CGM is a pre-emergent effecting seed root development. It doesn't work on established plants. It will have no effect on this plant. Wild onion is actually a wildflower, a bulb, that produces some very waxy leaves. You have to dig out the bulb, about 8 inches down. It dies back whether you spray it, or mow it or leave it alone. But spraying synthetic chemicals or 20% vinegar does virtually nothing to it. With an organic lawn care practice, you are going to have to accept that some weeds are going to show up from time to time. I have some wild onion growing in one of my flower beds. It comes up early in the spring and then dies back and you don't see it for the rest of the season. Corn gluten meal is not a registered pesticide because the U.S. EPA has granted an exemption for corn gluten meal as an herbicide.