The message seemed kind of cryptic: Meet in the oak collection, north of Valley Road at the northern end of Oak Path. But, the term “collection” was the tip-off that this mob meeting would be more horticultural than clandestine.

The Arnold Arboretum, a 265-acre botanical garden run jointly by Harvard University and the city of Boston, now hosts what it called “Tree mobs.”  Late on Thursday afternoon about 50 people wandered into the garden looking for the oaks. Some were guided by signs posted along the way that read “Join our Tree Mob. Casual learning in the landscape.” Others had scanned the QR code on the signs or the arboretum’s website for a GPS map that led the way to the oak-lined path.

(The codes on many of the garden’s plant labels –and the nod to the flash mob concept — reflect the Arboretum’s embrace of digital technology.)

Dave Barnett of the Mt. Auburn Cemetery talks oaks

Barnett’s affiliation allowed him to slip a little black humor into the talk, which went well beyond its scheduled half hour. The only way to maintain a collection like Mt Auburn’s is to keep planting new trees, he said.

“No matter how hard you try you can’t keep a tree alive forever, even a redwood,” he said, while noting “We have a saying around Mount Auburn – we think eternity.”

Many oak species are threatened by development, so Mount Auburn is part of an American Public Gardens Association effort to collect and share information on oaks

The goal is “to make sure if they disappear in the wild, they exist somewhere, “he said.

As the oak tree branches filtered the late afternoon sun, the presentation turned form conservation to observation.  Barnett and others said only way to determine a tree’s age is to cut or bore into the trunk. The rings can also tell the history of a tree, including the impact over the years of weather, fires and insects.

Elena Saporta, a Cambridge landscape architect and self-described “tree and plant nerd,” said the oak gathering was her second time at a Tree Mob event.  She needs to know the horticultural details of plants she chooses for her project.

            “It’s really important to research it as much as possible,” she said. “The Arboretum is just and amazing resource for that.”’