John Calvin Commentary Deuteronomy 16:21

"Thou shalt not plant thee an Asherah of any kind of tree beside the altar of Jehovah thy God, which thou shalt make thee." — Deuteronomy 16:21 (ASV)

Thou shalt not plant thee. It is clear from the end of this verse that it is part of the Second Commandment.

We know³⁰⁰ that among the heathen nations groves were sacred, so that with them no religious object would receive due reverence, except under the shade of trees. Therefore, so that conformity with this general custom would not corrupt the pure worship of God, this distinction was made; and this, then, is the intent of the prohibition: that the Jews should flee from all strange rites, so that by approaching the Gentiles too closely, they would not introduce a sinful medley.

But how necessary this prohibition was appears from their eager imitation (of the heathen), of which mention is constantly made in the sacred history. For there was scarcely any period in which they abstained from “high places.” Nor is it without reason that Isaiah and Jeremiah reprove them for playing the harlot under every green tree (Isaiah 57:5; Jeremiah 2:20; Jeremiah 3:6).

³⁰⁰ See Lucian in Dea Syria, sub initium; Πρῶτοι μὲν ὦν ἀνθρώπων, τῶν ἡμεῖς ἴδμεν, Αἰγύπτιοι λέγονται θεῶν τε ἐννοίαν λαβεὶν, καὶ ἱρὰ εἴσασθαι, καὶ τεμένεα, κ. τ. λ., — Tacitus Germ. 9. “— Tacitus Germ. 9. “Lucos et nemora consecrant.” Virgil. .” Virgil. Eccl. 6:73. “. “Ne quis sit lucus, quo plus se jactet Apollo."."