What is more efficent? Multiple passes or a single pass with iterative loop?

Hi! I'm making a GS(geometry shader) for draw grass on a superfice, but i have a problem. When I pass the vertexs from the GS to fragment shader, this drawing them together. I think that doing multiple passes through vertices this would not happen, but this would be ineffective. I tried to use a for loop to separate in each loop all the vertices belonging to each grass, but still joining them. What could I do?

Thank you in advance. Excuse my English, it is not my language and I have trouble writing it, I hope there are no problems in my writing and I hope you know how to apologize.