The Dutch tradition of laying flowers on all the individual graves in a cemetery began very soon after the war. The photograph here, taken at the Arnhem-Oosterbeek cemetery, was probably taken in 1946 or 1947.
In 1947, The Times recorded that ‘on one day recently […] over 30,000 Dutch people visited the graves of the fallen of Arnhem’. It added:
On occasions it is alleged that graves are cared for by those whose real interest is in the reward of cigarettes and food sent from England, but one has only to mark the many graves of the unknown that have been adopted to see without a doubt that this cynical charge applies only to a few isolated instances.
Special Correspondent, ‘The Dead of the Empire: Work of Army and Imperial War Graves Commission: Ideals and Achievement’, The Times, 11 November 1947.
WAR STORIES 
And to this day, the tradition of adopting a grave has continued. For example, all of the graves as well as all of the names on the Walls of the Missing at the American Cemetery in Margraten (the Netherlands) have been adopted. Currently there is a waiting list for people interested in adopting.