Further to TARAS B's post, it was in fact an American lady, a Miss Moina Michaels who worked at the YMCA HQ in New York who is generally credited with the origin of the use of the poppy as a symbol of remembrance. Apparently after reading "In Flanders Fields" she decided that from then on she would wear a poppy each year in remembrance of the dead.
So much for it being a symbol of British imperialism.