California Code, Food and Agricultural Code (Formerly Agricultural Code) - FAC § 5580

If the party upon whom service is required to be made does not reside within the county, service of the petition and citation may be made by mail, if he resides or has his office at a place where there is a delivery service by mail. If there is no delivery service by mail, or if his residence is not known, service may be made by posting a copy of the citation, together with a copy of the petition, in a conspicuous place on the property on which the neglected or abandoned plant or crop is situated, at least 20 days before the date specified in the citation.