The Secretary shall conduct a referendum among domestic producers and importers not exempt under section 4311 of this title who, during a representative period determined by the Secretary, have been engaged in the production or importation of flowers and plants, for the purpose of ascertaining whether the issuance of an order is approved or favored by such domestic producers and importers. No order issued pursuant to this chapter shall be effective unless the Secretary determines that the issuance of such order is approved or favored by not less than two-thirds of the producers and importers voting in such referendum, or by a majority of the producers and importers voting in such referendum if such majority produced and imported not less than two-thirds of the total value of the flowers and plants produced and imported by those producers and importers voting in such referendum during a representative period defined by the Secretary.