To render the practical application of this chapter possible in all drainage cases, it shall be sufficient for the petitioners to exercise due diligence to ascertain the names of all the landowners and those having title to land in the proposed district, and it shall not be necessary for the petitioners to list and include with the petition, filed with the clerk, the names of all the landowners in the proposed district; but after the filing thereof with the clerk, as the proceeding progresses, the petition may be amended as often as may be necessary to include the names of additional landowners when, and if others are subsequently ascertained, so that the petition may finally show the names of all the landowners it is possible to ascertain in the proposed district. The names of landowners, if any, in the proposed district, which it was impossible to ascertain after the exercise of due diligence, as aforesaid, shall be classified as unknown owners, who may, at any time, become parties to the proceeding as provided for in § 21-309.