On September 11, 2001, German artist Anselm Kiefer was in Vienna where his second wife was giving birth to their son. He was in a taxi when he heard what had happened in New York. He couldn't believe the news: 'I thought it was science fiction.'
The odd thing about this - pretty much universal - response is that if there's one person who might be expected to have shrugged it off, it's Kiefer. After all, he's spent a lifetime working on the ash-and-dust theme of fallen societies. A few years ago, when filmmaker Sophie Fiennes (sister of actors Ralph and Joseph) made a documentary on him, it was titled Over Your Cities Grass Will Grow. The inevitable decline of empires is both his chosen subject and his constant prediction; and, in the 1980s, he'd painted a plane flying into a skyscraper.
'Yes, yes,' he says, excitedly, one recent morning in Hong Kong, when reminded of it. 'When you see the twin towers, you feel it was demanding a horizontal.' A strange comment, of which more later. Perhaps he feels the same about Hong Kong's buildings? 'Over Hong Kong will grow grass, that is clear,' he replies, eyes gleaming behind his glasses at the prospect.
There's certainly a touch of the mad monk about Kiefer, if monks were allowed to marry (twice) and have five children.
For this interview, he's sitting - intermittently, as he's not keen on discussing his work and prefers wandering about, humming - in an upstairs room at White Cube, which is presenting his first exhibition in China. As Kiefer has insisted the lights be switched off and it's pouring outside, the ambience is gloomy but pleasantly intimate. Some of his work is on the walls, however, which means a third party is looming large over the conversation: Chairman Mao.
'I'm still fascinated by China,' Kiefer explains. 'The first time I went was in 1993, for three months. When I was asked to do a show in Hong Kong, I was re-activating the mystery of Mao.'
This re-activation has taken the form of painting flowers on top of his own photographs of the spring fields of southern France, and dotting statues of Mao in various corners. As is usually the case with Kiefer, the work is so massive and so thickly layered you need to look at it sideways to appreciate the entire effect; the tactile force is immense, Kiefer being the least delicate and most physical of artists. (He's the only person in this interviewer's many professional encounters who has ever signalled farewell by scrunching up her hair.)
In some cases, the appearance of Mao - stiffly popping up via an additional panel - looks like an afterthought. Kiefer admits as much in an interview on White Cube's website: 'When I did this, Mao comes later ... I like very much flowers but I needed something to break these flowers.'
The most breathtaking work in the show, Der Lange Marsch (The Long March), doesn't feature Mao at all, just a huge, withered landscape, receding into the distance, powerfully conveying a society's - any society's - suffering and loss.
That ruins should prove such an obsession for Kiefer is hardly surprising considering he was born in one: his mother gave birth to him in the cellar of a bombed house in Germany, in March 1945, during the final weeks of the war.
'My mother couldn't give me milk,' he says. 'I think I was dying already.' But he survived to grow up playing amid a devastated landscape: 'I built a nice house with two floors in the ruins and I sat there when there was a tempest.'
The taste for tempests has proved useful. In 1969, when he was 24, he exhibited a series of photographs of himself giving the Hitler salute in various European locations. The title was 'Besetzungen' or 'Occupations'.
Deliberate provocation thereafter became his stock-in-trade. He has described seeing an aerial film of bombed, postwar Germany and loving the vision of the vertical being made horizontal; you get the feeling that his depictions of obliterated societies, his toppling towers, are a personal bombing campaign over which, this time at least, he has some control.
'Nothing is stable, nothing is still, all is fluid,' he says. 'History, in general, is like a fish. Even the paintings are not finished. I do a lot of electrolysis, I put the paintings in the bath, I leave them outside. And they change.'
From 1992 until 2008, he worked in a derelict silk factory in Barjac, near Avignon in France. (He left Germany in 1991 and says, cheerfully, 'I don't trust the Germans'.) There he built his own kingdom. This is what the Fiennes documentary depicts: a vast complex of towers and tunnels amid which Kiefer toils. He's not altogether pleased with the film.
'It's not bad, but it could be better. It could have been the film of the century! I would change the music, I would tell people where you are, I would show it from above, from the air ... '
Barjac has been abandoned but Kiefer still owns it and you can visit it. In photographs and film clips, Barjac looks truly extraordinary, like a post-apocalyptic wilderness. It may not surprise you, therefore, to learn that last autumn he was trying to move into a decommissioned nuclear reactor, on the banks of the Rhine. But the Germans, it seems, don't necessarily trust Kiefer either. 'There's more horrible nonsense,' he declares. 'The media took it very badly, the Green Party [in Germany] wrote to me saying that they want it to be green meadows.'
By now, you will understand that Kiefer is not exactly of the sunny-pastoral school of life. 'I'm a realist,' he insists. 'Poetry and art are real. All other things are illusions. You are only real on canvas.' In that case, has he ever done a self-portrait?
Kiefer pauses, then laughs, tickled by the thought. 'No. I don't have any existence. I'm a medium for all these different things of culture - history, poetry, music. It forms something new through me.'
'Anselm Kiefer: Let a Thousand Flowers Bloom', White Cube gallery, 50 Connaught Road, Central, Tue-Sat, 11am-7pm. Inquiries: 2592 2000. Ends August 25