Growing against the south wall of St Paul’s Cathedral is a citrus tree fully 10 feet tall which around October is smothered with small, edible (just) oranges. The tree is Poncirus trifoliata or Citrus trifoliata, the Japanese Bitter Orange. There are in fact, several Poncirus trees around St. Paul’s which were planted around 25 years ago. In the autumn the leaves become a brilliant, translucent butter-yellow with shades of red, as if touched by fire. When they fall (other citrus plants are evergreen) the branches and trunks reveal three separate plants which have a wonderful, contorted, architectural profile. Then later, in the spring, the trees produce small and variable white flowers in a typical citrus shape which light up the dark trunks and perfume the air.
It sounds like the perfect plant, but wait; there is a catch – literally. The branches are smothered with the most large and vicious thorns imaginable. These weapons make it ideal for an animal-proof barrier or hedge (or student-proof as it has been used to effect on some American campuses).
Poncirus is closely related to other oranges (although called Japanese, like so many other plants it originated from China) and was already growing in Japanese gardens when it was discovered by Englebert Kaempfer in the late seventeenth century. Kaempfer was a German doctor, explorer and keen botanist, who had already travelled to Russia, Persia, Arabia and India before setting out for Japan in 1690 with the Dutch East India Company. He was the first westerner to describe the Ginkgo biloba, and the Camellia, and to write about the history and culture of Japan, at a time when it was virtually closed to the rest of the world (apart from Dutch traders).
It was not until the mid-nineteenth century however that the Poncirus arrived in Britain, introduced by the Scottish plant hunter Robert Fortune (who famously smuggled tea out of China), but has never become hugely popular, maybe because of those thorns.
Being frost-hardy and disease-resistant the Bitter Orange has been used as a root-stock for other citrus, particularly the satsuma, and has been crossed with another Chinese orange to produce a ‘Citrange’ which was at one time popular in America to make marmalade. But we now overwhelmingly use the bitter Seville orange for our marmalade (a legacy of the Moorish invasion of Andalucía in the eighth century). But maybe it’s time for us to grow our own (small) oranges to make our marmalade?