There’s something strange happening in South Africa’s Semillon vineyards. Green bunches turn red the one year, then back to green the next. Hung like Christmas baubles on gnarled bushvines, the shapeshifting clusters are an enduring mystery.

Viticultural juggernaut and protector of old vines, Rosa Kruger thought she was losing her mind when she first noticed this phenomenon known as red Semillon or Semillon Gris.

‘I started marking the mutated [red] vines while taking cuttings for planting material,’ she explained when I asked her about the variety. ‘The following year the bunches on the vine would be green again. I thought I was making a mistake, but after a couple of years I realised that Semillon Gris can actually mutate back to Blanc. In general though, the Gris vineyards are fairly consistent once they’ve mutated.’

Semillon Gris is largely unique to the Cape winelands and appears to be an old-vine oddity. One of the Cape’s oldest varieties, Semillon was widely planted in the early 1800s. It was so ubiquitous it was simply called Groen Druif translating to ‘green grape’ from Afrikaans.

These days plantings of Semillon have fallen massively, though there are still pockets of heritage vineyards, some more than 100 years old, carefully guarded by viticulturists and winemakers.

In the early 1800s it’s said that 80% of the vines in South Africa were thought to be Semillon. By the mid-1800s half of these had mutated into Semillon Gris.