We have the same problem in San Antonio. It's a good question. The time you are looking for is when the soil temps are warm enough to allow certain things to happen.

There are too many winter temp zones in SoCal to make a sweeping statement. Do you have redbud trees, cherry trees, or any other trees that bloom in the early spring? Flowering trees are more obvious than trees that just leaf out, but those would work, too.

This from Iowa State Universtiy may be of some help.
"HOW TO USE CORN GLUTEN MEAL

NICK CHRISTIANS
Iowa State University

Corn gluten meal works by inhibiting the root formation of germinating plants. It generally does not inhibit the roots of mature plants or transplants until your reach very high rates (80 pounds/1000 ft2 or higher). It should be applied before germination of the weeds. The weed will germinate and usually forms a shoot, but does not form a root. After germination, a short drying period is needed to kill the plants that have germinated but have not formed a root. Timing is critical. If it is too wet during germination, the plants will recover and form a root. (This is also true of chemical preemergence herbicides).

It is preemergence only, there is no postemergence effect on established weeds. In fact, it makes a great fertilizer for germinated weeds.

If it does not rain in 5 days of application, water it in with approximately .25 inches of water. Then leave a drying period after germination.

It will usually work for about 5 to 6 weeks following germination.

Rates will vary depending on crop and target weed. I generally recommend 20 lbs product per 1000 ft2. This provides about 1 lb of nitrogen per 1000 ft2. Some crops that are grown in rows can be treated in bands in the row and weeds can be tilled between rows. This makes it more economical to use in crop production. Test the material at rates from 10 lbs/1000 ft2 in 10 pound increments to as high as 80 lbs/1000 ft2.

It does not work well with seeded garden crops unless they are seeded deeply (radishes seem to be the exception and there may be others). Transplants or mature plants generally work well. Some producers put down a band, work it into the upper inch of soil, and then put the transplant in the band.

In garden and crop production, growers generally work out their own system, depending on their understanding of the crop they are growing and the weeds they are trying to control.

The material is generally about 10% nitrogen by weight. One hundred pounds has 10 lbs of nitrogen.

The nitrogen will release slowly over a 3 to 4 month period after application."

"I generally recommend 20 lbs product per 1000 ft2. This provides about 1 lb of nitrogen per 1000 ft2."

"The material is generally about 10% nitrogen by weight. One hundred pounds has 10 lbs of nitrogen. "

I'm having a little trouble following the math here.

If 20 lbs provides 1 lb of nitrogen, how can 100 lbs provide 10 lbs of N?

LOL. That's a good catch, bpgreen.

The problem in SoCal is that if you live south of the Tehachapi range and within 60 miles of the beach, the soil temps may be warm enough to allow seed germination all year round.

thanks for the replies. i keep going back and forth about trying cgm. the info kimmsr posted was especially helpful about cgm, but i keep hearing it's only about 65% effective as traditional preemergents, on the maximum end, and i have a terrible oxalis problem. as in, i practically have an entire lawn of oxalis. but i am an eternal optimist and hope springs eternal, so next uh...february? i might try it. it's just that now it sounds like i'm going to have to apply it 2-3 times a year instead of once. that's very expensive, even if it does double as fertilizer.

The link below goes to the Iowa State University web site that connects to a lot of information about Corn Gluten Meal and what these guys at Iowa State found as they researched it.

Here is a link that might be useful: About Corn Gluten Meal