C. F. Keil and Franz Delitzsch on Deuteronomy 16:21

Deu 16:21

“Thou shalt not plant thee as asherah any wood beside the altar of Jehovah.” נטע, to plant, used figuratively, to plant up or erect, as in Ecc 12:11; Dan 11:25; cf. Isa 51:16. Asherah, the symbol of Astarte (see at Exo 34:13), cannot mean either a green tree or a grove (as Movers, Relig. der Phönizier, p. 572, supposes), for the simple reason that in other passages we find the words עשׂה, make (1Ki 14:15; 1Ki 16:33; 2Ki 17:16; 2Ki 21:3; 2Ch 33:3), or הצּיב, set up (2Ki 17:10), העמיד, stand up (2Ch 33:19), and בּנה, build (1Ki 14:23), used to denote the erection of an asherah, not one of which is at all suitable to a tree or grove. But what is quite decisive is the fact that in 1Ki 14:23; 2Ki 17:10; Jer 17:2, the asherah is spoken of as being set up under, or by the side of, the green tree. This idol generally consisted of a wooden column; and a favourite place for setting it up was by the side of the altars of Baal.