Powtórzonego Prawa 12:3 Komentarz: Rashi, Chizkuni, Ibn Ezra & Siftei Chakhamim

מזבח consists of many stones,

ונתצתם את מזבחותם, “you shall break down all their altars;” the phrase is a repetition as it is needed for the other half, the commandment to wipe out the names of these deities, (at the end of the verse). ואשריהם תשרפון באש, “and burn their asherim by fire.” In Deuteronomy 7,5, the Torah instructed that the asherim ought to be hewn down, מגדעון. Our sages explain this difference by saying that if these trees had been planted before they had been worshipped whatever had grown after that has to be cut down. If they had been planted after on that site idolatry had already been practiced, the entire tree or trees have to be burned.

their asherim these are trees.

מצבה of one stone only. It is the בימוס, the pedestal for idolatrous statuary, of which we learn in the Mishna (Avodah Zarah 3:7): A stone which one originally hewed for an idol’s pedestal is forbidden for use.