a. If personal service of any such order cannot be made by reason of absence from the district, or inability to find one or more of the owners, occupants, lessees or tenants of the subject matter to which such order relates, or one or more of the persons whose duty it was to have done what is therein required to be done, as the case may render just and proper in the opinion of the board, to be shown by the official certificates of the officer having such order to serve, then service may be made through the mail, or by copy left at the residence or place of business of the person sought to be served, with a person of suitable age and discretion.
b. In any case where personal service of any such order cannot be made for the reasons stated in subdivision a of this section and service cannot be made as provided in such subdivision through the mail or by leaving a copy with a person of suitable age and discretion, because of inability to obtain the name or address of the person sought to be served, and such inability to effect service is shown by the official certificate of the officer having such order to serve, service may be made by conspicuously posting a copy of such order upon the property to which it relates. The posting of such order shall be sufficient notice of such order and of the nuisance therein mentioned to all persons having any duty or liability in relation thereto under the provisions of this chapter.
c. Whenever the board shall have declared any condition, matter or thing to be a nuisance, including ragweed or any other species of weed, plant or growth, and has entered the same in its records as a nuisance, the board may also take and file among its records what it shall regard as sufficient proof to authorize a declaration that such nuisance is widespread throughout the city or in any area thereof, and that personal service or service pursuant to subdivision a or b of this section of an order or orders requiring the abatement, removal or correction of such nuisance would result in delay prejudicial to the public health, welfare or safety, and upon the filing of such proof and the making of such declaration, the board may order that such nuisance be removed, abated or corrected, as prescribed by the board, by an order addressed generally, without specification of names or addresses, to all persons who, pursuant to the provisions of this chapter, have any duty or liability in relation to any such nuisance which may exist upon or in any real or personal property or place located within the area or areas specified in such order. Such order may be served by publishing the same for a period of not less than three days in the City Record and in a newspaper circulated in the area or areas mentioned in such order. Service of such order shall be complete at the expiration of the third day of such publication and such publication shall be sufficient notice of such order and of the nuisance therein mentioned to all persons having any duty or liability in relation thereto under the provisions of this chapter.