The National Arboretum will celebrate its 50th anniversary this year by expanding and opening a new entrance along New York Avenue, near the Route 50 entrance to Washington.

The new entrance, to open next month, will replace the small, side-street entrance on R Street, off Bladensburg Road, that visitors have had trouble finding ever since Congress created the scenic 412-acre park in Northeast Washington in 1927.

A $5.5 million, 33-acre addition to the Arboretum - a former brick manufacturing plant purchased by act of Congress last summer - has made the new entrance possible.

A new $400,000 national herb garden also will be started this year at the Arboretum, the donation of the Herb Society of America. The Arboretum, which only last year opened a new Bonsai pavillion to house the $4 million Bicentennial gift of Bonsai trees from Japan, last year received a change in its charter to allow private gifts.

Despite the present awkwardly placed entrance, in an area of small homes and industrial plants, this Arboretum had more than 500,000 visitors last year and already this year the number of tours requested has been 50 per cent greater than during the Bicentennial, according to Arboretum Director John L. Creech.

Creech said this week the additional 33 acres and new front gate represent "a major addition to the Arboretum, culminating more than two years effort," and a plus for Northeast Washington. The Aboretum, with its extensiv shrub, tree and flower collections planted on hilly woodlands and meadows overlooking the Anacostia River, is one of the few "asethetic" things in the Northeast section of the city, Creech said.

Included are extensive azalea, camelia, crabapple and dwarf conifer collections, some among the largest and best in the nation, as well as just about any domestic and foreign plant most gardeners name. Its flowering magnolias and plum trees are already in bloom, as are many of its daffodils and wildflowers.

The Bonsai collection, housed in a $300,000 outdoor pavillion opened last spring, is one of the largest in this country, with some of its 53 plants more than 270 years old. One of the trees was stolen last year but returned unharmed. The Arboretum has since hired guards on a 24-hour basis, which accounts for most of the $170,000 increase in this year's $1.4 million operating budget, Creech said.

The 33-acre that used to belong to the United Clay Products Co. until they stopped brick-making operations in 1971, is now mostly hills of bull-dozed red clay and half a dozen brick-firing kilns which the Department of Agriculture - may keep for historical purposes.

The New York Avenue entrance will thus be very visible but not very pretty for several years, until the Aboretum has hired a landscape planner to design the shape and plantings of its front 33 acres.

One major attraction planned there is a "synoptic" garden, a comprehensive living index to the plants displayed throughout the Arboretum, and a research-educational center which will have displays such as the history of American crops and flowers, similar to Moscow's famed Botanic Gardens. Most of the buildings at the Arboretum thus will be clustered at the north end, near New York Avenue.

Ultimately the Arboretum may close its 9 miles of roads to cars and institute a shuttlebus system, such as is used around the Mall. On spring weekends its roads and limited number of parking areas are jammed with thousands of cars.

"We also hope to create a picnic area, since picnicking is presently prohibited on Arboretum grounds," said Creech, "and we hope to install more public restrooms." There are now only two within the Arboretum.