Sefer HaChinuch ספר החינוך

To not plant any tree-idols: To not plant any trees in the Temple or next to the altar. And about this is it stated (Deuteronomy 16:21), “You shall not plant for yourself a tree-god, any tree next to the altar of the Lord, your God, etc.” Rambam, may his memory be blessed, wrote about the reason for this prohibition (Sefer HaMitzvot, Negative Commandments 13) [that it is] since the worshipers of idolatry would do like this in their houses of idolatry to plant beautiful trees there. And [so] to distance anything that is similar to them from the thoughts of people that come to the worship of God, blessed be He, in the chosen place, we were prevented from planting any tree there. And from the angle of the simple understanding, [this explanation] is likely.

From the laws of the commandment — that which they, may their memory be blessed, said (Tamid 28b) that the prohibition is not [only] to plant the tree exactly next to the altar like the simple understanding of the verse, but rather even one who plants it anywhere in the [Temple] yard is lashed, as the whole yard is called “next to the altar”; [that] whether it is a [fruit] tree or a fruitless tree, it is all included in this and he is lashed for it; [that] to make a fence for this prohibition, the Sages also said that it is forbidden to make a veranda of wood in the Temple in the way that people do in their [own] courtyards, and even though it is by building and not by planting, as it is stated, “any tree (which is also the word for wood),” but rather all fine structures and verandas that extended from the walls that were in the Temple were of stone; and all the rest of its details — are elucidated in Tractate Tamid.

And this prohibition is practiced whether by males or whether by females. And even at this time, one who plants a tree anywhere in the [Temple] yard is liable for lashes.