What is the oldest tree in Frankfurt?
Frankfurt's oldest documented tree is the Eschersheimer Linde on Eschersheimer Landstrasse, a large-leaved lime dated to the end of the 17th century and protected as a Naturdenkmal since 1937. It stands beside the U-Bahn station named after it, Lindenbaum, on a stretch where the line curves slightly rather than run under its roots. Its only rivals are the Schwanheim oaks, a group the city dates to up to 500 years without dating any single trunk.
Frankfurt dates its protected trees through a municipal register rather than through folklore, which makes the answer unusually checkable and unusually modest. The Eschersheimer Linde, from the end of the 17th century, is the oldest single tree here with a dated record; nobody cored it, so its age is a period rather than a year. The Schwanheim oaks may be older: the city's own page gives the group up to 500 years, the register as mirrored on Wikipedia 600 to 1,000, and no single trunk has been dated, so they stay a rival rather than the answer. The Goethe Ginkgo in Rödelheim was planted around 1750, half a century or more after the lime. Everything else with a date is younger: a copper beech set in 1815, a plane at the Nebbiensches Gartenhaus around 1820, a weeping beech in 1845, the Sossenheim peace oak of 1871, and a dawn redwood from about 1950, as old as any in Europe can be, because the species was known only from fossils until living trees were found in China in 1943. One tree here has no age at all: the eight-stemmed lime in Gruneburgpark. If you know it, tell us.
One of Frankfurt's trees also appears in Autumn Worth the Trip.