Mont. Code Ann. § 85-7-2108

Any special tax or assessment levied for any purpose is a lien upon the entire 40-acre tract or fractional lot as designated by United States government survey or platted lot, if land is subdivided in lots and blocks (or whenever land is owned in less than 40-acre tracts or in less than the platted lot, against each such tract) of land in the district or subdistrict of which the irrigable area forms a part, and the lien shall attach to the entire tract as of January 1 in the year in which the special tax or assessment is levied.