You're right about the corn gluten meal (CGM). It is very high in nitrogen and, so, is a great fertilizer. I'm curious as to where you heard that you should apply it in August. From what I understand about it, as below, I think you did indeed put some high-powered fertilizer on your weeds.
As to applying it for its pre-emergent benefits, it took me quite a bit of research to know how & when to apply it.
For weed control, it needs to be applied in the early Spring (usually in late February, depending on the weather), before any weed seeds germinate. Same for winter weeds -- apply in the late fall, before those seeds germinate.
I haven't done a fall application on our yard so I'll hold off making any recommendations as to timing on that. Howard says September/October, but I'm not sure what it is that drives when you apply it. Hopefully, this forum's moderator will enter the thread and shed some light on that for both of us.
For the spring, apply in February (must be done before March 1st). The CGM should be lightly watered in, but needs to be dry for a period of time after application to increase effectiveness. I never did find a firm definition for the "period of time" but I aimed for 4-5 days, and that is what I got last year. Lastly, any other fertilizers or stimulants should be applied 1-2 weeks after CGM application.
We have seen a significant reduction -- from just that one application -- in dandelions, and a few other weeds that I haven't ID'd yet.
And, with our 100% organic treatment of the yard, we've seen hardly any grass burs.
Don't give up. It really does work!
Happy Gardening!!
_________________
God speed!
Cara
**
Take time to stop and smell the flowers!
(or... as my ladybug refrigerator magnet says
"take time to stop and
eat the flowers!"
)