If you apply it in the spring, any pre-m effects will be long gone by fall.

Assuming you are talking about corn gluten meal used as a weed controlling herbicide. Any affect will have worn off by fall. The thing you should know is that it takes time and numerous applications of the stuff over a number of seasons for it to be effective. Additionally timing and irrigation are critical for effective control of crabgrass using CGM. It is however a great fertilizer. Soybean meal may be a lot cheaper for you and is also an excellent lawn food. Cutting long and deep but infrequent watering goes a long way toward controlling many lawn weeds.
Bill Hill

Here is a link that might be useful: Organic lawncare FAQ

Yes, as a herbicide. I have to start somewhere aside from hand pulling. We have about 1/3 - 1/2 acre lot and the previous owners had a bad grub infestation and didn't replenish the grass after the fact so there are huge patches of weeds. A lot of places I would dig up, replant grass and spend hours manually pulling. I almost give up. My husband is pro-chemicals - I am not. As it stands our lot has been chemical free since we have been here so I think I need something more than handpulling. He gave in to the nemetodes and is satisfied with the results. Yikes, too much history. Anyways, I need a little more than just fertilizing as we live near a forested area and alongside our fence is crabgrass galore. Over the past few years I have dumping my leaves over the fence trying to smother it but a lot of the seeds have made their way into our yard. Thanks.

The best non chemical control for weeds is mowing the grass long, 4 inches long if you can. Crabgrass dies out in the heat of summer. Strengthen you lawn with organic fertilizers and keep it long to force out and shade out most of your weeds. Mulch mow your lawn clippings and all your Autumn leaves as a great addition of organic material to you lawn. CGM is quite expensive. Do some research on it's effectiveness. Use it but do not expect miracles from it. If you think you need a little more than just fertilizing, then you may need some help from chemistry. Your decision.
Bill Hill

Here's my thing. We are going to have some work done at the very end of our yard which will include hauling of heavy material so I know everything will get torn up. I am afraid the exposed dirt will be a playground for weeds and am looking for something to control the crabgrass as the grass really won't have a chance. I manually pull the plantains and dandelions when the kids are playing. I can get up to 10 recycle bags a season - it's crazy! I'm trying but seeing the instant results everyone gets with chemicals is frustrating but with 3 kids and pets, it's not for me.