Kalın's bower has become a popular attraction for thousands of tourists in search of Cold War mementos that have survived the fall of the wall in 1989 more or less unscathed.
One day in 1983, the recently retired Turk decided to clear up a spot of rough ground littered with rubbish on the western side of the Cold War barrier so he could grow vegetables there.
He had no idea the land actually belonged to East Berlin.
"He was very lucky," said Kalın's son Mehmet, speaking on behalf of his 84-year-old father, whose memory is now failing him. "Without the Wall it might never have happened. He never dreamt of having a garden like this when he came to Germany."
The plot lay on a slight kink in the postwar partition of Berlin that East German authorities chose to leave on the other side of the Wall to make construction simpler.
His enclave within the former exclave has since become a symbol of the city's unique divisions and rebirth after the war.
And, like much Berlin real estate occupied by squatters in the years after World War Two, its legal status has never been fully clarified.
"I want to have it all legalised," said Mehmet. "Who knows what'll be here in 100 years. But if they try to push us off, I'll fight just like my father did."
Teeming with fruit and vegetables, the 350 square metre allotment now features in a number of local tourist guides.
"It's become an institution," said Joerg Flaehmig, a spokesman for the mayor of Kreuzberg, the Berlin district where the garden lies. "Not exactly a high-gloss Berlin tourist attraction, but it shows a very special side of the city."
Bus parties, cyclists and pedestrians regularly come to admire the fertile strip, where the white-bearded Osman still comes to sit, though his family now does the work.
Surrounded by a wire fence converging on a wooden hut made of old furniture and household paraphernalia, the enclosure contrasts starkly with the shiny multi-million-dollar edifices built in central Berlin since the Wall came down.
Though surveyors' estimates show it could be worth well over 100,000 euros ($141,000), the Kalın family has rejected offers to sell.
"We've always said no. If my father sold up, he'd probably be dead in a month. It'd be like he was selling himself. For us, it's like being in Turkey when we're here," said Mehmet, 45.
Tourists from as far as Japan have sought out the greenery laid out by Kalın, who was born in an Anatolian village in 1925, emigrated to Austria in 1964, moved to south Germany six years later and finally settled in Berlin in 1980.
Around three percent of Germany's population is of Turkish origin, many of whom first arrived in the 1960s during Germany's postwar Wirtschaftswunder, or economic miracle.
Over time Kalın's garden grew, as did the hut: once just a tiny shack, it is now a two-storey summer house with toilets, running water and a veranda looking west over the city.
"There were old fridges and all sorts of junk lying around there to begin with. He worked on it every day, including Sundays," Mehmet, a stone mason, said from inside the house with German and Turkish flags fluttering from its upper floor.
Digging the soil, Kalın began planting onions and garlic, until one day border guards from West Berlin arrived and attempted to drive off the former construction worker.
"But he sent them packing. He told them God had given him the patch," said Mehmet. "A few weeks later, two soldiers and an officer from the other side turned up with Kalashnikovs."
Fearing he could be a western spy or trying to help defectors escape by tunnel, the East Berlin guards began questioning Kalın, who quickly lost patience, his son recalls.
Resorting to a favourite Turkish expression, Kalın described the East Berlin officer as "a son of a donkey", and threw down his identification at their feet when they demanded to see it.
"He said to them: 'There you go. It's just a piece of paper. Let's talk like human beings'," said Mehmet.
At length, his interrogators relented.
"The officer said: 'You can stay as long as your fence doesn't get higher than my wall'," said Mehmet. "Afterwards they became friends. My father got sent wine and biscuits each year."
When the Wall fell Osman extended the plot, but nearly lost it when it came under the auspices of the central Mitte district after reunification a year later.
The garden later became part of neighbouring Kreuzberg, a leftwing bastion of West Berlin, to which Mehmet says it brings in at least 100 tourists a day during the summer.
"The city earns from it too," he said, adding that he hoped it would one day be recognised as an official monument.
Germans are still campaigning for the return of property requisitioned for the Wall and the adjoining "death strip", arguing that government compensation has been inadequate. The Kalın family says it has not been affected by this dispute.
More than 4,000 claims for restitution have been made on Berlin Wall territory and 147 remained open at the end of last year, according to the Federal Agency for Real Estate.
Whether the Kalıns will ever secure state backing to preserve the old gardener's legacy is unclear.
"Let's just say no one is looking to alter the status quo," said Flaehmig from the Kreuzberg mayor's office. "If we start having to ask health and safety questions to do with the state of the hut we could end up with a situation nobody wants."
"People don't want it to go, because it's part of the reunification era and belongs to the fabric of the city now."