Azaleas are what set spring's blush ablaze.

"Down south, every town calls itself the Azalea Capital of the World, and when the azaleas come out, they put all the little girls in hoop skirts . . ." says Frank White of Azalea Acres Farm in Lanham. "In contrast, around the Washington Beltway, among homeowners and collectors we have a greater number of evergreen varieties of azalea than anywhere else in the world." The blossoms -- in hues of red, coral, white, pink, purple and more -- thrive in our slightly acid soil.

This weekend and the first week in May are the peak this year for the azaleas at the NATIONAL ARBORETUM. When azaleas are in bloom, up to 15,000 people make the pilgrimage each weekend, says curator Erik Neumann. It's no wonder about the crowds, as there are 70,000 plants on the slopes of Mount Hamilton above the roadway.

Many of the massed blooms are crosses, unnamed varieties. But there are 400 named ones in the Ben Morrison Garden -- a sort of azalea library, named for the man who developed the Glenn Dale hybrids when he was chief of the Plant Introduction Center and later the first director of the Arboretum. The hours of the Arboretum, at 24th and R Streets NE, are weekdays 8 to 5 and weekends 10 to 5. Just follow the stream of traffic through the grounds.

The azaleas at BROOKSIDE GARDENS, farther afield at 1500 Glenallan Road in Wheaton, will be a little slower coming out: the expected peak is mid-May. A thousand varieties may be seen in a 71/2-acre garden, open from 9 to 5. Weather permitting, an evening walk through the garden will be punctuated with azalea talk, May 13 at 6:30. And the Brookside Chapter of the Azalea Society of America plans to have a show of old varieties and arrangements in the conservatory there, May 15 from noon to 5 and May 16 from 9 to 5.

The Brookside Garden folks also care for McCRILLIS GARDENS, 6910 Greentree Road, Bethesda. Here there's nothing but azaleas, expected to be in full bloom the first two weeks in May. The five-acre garden is open daily from 9 to sunset. Weekends from 10 to 4 a small art gallery exhibits part of the Montgomery County Recreation Department's art collection. Parking is available in the lot at Our Lady of the Woods Academy across the street.

At BRIGHTON AZALEA GARDENS, overlooking Triadelphia Reservoir in Maryland, is another flashy display. The gardens are kept by the Washington Suburban Sanitary Commission. To get there take I-95 north to Route 198. Go west on Route 198 to Route 650 at Spencerville. Then go north on Route 650 to Brighton Dam Road and turn right. Brighton Dam is a mile down the road.

This Friday, Saturday and Sunday is the 29th annual LANDON AZALEA GARDEN FESTIVAL. At 6101 Wilson Lane in Bethesda, the school campus boasts gardens with thousands of azaleas and rhododendrons, tree peonies and wildflowers, and a 150-year-old farmhouse to tour. Azaleas will be for sale, along with rose bushes, wildflowers and other plants. For kids, there's an electronic game room, for adults, an antique show ($2). Lunch will be served from 11:30 to 2:30 daily, and on Saturday and Sunday the Landon Chorus and Band will perform, as will bellringers and a barbershop quartet. Admission is free.

Here are a couple other places to buy azaleas: An AZALEA MART will be held the morning of May 8 by the Brookside Gardens Chapter of the Azalea Society of America. The mart will be in the Suburban Trust parking lot at University Boulevard and Grandview Avenue in Wheaton, from 8 a.m. until sellout. Information on azalea care and culture will be available.

Every weekend in May, the TAKOMA AZALEA COMMITTEE has a sale of azaleas grown from local cuttings, at the Public Works, 31 Oswego Avenue in Takoma Park. Hours are 9 to 1 Saturdays and noon to 3 Sundays.

Everywhere you look, someone has brightened up a dreary lot or a dull corner with azaleas. Some favorite spots to see azaleas are along Reno Road in Northwest Washington and in the Woodside area of Silver Spring off Colesville Road. But for the real enthusiast to see local private collections close up, the best bet is to join the Azalea Society. The members open their gardens to each other. There are three chapters in the area: For details, write the Azalea Society of America, Box 6244, Silver Spring, Maryland 20906.