After years of planning, the National Arboretum is preparing for a $2 million expansion that will upgrade the arboretum's front entrance and convert a crumbling turn-of-the-century brick factory there into a demonstration garden and teaching exhibit.

The first step, construction of an 800-space parking lot near the New York Avenue entrance, has already begun. The lot will nearly quadruple the parking available in the 444-acre garden park that attracts more than 1 million visitors each year.

The rest of the expansion, still mired in environmental and archaeological impact studies, calls for demolishing 10 of 12 two-story brick kilns near the entrance and turning their foundation outlines into round demonstration gardens of hardy plants and vegetables.

The other two kilns will be converted into indoor teaching exhibits on practical home gardening.

The dome-shaped kilns had been owned by United Clay Products Co., which operated for nearly 75 years before it closed in 1971. Four years later, the federal government purchased it and the surrounding 33 acres for $5.5 million with the idea of providing a move visible arboretum entrance.

Henry Marc Cathey, director of the arboretum, said the interest today remains attracting more visitors, expecially from among the 100,000 cars that travel New York Avenue daily. For years, only a small sign on a chain-link fence has announced the sequestered garden park, the only federally-funded arboretum in the country.

"Most of {the commuters} have no idea this wonderful place is here," Cathey said.

The new entrance as proposed would be widened and flanked by four 15-foot sandstone pillars that once helped support a heavy iron fence around the U.S. Capitol.

The carved sandstone monoliths were among about 30 that supported a fence with a gatehouse erected after British soldiers set fire to the Capitol in 1814. The pillars, designed by then-architect of the Capitol Charles Bullfinch, were removed in the 1870s when the Capitol grounds were enlarged.

Cathey said a telephone tip alerted him that some of the old pillars had been stored back among the trees behind Fort Totten, and he knew he had his gateway motif. "I wanted a monumental entrance, but I didn't want to have anything specially built," he said.

Washington historians say they don't know what happened to the rest of the pillars, although years ago, several were moved, along with the gatehouse to the corner of 15th Street and Constitution Avenue NW.

The rest of the expansion plan calls for converting the overgrown brickyard into a permanent, practical garden exhibit patterned after a short-term exhibit called the National Country Garden there that won rave reviews a few years ago.

The new version, to be called the New American Garden, will feature what Cathey called "tough plants for tough times," vegetables and flowers that thrive with minimal care in harsh environments.

A container garden, for example, would show urban apartment dwellers how to successfully grow flowers and vegetables in small spaces and without soil.

An engineer's report two decades ago called the brickyard buildings unsafe and recommended tearing them down, but Congress had agreed to save at least two kilns as a condition of purchase.

Cathey said the expansion will honor that agreement. "We kept as much of the old brickyard as we could, but most of those buldings were beyond restoring," he said of the plan.

Demolition must still be voted on by the D.C. Historic Preservation Review Board. Environmental impact studies are also underway, as are archaeological impact studies. Records show that the area was inhabited by several Indian settlements over the years.