Cologneで最も古い樹木はどれですか。
Nobody has measured the age of any of Cologne's five registered trees on this page, so there is no confirmed oldest yet. The strongest circumstantial case belongs to the Great Plane of Hohenlind, a London plane 5.30 metres round standing in a former country estate that was already reckoned about 150 years old in 1932, putting some of the estate's planting back into the late 1700s. That figure describes the park the tree grows in, not a dated record for the plane itself, so treat it as context rather than an age. If you know how to date any of these trees, or can supply a girth for the ones still unmeasured, tell us: this is exactly the kind of gap the site is built to close with readers rather than guesses.
Germany designates individual trees as Naturdenkmal, natural monuments, under state nature-conservation law, and Cologne keeps a public list of its own, reproduced in full on German Wikipedia as Liste der Naturdenkmale in Koln. It runs to roughly two hundred entries, mostly ordinary protected street trees, and none of them carry an age field: the register records girth, species and location, never when a tree was planted. Two of the five trees here were measured by the city in May 2018, at 5.30 and 3.75 metres round; the other three have no girth on file either, an honest gap rather than an oversight. What the register does supply reliably is provenance: three of these trees stand inside larger green spaces, the Flora botanical garden, the Volksgarten and the Stadtwald, each with its own documented history that explains why an old tree might be there, without dating the individual specimen itself. Ages across this page are left blank rather than guessed, and each is a real question a reader with a tape measure or a local record could help answer.