Deuteronomy 16:21

21.Thou shalt not plant thee. It is plain from the end of this verse that it is part of the Second Commandment. We know (300) that amongst the heathen nations groves were sacred, so that with them no religious object would receive due reverence, except under the shade of trees. Wherefore lest conformity with this general custom should vitiate the pure worship of God, this distinction was made; and this then is the intent of the prohibition, that the Jews should fly from all strange rites, lest by too closely approaching the Gentiles, they should 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.)