Iowa Code § 468.82: Payment.

The board, at the time of making the levy, shall fix a time within which all assessments in excess of one hundred dollars may be paid, and before any bonds are issued, publish notice in an official newspaper in the county where the district is located, of such time. After the expiration of such time, no assessments may be paid except in the manner and at the times fixed by the board in the resolution authorizing the issue of the bonds.