Written by Mick Canevari and Brent Holtz, UCCE San Joaquin. Weeds have a tremendous capacity to spread within an orchard.  The first line of defense is  identifying the weeds you need to control, and selecting the best herbicides or cultural practices to control those weeds.  If you use the same herbicide(s) each year, a shift to tolerant weed species will ultimately take over and a loss of herbicide effectiveness will occur.  Alternating products with different modes of action at least every couple years will improve results and insure herbicides long term viability.  The UCIPM web site has charts that show which weeds are controlled by what herbicides, and an excellent weed photo gallery that includes many weed species commonly found in California for easy identification and reference http://www.ipm.ucdavis.edu/.