Big green things start tiny

I study groups of growing green things -like tall grasses and bright flowers- that are very new. They are built by people to make up for breaking really old green groups to grow food and build cities in the middle states. Big green things start out as tiny (usually) brown things, but not all tiny brown things grow. I want to learn which tiny brown things will grow into big green things, and which won't. Also, I want to learn how to make a mix of tiny brown things that will grow into many kinds of green things living together.