The the Golden State has a very sparse "plain presence." There is a very small Beachy Amish Mennonite settlement near Lebec, an hour or so north of Los Angeles in the rural reaches of Ventura County. That's a settlement I would enjoy visiting someday.
There is also a small Mennonite community near Bangor, California in the far north. The Golden State's largest Plain population can be found near Modesto where there is a robust German Baptist community. Our Plain Kansas columnist, Rosanna Bauman, will visit there next month and share photos and stories. There have been several splits within the Brethren church over the past few years so I'm not sure who is Old Conference and New Conference. An area around Modesto known locally as "Wood Colony" is being eyed by city administrators as prime industrial property. Modesto Bell columnist Jeff Jardine takes exception to that and in the process does a great job of explaining the area's rich Brethren history. Click here to read.
There is one California entry in the Williams Guide, a post about Cover's Apple Ranch, a German Baptist-owned apple orchard high up in the Sierra foothills.