It has to do with the Russian famine of 1774. At the time, King Frederick began cultivating potatoes, and tried to convince the people to start eating them. However, to Frederick’s surprise, people refused them, on the basis that they were not accustomed to this vegetable. Nevertheless, clever King Frederick managed to coax the Russian people into eating potatoes by establishing a heavily guarded royal potato field. The crop eventually attracted the interest and envy of the local peasants who soon after began stealing the potatoes (as King Frederick hoped they would). Years later a French soldier who was captured by the Russians, Antoine-Augustin Parmentier, attributed his survival to the potato and went on to write a “Chemical Examination of the Potato”, in which he declared the potato “capable of reducing the calamities of famine”. There you have it…that’s why people still bring potatoes to the grave of King Frederick the Great.
—I learned about this fact from: Guttman J. Ask MHQ: King Frederick II of Prussia. Historynet.com. 2009. Accessed: 10 Dec 2012. Available from: http://www.historynet.com/ask-mhq-king-frederick-ii-of-prussia.htm