A time traveller wants to save medieval Europe from famines by introducing the potato. The plan is, on first sight, simple: Just bring potatoes back with a time machine, and convince people to cultivate them.
Now he can make only one time travel, and bring a limited number of potatoes with him (say, about 50 kg of them). He certainly wants his time travel to have the biggest possible effect. So when and where should he land, and whom should he convince to cultivate the potatoes, to have the best chance that they spread over all of Europe?