KAREN MALLARY of Anacortes, Wash., had been painting professionally for two years when she dashed off a letter to the Postal Service, suggesting she should design some stamps.

"I just wanted to be considered as an artist," said Mallary, who had collected stamps as a child. Mallary did not tell the Citizens' Stamp Advisory Committee what she wanted to paint, but her specialty left them little choice. "I do flowers; that's all I do."

Later this month, six years after Mallary's brash proposal arrived at Postal Headquarters, she will make her debut as a stamp artist. It promises to be a spectacular one.

Fifty different stamps, each bearing Mallary's watercolor image of a native American wildflower, will go on sale July 24 in Columbus, Ohio. They are the latest in a series of stamp sheets that have carried 50 different images, including state flags, state flowers, and birds and wildlife. All have been among the most popular stamps that the Postal Service has issued.

Postal officials are expecting that Mallary's wildflower stamps, being issued at the American First Day Cover Society's national convention, will continue that tradition. The service has been so taken by her paintings that it reproduced them in a 64-page album featuring photographs and details about each of the flowers.

It wasn't until 1989, three years after Mallary had offered her services, that the Postal Service accepted her idea. Initially, its plans called for a block of four garden flowers, Mallary recalled in an interview.

Postal officials let Mallary pick the flowers and she decided to go by region. Fragrant waterlily represented the South; Jacob's ladder, the North; California poppy, the West; and large-flowered trillium, the East.

When members of the stamp committee reviewed her work, they became "very enthusiastic," Mallary said. They decided to expand the project to 50 separate stamps, an unprecedented honor for a first-time stamp artist.

The assignment came with a caveat that made Mallary uncomfortable. She could not tell anyone about her project.

That made life at the Lady Bird Johnson National Wildflower Research Center in Austin, Tex., where she was doing research, somewhat difficult, Mallary said. "They assumed I was working on a book and I didn't tell them otherwise."

Her designs were completed last year, but uncertainty over the first-class postage rate led postal officials to delay the issue. Mallary said she has yet to see the printed stamps and is "so worried" about how they will be received. "I hope they'll be bright and colorful."

Unlike the previous sheets with 50 different designs, her wildflowers were printed by a private printer in a process that could produce several varieties. Ashton-Potter America Inc., a newly formed U.S. subsidiary of an Ontario firm that prints 80 percent of Canada's stamps, is producing the stamps at its Amherst, N.Y., plant.

The contract fulfills the hope postal executives expressed last year in the midst of a controversy over the printing of two U.S. stamps in Canada. After Congress threatened to outlaw the practice, postal officials said they believed they could get a Canadian company to move south and print stamps in the United States.

Ashton-Potter America is printing the stamps simultaneously on two offset presses, one of which produces four sheets of stamps and the other six sheets at a time.

Stamps printed on the four-sheet press will carry odd plate numbers and those from the six-sheet press will carry even numbers. To help collectors who collect sheets and plate number blocks, a diagram is being printed on the sheet margins to show where the sheet was printed on each press, another first.

The use of two presses creates the possibility of printing variations and the different plate numbers and the diagram should tell which press may have produced any variety.

The wildflowers on Mallary's stamps literally tumble off their black backgrounds and onto the white borders of each stamp. The designs are as colorful as the name of each species: showy evening primrose, claret cup cactus, Turk's cap lily, yellow skunk cabbage, Dutchman's breeches, smooth Solomon's seal . . . .

Mallary, a San Francisco native, said she doesn't have a favorite. "Like an artist, they're all my children," she said. But when pressed, she paused and said, "the cactus."

INDIVIDUALS who wish first-day cancellations of the wildflower stamps may purchase the stamps at local post offices, place them on addressed envelopes and send them to: Customer-Affixed Envelopes, Wildflower Stamps, Postmaster, 850 Twin Rivers Dr., Columbus, OH 43216-9991. Postal workers will affix randomly selected stamps at a price of 29 cents per stamp on up to 50 envelopes at: Wildflower Stamps, Postmaster, 850 Twin Rivers Dr., Columbus, OH 43216-9992. Orders must be postmarked by Sept. 22.

A full sheet of all 50 stamps affixed to a 9 1/2-by-11-inch envelope with first-day cancellations is available for $17 from: Wildflower Stamps, Philatelic Sales Division, P.O. Box 449997, Kansas City, MO 64144-9997. The wildflower album with a mint sheet of the stamps is available for $22.45 from: Wildflower Album, U.S. Postal Service, P.O. Box 14328, St. Paul, MN 55114-0328.

Bill McAllister is a member of The Washington Post national staff.