Stand at an overlook along the mile of paths winding through the enchanting 11-acre Azalea Garden that opens on Saturday at the New York Botanical Garden in the Bronx, and look down over the lush plantings. Broad sprays of white, pale pink and lavender seem almost haphazardly arrayed. Some flowers are just beginning to reveal themselves; others are extravagantly pronounced, sweeping across the hill in bands, punctuated by coarse foliated rock formations and tall trees that have yet to display robust signs of spring.