I run these down to 15 degrees or so if active.
http://www.cabelas.com/product/Cabelas-Alaska-Hunter-Boots-by-Meindl/1995551.uts?searchPath=%2Fbrowse.cmd%3FcategoryId%3D734095080%26CQ_search%3Dmeindl

If its colder than that or if sitting/glassing/riding a horse for long I use a pair of Schnees pacs.