The Blue Gum of the Botanical Garden

Thirty metres, which makes this the tallest tree the registers record inside the garden, and the reason it stands here at all is a nineteenth century medical theory that turned out to be wrong. Eucalyptus globulus was planted across southern Europe from the 1860s onward because it grows fast, drinks enormously, and was believed to dry marshy ground and so drive out malaria. Some authorities went further and credited the aromatic oil in its leaves with cleaning the air. The drinking part was real. The rest was not, since malaria is carried by mosquitoes rather than by bad air, which is what the word literally means. By the time that was settled the tree had been planted from Portugal to the Caucasus, and it is now the most widely grown eucalyptus on the planet. This one measures 440 centimetres round. Look for two completely different leaves on the same tree. Young shoots carry rounded stalkless pairs clasped around the stem, coated in a waxy bloom that reads as pale blue, which is where the name comes from. The mature crown carries long dark sickles that hang edge-on to the sun, so a tree this size throws almost no shade. Crush one between your fingers and the smell arrives instantly, sharp and medicinal, the oil the nineteenth century was banking on.