The avocado tree in the backyard of Ammar Swalim’s San Francisco hardware store is four stories tall and still bearing fruit.

When Ammar Swalim took over a small Pacific Heights hardware store two weeks ago, he knew he’d be inheriting tightly packed aisles filled with hammers and bleach and light bulbs. He had no idea what was growing in the backyard of the California Street business: a massive avocado tree dripping with fruit so plump that each one makes a loud thump when it falls to the ground.

The avocado tree — lush, verdant and with too many branches to count — towers over the backyard with a magical presence. The fact that an avocado tree this large, estimated at nearly 50 feet tall, is bearing softball-sized, lime green avocados in foggy San Francisco adds to the mystique. They typically grow best in warm, humid climates, and it can take as long as 15 years for an avocado tree planted from seed to yield fruit.

So when Swalim took to Nextdoor.com on Sunday to offer free avocados from the tree, he unexpectedly opened the floodgates to a flurry of excitement. His post on the neighborhood platform drew more than 250 comments in less than 24 hours. His inbox flooded with pleas from all over the Bay Area. Several people (including this reporter) hurried over on Monday to claim some of the fat avocados from the tree, which shares the yard with a lemon tree and rosemary bush.

Ammar Swalim next to the trunk of the huge backyard avocado tree from which he’s giving away fruit to neighbors.

Each day at Pacific Heights True Value Hardware Store, Swalim makes his way through its aisles and up a flight of stairs into the backyard to collect avocados that have fallen to the ground. He’s eaten more than a few himself, and says they’re delicious.

But he found he had so many, he thought he’d offer them for free to people in the neighborhood — and perhaps drum up some business for his store. But when he posted on Nextdoor, he accidentally alerted all of San Francisco.

“I think we have the biggest avocado tree in the USA located in the heart of Pacific Heights,” Swalim proudly proclaimed below blurry photos of the tree in his post. “All neighbors eat (them) and they are tired of eating so much avocados.”

Almost immediately, people started clamoring for a locally grown version of the beloved, quintessentially California fruit.

“Let the games begin,” one Nextdoor poster quipped.

Some offered to trade fruits and herbs from their own backyards. A representative from San Francisco Public Works suggested Swalim register the tree with the city’s Urban Harvesting Program so the avocados can be distributed to shelters and food banks. Some sent Swalim messages about avocado trees in their own lives, like one woman’s mother’s tree in Hawaii, or another San Francisco tree that’s also enormous but never bears fruit.

The most eager, though, just showed up as soon as possible to secure their prizes, which Swalim keeps in a crate below the store’s cash register.

“They’re excited. They’re happy to have free avocados,” Swalim said of people who came by the store on Monday. He gave out two or three avocados to each person. Several bought something from the hardware store as a thank you.

Elsa Bronte was one of the Nextdoor users who saw Ammar Swalim’s post and came by to snag some of the fat, home-grown avocados.

Elsa Bronte was among the lucky ones to snag some avocados. She planned to smear the creamy flesh on leftover pork tenderloin sandwiches and mix it into arugula salads. Seeing the avocado post on Nextdoor, a website better known for long threads about crime and neighborhood complaints, was a welcome change.

“It is refreshing to see a positive and generous post on Nextdoor,” Bronte said. “I think that’s also what drew me in.”

Swalim, who took over the store after working for more than a decade as a barista in San Francisco, doesn’t know how old the tree is. He said the previous owner, who ran the California Street store for 15 years, didn’t know, either.

Gary Gragg, who grows avocado trees at the nursery Golden Gate Palms in Richmond, estimated that the tree could be anywhere from 40 to 100 years old. While avocado trees don’t like the oceanic winds that blow through San Francisco, Gragg said, they can grow if shielded by other trees or a building, as Swalim’s is. All avocado trees flower from March through May, he said. Depending on the variety, the fruit is ready to eat in anywhere from six to 14 months.

Though it’s a misconception that avocado trees can’t thrive in Bay Area climates, Gragg said, this tree is unusually tall. It’s also distinctive for another reason: A tree this large and this old means it likely grew from a seedling decades ago, Gragg said, when there were many more varieties of avocado. Though Hass makes up the vast majority of current production, there are actually more than 1,000 named varieties of avocado and many more that have been “lost to time,” he said.

“A lot of these really old trees came from a much larger genetic bank of possibilities than what people are throwing out in their backyards today from seed,” said Gragg, who in his spare time drives around the Bay Area hunting for unusual avocado varieties he might spy.

It’s not clear what kind of avocado Swalim’s tree produces. The fruit has the smooth skin of a Mexican avocado, but its round shape and hefty size more resembles a Guatemalan avocado, Gragg said. It could be a hybrid of the two, he added. He suspects it’s a seedling hybrid, which means a seed germinated and created a unique fruit.

“It makes me wonder — and you’ll never know, it’s always a mystery — as to what the exact lineage of the fruit is,” Gragg said.

Ammar Swalim holds one of the avocados that grew on an enormous tree, close to 50 feet tall, in the backyard of his Pacific Heights hardware store.

Ditka Reiner, who lives near Lafayette Park, was floored by the news that an avocado tree four stories tall was bearing fruit in her neighborhood. On Monday afternoon, she headed over to see it for herself.

“Unbelievable! Here?” she asked Swalim. “I have to see it. Can I see it?”

She planned to use her avocados for omelets, guacamole and salads. (Avocado toast was off the table, she said. Too trendy.) Reiner is an avid home gardener and cook who makes her own vanilla extract and strawberry syrup. (Coincidentally, her cousin is the New Yorker food writer Helen Rosner.)

But even more than the avocado’s tempting flesh, she was affected by Swalim’s generosity.

“I just thought it was really, really generous and neighborly,” she said — an example of a social media platform at its best. She told Swalim when her own lemon tree is in season, she’ll be back to return the favor.

Not everyone was as enthused about Swalim’s post. The store’s upstairs neighbor got upset, he said. (He mistakenly thought she locked his door to what he said is their shared backyard, but he now has access and is giving out avocados again.) The neighbor declined to answer questions for this story.

Still, while the Pacific Heights tree may be unique, Gragg said, anyone who lives in the Bay Area should try their hand at planting a tree of their own.

“I’ve had a huge crusade over the last 25 years: There should be an avocado tree in everyone’s backyard. It’s one of the only fruits that gives you highly nutritious caloric value. Most fruits are sugary. But the avocado,” he said, “gives you everything.”

After news of the tree spread, a woman stopped by the Pacific Heights business. She told Swalim, he said, that her grandfather had run a hardware store there long ago, and she worked there as well. (The Chronicle was unable to reach her for comment.) When her grandfather opened Boegershausen Hardware on California Street in 1908, he planted an avocado tree in the backyard for good luck.

Elena Kadvany is a San Francisco Chronicle staff writer. Email: elena.kadvany@sfchronicle.com Twitter: @ekadvany

Elena Kadvany joined The San Francisco Chronicle as a food reporter in 2021. Previously, she was a staff writer at the Palo Alto Weekly and its sister publications, where she covered restaurants and education and also founded the Peninsula Foodist restaurant column and newsletter.