Which grass species flowers once every several decades, then dies across a whole continent at once?

Bamboo. Many species flower on a fixed cycle of decades - some as long as 120 years - and when they do, every plant of that species flowers simultaneously wherever in the world it is growing, sets seed, and dies. Cuttings taken to another continent a century earlier flower on the same schedule.

That implies a clock inside the plant rather than a response to conditions, since climate, latitude and hemisphere make no difference. Nobody has found the mechanism. The evolutionary logic is better understood: flooding the forest with seed all at once means the predators cannot eat it all, and dying afterwards clears space for the seedlings.

The consequence for people is a famine cycle. In north-east India the flowering of one bamboo species roughly every 48 years produces an explosion in the rat population feeding on the seed, and when the seed runs out the rats move into the rice - a cycle local communities have documented for generations and which contributed to an insurgency in Mizoram after the authorities dismissed warnings before the 1959 event.