My sunflowers died rotten within autumn. Will they grow put money on subsequent year or do I hold to re-plant seed? ...
They're a seed crop, so you necessitate to reseed every spring.
You will have to replant the seed for best results. You may get a few volunteers from seed that dropped off this years plants, but to be sure, basically plant more next spring.
You could dry the seed off your existing sunflowers and use those to plant subsequent spring.
Answers: You'll have to replant from nut. They're not annuals.
Good Luck. ;-)