Deuteronomy 16:21

As it is not allowed you to plant a grove by the side of the Lord’s altar, so is it not allowed you to associate in judgment a fool with a wise judge to teach that which you are to do.

THOU SHALT NOT PLANT THEE AN ASHEIRAH. Any tree planted at the entrance of a house of G-d is called asheirah. Possibly because it ordereth the way aright, G-d is called etc.” Ramban’s further language about the prohibition of planting a tree near the altar of G-d, clearly shows that he is speaking here of a House of G-d. directing people’s “steps” [to worship], it is called [asheirah] from the word ‘ashurai’ (my steps) have held fast to Thy paths., 17:5. Scripture thus admonishes not to plant a tree beside the altar of G-d for beauty and to think that it is an honor and glory to G-d’s altar. He prohibited it because it was a custom of the idolaters to plant trees at the entrances of their idol’s temples, as it is written, further, and throw down the altar of Baal that belongs to thy father, and cut down the asheirah that is by it.
Now Rashi wrote: “Thou shalt not plant thee an asheirah. This is intended to make one liable [to punishment] from the very moment of planting; and even if he does not worship it, he transgresses a negative commandment for planting it. And thou shalt not plant any kind of tree beside the altar of the Eternal thy G-d. This is an admonition against planting [trees] or building a [wooden] house on the Temple mount.” If so, Scripture is stating: “thou shalt not plant thee an asheirah ‘nor’ missing in the word kol (any) which should be v’kol (nor any). Thus, Rashi interprets the verse: we may not plant a tree for an idolatrous purpose anywhere, “nor any” tree, even for beauty’s sake, beside the altar of G-d. Ramban agrees to all that. However, Rashi adds that “building a [wooden] house on the Temple mount” is likewise prohibited; Ramban points out that this is forbidden only by law of the Rabbis. any kind of tree beside the altar of the Eternal.” But building a [wooden] house on the Temple mount is [prohibited by] Rabbinic ordinance based on Scriptural support, for Scripture prohibited only the “planting” — “thou shalt not plant thee an asheirah [prohibiting the planting anywhere of trees intended for idol-worship], nor shalt thou plant thee any tree [for whatever purpose] beside the altar of the Eternal thy G-d.”

אצל מזבח, for this is where they usually planted these asherot. We are told in Judges 6,25 that G’d told Gideon in a dream to destroy the asherah next to the altar of the Baal belonging to his father. Jeremiah 17,2 laments the fact that his own people had taken to planting such asherot near every green tree.

לא תטע לך אשרה, כל עץ, no tree must be planted in the courtyard of the Temple as its proximity to the altar was liable to lead the people to perform some kind of worshipful gesture as they were familiar with having observed the Canaanites do so.

לא תטע לך אשרה, in this verse the Torah lists three items whose common denominator is that they appeal to the senses, are desirable, but at the same time are all spiritually negative, harmful.
The first one is האשרה, something beautiful and decorative for buildings, but at the same time something ugly from the vantage point of holiness as it is usually a conduit leading to idolatrous practices. Seeing that this is so, we are commanded whenever faced with such choices to give preference to that which leads to spiritual righteousness at the expense of physical perfection or beauty. By the same token, we are to prefer spiritual qualities possessed by someone chosen as a judge to external features, impressive though they may be.
The second item is המצבה, even though such a kind of altar was welcomed by G’d as an outlet for someone who wanted to bring an offering to G’d, this was before the Torah had been given. Consider, for example, Exodus 24,4 where the Torah welcomed 12 such monuments erected by Moses, where these monuments symbolised the fact that the person offering a sacrifice considered himself as constantly in the presence of the divine. (compare Psalms 16,8 שויתי ה' לנגדי תמיד, “I feel myself constantly in the presence of the Lord.”) The Jewish people had not been able to maintain this spiritual level after they had committed the sin of the golden calf. Even when such a מצבה is dedicated to heaven it had become hateful in the eyes of the Lord, i.e. G’d had expressed His distaste in Exodus 33,3, after having provisionally “forgiven” the people by appointing an angel to lead them to the Holy Land, but refusing to lead their ascent by His presence being among them. We encounter a similar concept described as the difference between an old man whose beard proclaims his respectability, as opposed to the old man whose beard is disheveled, i.e. reveals traces of a youth which was spent irresponsibly. We look for people whose exterior testifies to their blameless interior, character. [This metaphor is used by the Chazzan in his private introductory prayer on Yom Hakippurim. Ed.]
The third item, also reminding us of the rejection of an externally basically beautiful animal as a sacrifice, is a series of blemishes, some quite minor, not affecting the value of the animal in question in the market place at all. Such a blemish in an animal worth 1000 dollars disqualifies it as an offering, whereas another similar animal worth one single dollar, but without such a blemish, is given preference over the far more expensive animal, which is rejected. The Torah gives us three examples to teach us basically the same lesson. What is true of the blemished animal for presentation on the altar, is equally true for the venerable old scholar who is afflicted with some character fault. We are to look further in order to find a less impressive individual not afflicted with such character fault.

לא תטע לך אשרה, “do not plant for yourself an asherah; (tree to serve as something to be worshipped.) The wording of the Torah makes it plain that even the mere planting of such a tree is a culpable offense. The Canaanites used to surround the altars for the baal with trees. We know this from Judges 6,28 where Gideon is reported with uprooting those trees and destroying the altar. It appears that when these idolaters sacrificed on their altars the sacrifices were in honour of these trees.