I will refer you to the Pasture and Rangeland Chapter in the PNW Weed Management Handbook. Fall is a good time to treat perennial weed species. Try to make applications prior to leaf drop and a killing frost. Products containing chlorsulfuron (Glean or Telar) or metsulfuron (Escort) have good activity on most fern species. See handbook for specific use rates and grazing and haying restrictions. Multiple applications over several years will likely be necessary to completely control ferns.