I found an article on the American Nurseryman Magazine web site discussing the control of weeds in container production I would like to share. In this article, OSU associate professor Hannah Mathers has some great insight on weed control, along with several great photos.
American Nurseryman Magazine: Containing Weeds
by Hannah Mathers
If your nursery container weed control program is working optimally, the only time you should need to hand weed would be one to two days before each herbicide application. Your scheduled applications would be in the spring, summer and fall for most Midwest container nurseries. With longer lasting pre-emergence herbicides, your scheduled applications may only be spring and fall. In regions with longer growing season's applications, two or three applications may be required over the summer. Sometimes, however, weeds may go to seed before a scheduled application. In this event, supplemental hand weeding would be required followed by a half rate of pre-emergence herbicide. Only a half-rate is required as you are only re-establishing the chemical barrier in the top half-inch of the soilless media.