We just worked on such an issue. Dealing with the exceptions is always difficult, however, one simple standard will not work:
This provision does not apply to land actually used for bona fide agricultural purposes. This provision does not apply to cultivated and maintained gardens. Areas between property lines and centerlines of adjoining streets, alleys and easements shall comply.
On vacant and undeveloped property, weeds and indigenous grasses shall not exceed 12 inches in height for 100 feet inside the perimeter of the property where adjoining an improved street, alley, or developed property (plus any areas between the property lines and centerlines of adjoining streets, alleys, and easements). For native prairie gardens, wildflower gardens or other natural areas designated by the Development Services Department Director, weeds and indigenous grasses shall not exceed 12 inches in height for 50 feet inside the perimeter of the property where adjoining an improved street, alley, or developed property. (Areas between the property lines and centerlines of adjoining streets, alleys, and easements shall comply.)