For years I have wanted to grow those HUGE sunflowers everyone else in the world but me has. We put black oil sunflower seeds in one of our bird feeders. This year we have SUNFLOWERS!!! First time. My questions: Are they perennial? Do they get a bigger diameter as they grow? And how come the seeds from the packs you buy at the store have a different stem and don't look as healthy? Thanks!!! Janni