The last Sunday of last month I met a man selling oranges and mandarins at Shirahige Park, north of Asakusa, from a truck marked Kitagawa Pig Farm. His wife makes jams from mandarins and figs. Her mandarin jam is something else. Sweet and bitter like marmalade, but delicate.

Yoshihiko Naito is a fourth-generation farmer from Shizuoka. The first generation grew many things, but Naito’s grandfather staked the business on citrus. They went deep into their craft, growing mandarins, navel oranges and other citrus including dekopon, buntan and ponkan. Naito told me about the creation of the harumi, an unusual cross of a female kiyomi orange and a male ponkan. (He calls them “mother” and “father”.)

He says, The kiyomi was hard to peel, so someone said, what if we crossed it and made things easier?

The harumi is hefty, like an orange, but it’s lighter than it looks – tap it and it feels hollow, perhaps that’s because of the air-lock between the thick peel and the flesh. The plump segments have a thin parchment-like outer membrane that tears easily and the flesh is juicy yet somehow almost crisp, textured like grapefruit. With its sweet-and-sour orange tang, it’s the best citrus I have eaten.

Every week, Naito and his wife drive to Tokyo in their borrowed truck and on Saturdays set up at the farmers market at Hikifune Station, and then two Sundays a month at Shirahige, in the shadow of the bousai danchi, “disaster protection condominiums.” The 18 towering apartment blocks are so named because of their supposed role in emergencies. External sprinklers supplied by massive water tanks turn the buildings into waterfalls if there’s a catastrophic fire in the densely populated streets behind. They also have multi-storey floodgates between them that swing shut to connect the entire chain of condos into a sort of dam, if the river floods from beyond the park.

The effectiveness of all this strikes me as implausible, but here the buildings stand, a memento of public works spending of the 1980s. They are at least solidly built and offer a sheltering escarpment above the park.

Naito the orange seller is a special feature here. I’m grateful to buy fruit you don’t see in the supermarket. Take the mandarins with blotchy skin, he says. They don’t look so good but they’re just as sweet inside, and cheaper. He gives me a big yellow pomelo, or buntan. He says, It fell off the tree with the first spring breeze.

The seasons bring more than strong winds. Naito says, It’s getting harder because of the climate. The past 20 years especially. Rain and temperatures have become impossible to predict.

He might as well be saying he faces a disaster of his own. I don’t know how long we’ll be able to keep going, he says. It’s something his grandfather could never have foreseen.