The members of Watchhouse, Punch Brothers and Leftover Salmon come from the non-traditional side of the bluegrass fence. But when they get together as Mighty Poplar, the grass is bluer on this other side.

Dressed in office attire but playing music suited for the front porch Sept. 17 at a sold-out Stuart’s Opera House in Nelsonville, Ohio, Watchouse’s Andrew Marlin (mandolin/acoustic guitar), Punch Brothers Chris Eldridge (acoustic guitar/mandolin) and Noam Pikelny (banjo), Leftover Salmon’s Greg Garrison (bass) and fiddler Shad Cobb (filling in for Billy Strings’ Alex Hargreaves) showcased their self-titled debut LP alongside other covers (“Little Sadie”) and self-written tracks, including the rapid-fire instrumental “Man Chicken,” which Pikelny facetiously introduced as his attempt to communicate heavy thoughts to his parents.

When the band stumbled over the intro and into a false start, the laconic banjoist and onstage spokesman blamed the emotion of the decidedly playful number.

Unplugged and amplified only with a single mic, Mighty Poplar epitomized ensemble precision, smiling, cuing one another for solos and giving the sonic illusion of a single instrumentalist as the players navigated toward and away from the mic as the music demanded. Despite being one of a rotating cast subbing for the busy Hargreaves, Cobb was so fully integrated, he shared harmony vocals with the entire group and lead vocals with Marlin and Eldridge as the Saturday-night-on-a-Sunday audience rabidly cheered band and individual passages, spontaneously erupting into a standing ovation and stomping the Opera House’s century-old wooden floors when the night ended.

Walking onstage casually just before 7:30 p.m., the band eased into a slow build as Marlin and Eldridge lay a soft foundation that was eventually cemented by Garrison. Once it had firmed up, Pikelny, then Cobb, chimed in and the music began ascending the first of many mountains of tension before sliding down a slope of release. This was the trend over 105 minutes of continuous performance as Mighty Poplar nodded to the Carter Family on “A Distant Land to Roam,” Bob Dylan with “North Country Blues,” Leonard Cohen’s “Story of Isaac,” John Hartford with “Let Him Go on, Mama” and Bill Monroe with “The Old Mountaineer.” At turns raucous, balladic, celebratory and melancholic, the songs’ diversity was matched by the band’s ability to inhabit them, confirming Mighty Poplar’s status as the rare supergroup that lives up to the superlative.