Also agree with what RC said.

If you get GC in before you have a weed problem, it's your best chance to prevent the weed problem.

I think that's why traditionally stated GC stocking numbers are so high. They assume people are only putting them in to address an already existing overabundance of weeds.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."