Mont. Code Ann. § 7-12-4338

Any special assessment levied and made for any purpose aforesaid, together with all costs and penalties, shall constitute a lien upon and against the property upon which assessment is made and levied from and after the date of the final passage and adoption of the resolution levying the same. The lien can only be extinguished by payment of such assessment, with all penalties, costs, and interest, or otherwise as provided by law.