Corn gluten meal, a by-product of corn processing, works as a pre-emergence weed control by suppressing germination of most seeds. The Wikipedia article explains it thus: “Proteins in CGM inhibit root formation on newly-germinated seeds, killing the plant. Applications must be timed so that the CGM is present and effective as seeds are germinating.” It also recommends applying corn gluten meal “at least 6 weeks before sowing, or 2 weeks afterwards”. This stuff is completely harmless; it has been traditionally used as animal feed and can even be consumed by humans (although I am given to understand that it is “unpalatable”). As it breaks down it leaves behind some nitrogen, amounting to a 10-0-0 rating of NPK.
I got a large bag of the granules for less than half price at a local nursery because the bag in question had mouse damage. (Speaking of which, I have only found evidence of mice on our property twice in almost five years, and once was when they broke into the corn gluten (again) and made one hell of a mess. Keep this stuff in a plastic or metal bin with a tight-fitting lid.) I sprinkle it liberally over a bed after I have done a really thorough weeding job and lightly rake it in. Water if you aren’t expecting rain in the next five days (i.e. you do not reside in the Pacific Northwest). There are two caveats: 1) Since this stuff suppresses germination don’t apply it anywhere that you want germination, such as an area where you have planted or will be planting within 6 weeks, and 2) You can’t expect perfection – there are some plants this stuff doesn’t effect, and if you left behind pieces of root they will still grow back (because they’ve already germinated). Overall, though, it is a definite work-saver. Most of the weeds do not return, so I have less work to do the next time I come through with my trowel and cultivator.