Iowa Code § 562.4: Tenant at will — notice to terminate.

A person in the possession of real estate, with the assent of the owner, is presumed to be a tenant at will until the contrary is shown, and thirty days’ notice in writing must be served upon either party or a successor of the party before termination of the tenancy. However, if a rent is reserved payable at intervals of less than thirty days, the length of notice need not be greater than the interval.