Washington is paying tribute this week to the first lady who helped dress the city in its spring finery of blooming tulips and blossoming dogwoods. Lady Bird Johnson "deserves the national applause and appreciation for what she's done for the country," says David Northington, executive director of the National Wildflower Research Center in Austin, Texas. Separate ceremonies were planned Thursday in the House, Senate and White House Rose Garden to honor Mrs. Johnson, wife of ex-president Lyndon Johnson and founder of the wildflower center.