I have some Sun flowers that I'm growing for my nephew. I have mine doing the samething. They do grow tall and right now you want to veg them and just them grow. This unlike weed you want to grow tall. I keep a fan blowing to keep the stem stong to support the flower that is to come. I believe that you would add the bloom nutes after the flower has started to develop. Are you going to grow them completly indoors? Mine will be going out side so I don't need to worry about switching to bloom. I know the ones I got are the titan variity that grow 5' to 6'. My sister inlaw didn't specify what she wamnted. She wanted me to grow 3' dawarfs. I just think like a kid and figure that bigger is better, lol. How tall will yours go?