First lady Michelle Obama will serve as honorary chair of the National Cherry Blossom Festival next year, organizers announced Wednesday, adding that the event will be expanded from two weeks to five.

The 2012 festival marks the 100th anniversary of the planting of the famous Japanese cherry trees in Washington, and the organizers hosted a luncheon Wednesday to unveil special centennial aspects to the celebration.

They announced that renowned artist Peter Max has executed a centennial festival poster — a kind of psychedelic rendering of the U.S. Capitol, cherry blossoms, and figures floating against green grass and a pink sky.

In addition, the festival is collaborating with the National Geographic Society to produce the commemorative “Cherry Blossoms: The Official Book of the National Cherry Blossom Festival,” due out in February.

Among other events, the National Gallery of Art is planning an exhibit titled “Colorful Realm of Living Beings,” a 30-scroll set of paintings never before seen in its entirety outside Japan, the festival said.

And the U.S. Postal Service has created a Cherry Blossom Centennial stamp that will be issued in March.

The annual festival marks Tokyo’s 1912 gift to Washington of 3,000 Japanese cherry trees, most of which were planted around the Tidal Basin.

Next year’s spring blooming will be augmented by the new Martin Luther King Jr. Memorial, which has been erected in the middle of the Tidal Basin’s belt of cherry trees, officials said.

The 2012 festival will run from March 20 through April 27, with the annual blossom parade set for April 14.

Last spring’s blooming period ran from March 26 through April 4, according to the National Park Service, and the previous blossom festival ran from March 26 through April 10.

The blooming period starts several days before the peak bloom date and can last as long as 14 days, the festival says on its Web site. But frost or high temperatures combined with wind or rain can shorten this period.

So next year’s festival will probably begin well before the bloom and last long after it is over.

Still, organizers and supporters expressed excitement Wednesday over the centennial, noting that the festival annually draws a million visitors to Washington and generates $150 million for local businesses.

Ichiro Fujisaki, the Japanese ambassador to the United States, noted that the festival was less than six months away.

“I’m so . . . excited,” he said. “ I feel like an 8-year-old boy.”

“There are many things good in Washington,” he said, but the festival “is the very best, because it’s nothing but beauty and friendship.”

Mayor Vincent C. Gray said: “Each year the world pauses, and certainly this city pauses, to celebrate the beauty of the Japanese cherry blossoms. . . . This celebration, and this event, is not just about the gift of trees. It is really about the gift of friendship — an enduring friendship over almost now 100 years.”

“When Japan made the gesture of goodwill on March 27, 1912,” he said, “I don’t think anyone could have known that this was creating a national treasure for generations of Americans and visitors from around the world.”

Max, the 73-year-old artist famous for his 1960s images, was not present at the luncheon but displayed his poster in a video message.

“This is the piece I’ve done,” he said, showing the image, which also depicts the Washington Monument, a blooming cherry tree, with doves and planets and people floating in the air.

“Take a look at it. This is going to be the poster you’re going to have. I’m really happy to have done it. And I can’t wait to see you all. Take care. Bye bye.”