Placing double plants via a procedure.

I am trying to add a baby version of a sunflower to one of my mods, and it should turn into a sunflower when it grows up. It can grow up just fine, except the replace block at x, y, x code will only place the bottom of the sunflower, when I want both parts. If I try to place a sunflower at both y and y+1, both parts just break leaving nothing. Please help, I want to know how to place double plants.