Mont. Code Ann. § 7-13-2289

Any special assessment made and levied to defray the cost and expenses of any improvement, together with any amounts imposed for delinquency and for cost of collection, constitutes a lien upon and against the property upon which the assessment is made and levied from and after the date of the passage of the resolution levying the assessment. This lien can be extinguished only by payment of the assessment, with all penalties, costs, and interest.