It was done at the outbreak of the First World War and, I understand, by 1915 many of the younger gardeners had gone to fight for King and Country. Adding to the poignancy was the discovery later of many of the same names on local war memorials
More details can be found, e.g., on Heligan’s website.
In recognition of the centenary, Heligan have sown a ‘Poppy Field of Remembrance’ on the West Lawn. And later this year they will be holding a Remembrance Sunday “Lest We Forget” day, on 8th Nov 2015.