volleyball in the middle of a garden, now gone, on the top of Radio City Music Hall. 

 "Without the gardens, Rockefeller Center wouldn't be what we call the Village Green," said Richard A. Voell, president and chief executive officer of the Rockefeller Group, which runs the complex for Mitsubishi, the controlling owner.

"The gardens are what make it a people-friendly place." 

Usually in suit and tie, Mr. Murbach presides over one of the world's most famous urban gardens, with the help of five workers. He designs and installs plantings in five private rooftop gardens, two small parks, five plazas and the lobbies of the center's 18 skyscrapers. The Channel Gardens, so called because it separates the British Empire Building from La Maison Francaise on Fifth Avenue, are the site of the larger shows, and also the center's most difficult horticultural space: a narrow box 32 inches wide, 18 inches deep and 120 feet long.

Mr. Murbach sees his mission as providing bold splashes of color to an elegant but monochromatic stone temple. "I look for lushness to offset the limestone of the buildings," he said.

He must also capture people's attention in a visually vibrant city. "I have to compete with yellow taxicabs," he said. "I try to be brighter and bolder than I would be in a quiet country garden."

His answer, delivered in 14 or so shows a year, includes a yearly tropical show with large palm trees and jungle flowers. For desert shows, cactuses and other succulents are surrounded by volcanic rock and desert driftwood. There are gargantuan topiary depictions of dinosaurs, or landscapes fashioned from rhododendrons, spring bulbs, Easter lilies and golden chrysanthemums.

This summer, working with a community gardening group called Bronx Green Up, he planted vegetables like squash, eggplant and corn in the Channel Gardens. "People were surprised to come around the corner and see a cornfield," he said.

Christmas is always the biggest spectacle, at least in terms of the number of visitors, more than 10 million. In addition to the Channel Gardens' traditional wire-woven angels, first introduced in 1954, and the giant toy soldiers by the skating rink, decorations this year include hundreds of poinsettias and a new display: piles of giant red tree ornaments, eight feet in diameter.

Mr. Murbach sees parallels in his work to the decorating of expensive store windows. "I delve in ornament," he said.

This year, the American Association of Botanical Gardens and Arboreta awarded him a citation: "From desert gardens to a native azalea collection, from ornamental grasses to pine woods, your talents as a designer are witnessed and enjoyed by millions of people living, working or visiting Manhattan."

Judith Zuk, president of the Brooklyn Botanic Garden, puts it plainly: "He probably gets about twice as much done as ordinary mortals might."

In his youth, Mr. Murbach visited his father's law offices overlooking the Fifth Avenue roof gardens, elegant formal gardens visible only from windows on higher floors and not open to anyone. Mr. Murbach answered a newspaper ad for the job in 1984. One of his first tasks was to restore those very same gardens, planting new hedges and shrubs and restoring the lawn using old photographs as a guide. Fifteen workers had to carry tons of soil in buckets up the elevators.

After graduating from the University of Arizona with a degree in forestry, he spent seven years supervising greenhouses and orchards at the 28-acre private Kraft-Gwydir Estate in Cold Spring Harbor, L.I. He then got a master's degree in public garden administration from the University of Delaware while working at the Longwood Gardens established by the duPont family.

Mr. Murbach said he often finds himself daydreaming about floral extravaganzas. He does not design at a table, but gets ideas from watching movies or operas. He got the idea for an all-purple show after seeing a boy drop a lavender balloon in the Miami airport that landed on purple flowers.

Sometimes, he doesn't decide exactly what to do until he sees the plants unloaded from the truck. In any year, Rockefeller Center may display more than 10,000 plants.

Mr. Murbach, who owns a T-shirt emblazoned "Hortisexual," is so much a plant lover that he avoids leafy vegetables, which he has suspects contain toxins that evolved to dissuade animals from eating them. A quart of milk and bag of cookies are his preferred foods.

"I can't eat plants," he said. "They're my friends."

The Christmas tableau at Rockefeller Center is quintessentially New York. The tree is to be lighted at 6 tonight, signaling the beginning of the Christmas season in the city.

THE TREE

This year's tree, a Norway spruce, is 80 feet tall, weighs 7 1/2 tons and is about 70 yearsold.

It was a gift from Alan and Maria Egler of Ridegefield, Conn. It was brought to New York on a custom-made trailer.

As with all trees since 1971, this year's will be recycled; the mulch is used for trails at a Boy Scout camp in New Jersey.

THE ANGELS

The nine-foot angels in the Channel Gardens leading up to Fifth Avenue were created by Valerie Clarebout, an English sculptor and first used in 1954. They are made of aluminum, brass andcopper wire.

THE DECORATIONS

More than 25,000 multicolored 7 1/2-watt bulbs are strung on over five miles of wire to decorate the tree. Each branch is individually wrapped. 

THE WORKERS

It takes 15 to 20 people and an 80-ton crane to erect and remove the tree.

THE STAR 

The 45-inch-wide plastic star has decorated the tree for more than 40 years.

"A LOOK BACK: Rockefeller Center Christmases Past"

1939. The tree was not trimmed, except for a lighted star on its top, and was illuminated by floodlights.

1942. During wartime, three trees were placed in Rockefeller Plaza. One tree was trimmed with white globes, one with red globes and one with blue globes.

1945 and 1946. The tree was decorated with fluorescent colored globes and lighted wiht black light ultraviolet rays.

1973. Thousands of multicolored reflective disks were used with small bulbs to help conserve energy.

Copyright 1994 The New York Times Company