The seeds for Bangor’s first sign of spring actually were planted last fall.

That’s when central Maine landscapers start firming up the designs for their exhibits for the Bangor Garden Show, which opens with Preview Night on Thursday.

Actually, some landscapers say, ideas began to germinate at the previous year’s show.

“While you’re there, you start coming up with things you could have added, then you file them away for the future,” said Steve Elliott of New Land Nursery and Landscaping Inc. of Ellsworth. “Then you start solidifying those ideas, and make them gel.”

“As soon as we were done last year, I started collecting rocks, and thinking about what plants I wanted to use in the display,” said John O’Keefe of Rocky Ridge Perennials in Orland.

Within six days, the landscapers transform the nearly 17,000-square-foot Bangor Auditorium from the empty mecca of basketball to a colorful, living thing, soon to be thronged by from 14,000 to 17,000 people over a four-day period.

The 10 landscaping concerns will compete for the People’s Choice Award (voted on by the show’s attendees), the Gwethalyn M. Phillips Landscapers’ Award (voted on by participating landscapers) and the Maine Landscape and Nursery Association Award (voted on by the association’s members).

O’Keefe finds himself with a hard act to follow, as he won the People’s Choice Award last year, while participating for the first time in Bangor.

His exhibit this year will consist of three rock gardens of varying sizes. He will mix annuals, perennials, roses and ornamental grasses, using a total of 110 different types of plants. This includes such common varieties as coral bells, delphiniums, salvia, hosta and roses.

“I want people to come through the door and say, `Oh, I can do something like that at my house,”‘ O’Keefe said. “I’ll use elevated rock gardens to show off smaller plants and trailing plants.”

O’Keefe began growing some of the plants from seed last year, overwintered them outside, then brought them into his greenhouse in January, to get them to bloom for the show.

“It’s a challenge to get the plants to bloom when I need them to,” he said. “Some will flower before, some after, but most are being cooperative, and doing their thing.”

Elliott, the co-chair of the new Landscape Exhibitors Advisory Committee, has taken part in six of the nine years the show has been held.

This year’s show has been organized by committee for the first time, which Elliott sees as a positive development.

“It’s helped with the diversity of the show overall,” he said.

While somewhat close-mouthed about how his finished exhibit will look, Elliott described it as a semiformal garden tucked into the corner of the Maine woods.

“It incorporates native materials with the ornamental,” he said. “Annuals, perennials and woody materials abound. Then plants and building materials will be combined to complete the exhibit.”

Elliott has been forcing his plants since January. He has plants spread throughout the Ellsworth area at different greenhouses.

“We depend on a lot of other people to help us out,” he said.

The two landscapers have different motives for participating in the show.

“I love doing it,” O’Keefe said. “It gives me a chance to be creative. It also gives me a chance to give people ideas, and to get ideas, too. Plus it’s good for my business.”

“I like the competition, and the challenge of designing and putting it together in a limited period of time,” said Elliott. “Also I like the recognition we receive.”

The theme for the Preview Night event, held 6-9 p.m. Thursday, April 8, will be a coastal summer garden party. Against a background of classical guitar and bass music, patrons will be able to view the fresh landscape exhibits and nibble on hors d’oeuvres catered by Killarney’s in Bangor. Tickets for Preview Night are $25, and are available in advance from Bangor Beautiful at 990-1201. Each preview ticket includes one admission to the weekend’s show and lectures.

A newcomer to the show’s roster of lecturers will be Cassandra Danz, the garden writer, author and TV personality better known as Mrs. Greenthumbs. She appears regularly on “Live with Regis and Kathie Lee,” is the winner of the Garden Writers Association’s Quill and Trowel Award, and writes a column for Country Living Gardener magazine. On Sunday, Danz will sign books at 10:30 a.m. and will give a presentation at 3 p.m.

Returning to the lecture series are Roger Swain, author and co-host of PBS’s “The Victory Garden,” and Barbara Damrosch, author and co-host of “Gardening Naturally,” airing on The Learning Channel. (See accompanying complete lecture listings.)

Local gardeners are invited to enter their plants in the Houseplant Contest. For rules and an entry form, call Bangor Beautiful at 990-1201.

This year’s theme for the Children’s Room, located in the mezzanine above the lobby of the Civic Center, will be the Maine Coast. Nokomis Regional High School students and staff are creating a 40-foot-long sand beach and saltwater pond. College of the Atlantic in Bar Harbor will provide a 20-foot-long whale skeleton. A touch tank of live aquatic creatures will be presented by the Maine Environmental Research Institute.

More than 70 booths will be set up at the Vendors Marketplace in the Civic Center. The centerpiece of the food court, located in the lobby of the Civic Center, will be a fresh-flower market, new this year. Proceeds from sales there and from the show itself will benefit the recycling and beautification programs of Bangor Beautiful.

Elliott said several things bring visitors to the show.

“The majority are coming to see spring early,” he said. “It’s like walking into a different world. A lot of people come looking for new ideas. Then there’s the miracle of the transition from concrete floor to a living, blooming garden. The biggest question we get is `How do you do this?”‘

Show hours are 9 a.m.-9 p.m. Friday and Saturday; 10 a.m.-5 p.m. Sunday. Ticket prices are $6 for adults, $2 for students and under 5 free. Tickets are available at the door. For more information, call Bangor Beautiful at 990-1201.