Longwood Gardens in Kennett Square is about to embark on its most extensive - and expensive - project ever: a $90 million restoration of the five-acre Main Fountain Garden, where shows featuring shooting water, and sometimes music, lights, and fireworks, have delighted visitors since 1955.

With more than one million visitors a year, Longwood - the former estate of the industrialist Pierre S. du Pont, who designed and built the Fountain Garden in 1931 for his own entertainment - is the most popular public garden in the country. And the iconic Fountain Garden is one of its biggest draws.

When it reopens in the spring of 2017, visitors may be surprised to note that from a distance, the overhauled garden will still look like its classic self. Longwood historian Colvin Randall - like du Pont a fountain aficionado - describes that as "a combination of French grandeur and Italianate ornamentation, surrounded by a green English landscape."

But every element of the garden will have been restored, updated, replaced, or expanded upon.

The fountains' original plumbing - known in the trenches as "a network of Band-Aids" - is high on the must-go list. The pumps are decrepit; many of the 224 water jets are clogged with fireworks debris; and 20 wall-mounted fountains have been inoperable for 25 years.

"It's beyond repair. Time's up," said Jack Dunbar, a Longwood plumber for 26 years.

The 380 fountains will be restored and enhanced with new controls allowing some of the jets to hit 175 feet, 45 feet higher than currently. Fountain shows will be able to do more contemporary choreography and special effects, such as a propane-fueled flame that travels up a column of water and remains lighted at the top.

Obsolete lighting will be replaced with more-efficient LEDs allowing a wider range of color combinations.

Boxwood hedges, some from the 1930s, will give way to modern disease-resistant varieties, and their design will be reconfigured, from tightly controlled to "cloud-like."

"The boxwoods will be billowing, like eyeliner that amplifies and accentuates and dramatizes the visual," said Adriaan Geuze, cofounder of West 8, the Rotterdam, Netherlands, firm designing the fountain project's new landscape.

The current landscape, he said, "is lacking in exuberance. We found it underwhelming, considering that the horticultural level of Longwood is outstanding. That level in the fountain garden needs to come up."

The project also includes:

Longer, wider allees. Linden trees, a European favorite, will take the place of Norway maples.

Restored sculptures and limestone reliefs. Many are crumbling or covered in algae.

Reopening of the south wall - which has been closed to the public for two decades - with a new walkway on top. This affords a spectacular view of the garden and the East Conservatory up on the hill.

White-oak benches painted dark green, and fine decomposed-granite pathways in pale grays and tans. Visitors have always been welcome among the fountains, but few ventured there, and there was nowhere to sit.

A new stone grotto behind the loggia, a 21-square-foot subterranean chamber that Richard W. Southwick, partner-in-charge of the New York architectural firm Beyer, Blinder, Belle, calls "a grand surprise." It will have a dripping circular "water curtain," carved stone seating, and a four-foot opening in the domed ceiling.

"Compared to the very formal garden," Southwick said, "this is quiet and contemplative, intimate . . . and cool."

An underground pump facility to replace the ancient Pump House, whose lobby will become exhibit space. A small plaza with tables and chairs will be added next to the Pump House. "This could be the spot . . . where you sit and have a lemonade and ice cream," said Claire Agre, West 8 project director.

All parts of the Fountain Garden will be handicap accessible, and when the fences go up in mid-October, visitors will be able to follow progress on the project from the conservatory terrace, which is where Terry and Lindsay Shelton of Wilmington were relaxing one recent afternoon with their toddler, Ezra.

"The plan is very exciting. [The Main Fountain Garden] is our favorite spot," Lindsay Shelton said.

"But it's not in great shape," her husband added.

The fountain project, which follows a $9 million meadow restoration that was completed in June, is yet another indicator that the good times continue to roll at Longwood.

Since Paul B. Redman, executive director, arrived in 2006, membership has grown from 16,000 households to 52,000; annual visitation has jumped from 770,000 to 1,100,000; and the endowment is $750 million, up from $557 million.

"We remind ourselves daily of our legacy, but Longwood does not have a long arm coming from the grave," Redman said. "We have the freedom to think big."