Charles Ellicott 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)

You shall not plant for yourself a grove. —Hebrew, ashêrah, sometimes referring to images, but here evidently to the grove itself. The worship of Jehovah allowed no secret rites, and nothing that could lead to the abominations of heathen idolatry could be permitted near Jehovah’s altar.