Etwas interessant, aber das wuchs ein Gehweg entfernt von einem Blumenbeet

simple answer., Seed dispersion. . Petunia seeds are very small. and can be carried by wind water birds etc.. and even insects. Ants may have carried it., in fact ants have a special relationship with violets, who’s seed coat has an iresistable food source, where they take the seed to the have eat off the coat and discard the seed by the hive to then start growing in the silt pile by the hive. perhaps something similar.