Here's what the Hardly Strictly Bluegrass festival brings to San Francisco during one glorious early-autumn weekend: bluegrass pioneers like Earl Scruggs and Ralph Stanley, soul men such as Otis Clay, folk heroes including John Prine, and country legends such as Merle Haggard.

During the three-day festival this weekend, 87 bands are slated to play on six stages in verdant Golden Gate Park. Yet here's the most amazing thing of all: The festival is greater than the sum of its parts.

It's a free celebration of diverse music, food (superb vendors and picnicking) and community. There are no tickets, no turnstiles, no sponsors, no billboards.

And musicians love to mix it up and join one another onstage. Though it's not on the published schedule, former Led Zeppelin vocalist Robert Plant will join Patti Griffin on Saturday and Buddy Miller on Sunday.

During the past couple of years, the three-day attendance has reached about 800,000, according to police estimates, yet the crowds have been manageable thanks to savvy planning.

This year the festival is expanding to include a full day of music on Friday (rather than just a few acts late Friday afternoon), as well as six stages running all weekend until the closing sets at twilight on Sunday.

As wonderful as this year's lineup is, it's missing two bluegrass stalwarts who've been regulars at Hardly Strictly: Doc Watson and Hazel Dickens. Bluegrass legend Watson, 88, wasn't well enough to travel and Dickens died at age 75 of pneumonia in April.

"We don't want to run out of old-timers," said HSB co-producer Dawn Holliday, noting that Dickens was a central inspiration for the festival and will be honored throughout this year's event.

A decade ago, when investment banker Warren Hellman met with Holliday to discuss a free bluegrass festival, Holliday asked if Hellman wanted to include Dickens.

"I love Hazel," Hellman said, but the pair quickly agreed they needed to get Emmylou Harris on the bill so people would come. Harris headlined that first year and has been at every Strictly Bluegrass show since then.

That first show drew about 20,000 people; this year's 11th annual festival may draw close to 1 million.

"Only in San Francisco," marveled Chicago Bluesman Otis Clay, who will bring his incendiary blend of soul, gospel and blues to the park on Saturday. "There's no place on earth like San Francisco."

Clay recalled he was scheduled to play at the San Francisco nightclub Slim's the weekend after the 1989 earthquake. He wasn't sure if the show would go on, but the club manager said, "We've been talking about nothing but the quake. Come on out. We're gonna party."

That spirit is alive and well at Hardly Strictly, where music and communal good will trump hardships. And in today's economy, when many can't afford to pay inflated concert ticket prices, the festival is a genuine gift to the Bay Area.

Pete Sears, who will be appearing with his jam band, Moonalice, says the park has a historic place in the annals of San Francisco music.

A former member of Jefferson Starship, he recalled that the Grateful Dead, Jefferson Airplane and other psychedelic bands that were rooted in folk and bluegrass often played free Golden Gate Park shows during the 1960s.

Speaking of Hardly Strictly, he said: "I can't think of anywhere else that you get such a fantastic concentration of musicianship in one place for free."

Hardly Strictly is also a place for bands to make a big splash when they're doing something special.

The Jayhawks, a 1990s Americana band that evokes the jangly sound of Neil Young, lost a founding member when Mark Olson departed in the mid-90s. A decade and a half later the band has reformed, released a new album and will be playing Sunday at HSB and tonight <NO1><Fri 9/30><NO>at Slim's.

Hellman, a banjo player, will play an early set Saturday with his band the Wronglers, joined by Jimmie Dale Gilmore. He modestly says the festival, whose price tag he wouldn't reveal, is a gift to himself.

"People keep saying thank you, thank you, but this is the world's most selfish gift," Hellman told The Press Democrat last year. "To spend three days with people I adore and musicians I'm nuts about, it's an unbelievable mitzvah to be in a position to do this."

Holliday notes that most of the bands at HSB aren't big headliners; the majority couldn't draw a thousand people to the Fillmore auditorium.

But others are well-known, and all this talent, not to mention the stages and security, doesn't some cheap. Holliday appreciates Hellman's generosity in letting her book any band she wants.

"There's only one person at the end of this festival who says, &‘Check, please,' and that's Warren Hellman," she said. "So far he hasn't told me no."

Michael Shapiro writes about entertainment for The Press Democrat. Contact him at michael.shapiro@pressdemocrat.com or see his site, michaelshapiro.net.