The 13th prohibition is that we are forbidden from planting trees in the Holy Temple or next to the altar for decoration or beauty, even if the intention is to serve G‑d. This is because they would also honor idols in this way, i.e. by planting beautiful, pleasant-looking trees in their houses of worship.
The source of this commandment is G‑d's statement (exalted be He),1 "Do not plant for yourself an Asherah2 or any [other] tree near the altar of G‑d your Lord."
One who transgresses this prohibition is punished by lashes.3
The details of this mitzvah have been explained in the Talmudic tractate Tamid.4 There it is explained that this planting is prohibited in the entire Temple.5
