The rich blue salvia is in bloom. Ditto the red yarrow, the yellow yarrow and the pure white bellflower.

Thousand of flowers stand at attention, like horticultural brigades waiting to march up the hill and pour through the columned west entrance of the Philadelphia Museum of Art.

They create a splashy parade ground — great squares of blue, red, yellow and white — where before had been a simple swath of green lawn.

These flowers, which will fade and be superseded by others coming into bloom (next up: blue false indigo, red blanket flower, yellow false indigo, white gaura), which, in turn, will fade as still more fluoresce, are the botanical conception of artist Sol LeWitt, imagined more than 30 years ago.

But only now has LeWitt's Lines in Four Directions in Flowers, commissioned in 1981 by what was then the Fairmount Park Art Association (now the Association for Public Art), been realized by the Philadelphia Museum of Art. It is a unique work of living public art conceived by a modern master (LeWitt died in 2007) specifically for the nondescript greensward that stretches past the Anne d'Harnoncourt Sculpture Garden. It has been a huge site — about a third of a football field — largely overlooked by all who saunter past.

Thursday the museum formally "opened" the garden, which will remain in place, evolving from season to season, for about two years. It's been a long time coming.

"Nobody ever said to me that Philadelphia moves expeditiously," Timothy Rub, CEO and director of the museum, told about 100 people assembled for the occasion near the Reilly Memorial Fountain. "But still, 30 years is a long time."

Penny Balkin Bach, the art association's executive director, said LeWitt conjured up the idea in response to her efforts to attract public art proposals from a variety of artists.

"Why wasn't it done then? Lots of reasons," Bach said. "The museum wasn't as outward thinking then as it is now. At the time we didn't have partners. … So we set it aside."

The idea went into a kind of hibernation, reviving when the museum began planning its sculpture garden five or six years ago. Alice Beamesderfer, now the museum's deputy director for collections and exhibitions, eventually urged Rub to consider implementing the project and he agreed.

OLIN, the design and landscape architecture firm involved in multiple projects that amount to a nature-based revival of the Benjamin Franklin Parkway, was brought in to interpret LeWitt's instructions for the piece, which were vivid in their own way, but left great latitude.

LeWitt wrote in his precise block hand: "Proposal: To plant flowers of four different colors (white, yellow, red & blue) in four equal rectangular areas, in rows of four directions (vertical, horizontal, diagonal right & left) framed by evergreen hedges of about 2' height …. The type of plant, height, distance apart, and planting details would be under the direction of a botanist and the maintenance by a gardener."

OLIN partner Susan Weiler said she was the botanist imagined by LeWitt.

"The challenge was to get the pixilation of it, to get the vibrancy of it," she said. "LeWitt gave us the dimensions and the height of the hedge and everything else was left to our imaginations."

Working with Michael Van Valkenburgh Associates, Weiler came up with a computer algorithm to determine placement of each flower — there are more than 7,000 individual plants — and the blooming sequence.

"How many times have I read [LeWitt's instructions] and wondered if I could or would get it right," Weiler said. "The landscape is such a dynamic medium."