Tucked away from the rush-hour gridlock on the Jamaicaway, a Massachusetts General Hospital radiologist was one of about 40 people huddled with an Arnold Arboretum researcher showing how the flowering Stewartia trees vary by bloom and bark.

On a day off, Jack Wittenberg briefly joined the “Tree Mob” with senior research scientist Peter Del Tredici at the Stewartia collection, which dates from 1918. Wittenberg, a fan of Harvard’s Arnold Arboretum since the years when his mother convalesced nearby, didn’t have to sign up in advance or make a big time commitment.

“I come to interesting things here and you know with Peter it’s the final word,’’ he said.

Amy Galblum said she decided to swing by the 5:30 p.m. gathering en route from her job in Brookline to her home in Roslindale. “I bike through here, and I didn’t realize there were so many Stewartias and that they are so varied.”

Many botanical gardens give tours and offer classes about their collections, but that is not enough for Arboretum Director William (Ned) Friedman, who took the helm of the oldest public arboretum in North America in January 2011.

“You can go to collections and see everything has a binomial [scientific name] and be around people who know the name of every cultivar, but that’s not the same as rejoicing in the biology and the stories of these plants,” said Friedman.

Friedman took the whimsical concept of a flash mob — a social media–driven spontaneous gathering — and applied it to outreach to the public to encourage interaction with the scientists, curators, and horticulturalists who work on the Arboretum’s 265 acres.

“I just hatched it, I’ve never heard of anything quite like it,” Friedman said of the Tree Mob concept. “It goes at what I’m trying to do — move away from some of the formality in the collection that comes with a tour, a lecture, or children’s education.”

Tree Mob attendees may be alerted by a sign or may notice a group gathered. They may drift by for only a few minutes of the presentation or they may stay for its entirety.

“It’s about getting back to learning about the biology of plants in a fun way,” said Friedman, who led a mob on the evolutionary development of thorns on a rainy 40-degree day. The 20 people in attendance asked him questions well after the expected 15-minute duration.

“This is about meeting an organism, to be in a spot and open people’s eyes to an aspect of the collection they may have never noticed before.”

The next Tree Mob, “Locust, Legumes, and Nitrogen Fixation,” is 5:30 p.m. July 19 and is hosted by horticulturalist and Arnoldia Editor Nancy Rose. Rose will discuss how plants and bacteria partner to produce fertilizer. For more information, visit the Arnold Arboretum website.

Henry Young '52 and his wife Chris Young of Andover, Mass., join others for the Arboretum's Tree Mob event last week.

Friedman’s plan is to increase the number of tree mobs so that they remain intimate and spontaneous.

Del Tredici, an Arboretum scientist since 1979, admitted he was dragged “kicking and screaming” into Friedman’s initiative of Tree Mobs and the mobile tour concept that lets anyone with a cell phone give themselves an informative tour of the prestigious collection.

But his deep knowledge about the collection in general and specifically about the new hybrid of Stewartia — Scarlet Sentinel — developed in the Arboretum, is compelling for mob attendees.

“You can come here and see these plants, but you don’t know anything about them. You could hold your cell phone up [to the bar codes on the signs posted by many plants]. But it’s better to have someone tell you,” said Del Tredici.

He showed the group a half dozen varieties of the camellia-like flowers that were blooming on the woodland trees, with anthers — the filaments holding the stamen — that vary from lavender to mustard yellow. And the bark of the trees, which hail from China, Japan, Korea, and North America, ranges from a mottled desert camouflage to the thick, scaly husk typically seen on deciduous trees.

The white or pink blooms and interesting bark make the Stewartia a sought-after garden acquisition, but many did not survive until plant propagators at the Arboretum unlocked some of the mysteries of growing them. That’s why Del Tredici found himself fielding questions from his group about how to ensure their survival in a typical garden. “It is kind of ‘miffy,’ like it is easily miffed if something is not quite right.”

The Tree Mob participants range from ardent gardeners to city dwellers seeking refuge from the blazing asphalt of a summer day. But one reality that has come with promoting the events is that participants are apparently not Facebook and Twitter devotees.

Pamela Thompson, manager of adult education, said attempts to use social media to invite people to the Tree Mobs largely fell flat. But success has come from posting signs close to the event on the Arboretum grounds and notifying the email list of more than 7,200, she said.  Without the need for much lead time, the Arboretum can respond to weather and blooming patterns to come up with a last-minute mob, she said.

The rewards of that spontaneity hit home for Thompson one evening when she was in a tiny mob with Friedman that spent 20 minutes watching a magnolia open.

“You look at it and you think nothing’s moving and then it’s like, ‘Wait a minute, did that petal move?’ and then it just popped open.”

Doctor knots