Deuteronomy 16:21 "Do not plant for yourself an Asherah of any tree near the altar of the L-rd thy G‑d"

The "Asherah" is a tree that is planted near the house of idol- worship in order to make the area more beautiful.

We are forbidden to plant trees near the altar of the Temple - the Beit HaMikdash.

The Torah wants our service of HaShem to be holy and special.

It should, in no way, resemble any form of idol-worship.

Because of this, there were no trees planted on the Temple site.

As with the "Matzevah" (Negative Mitzvah 11) and the "kneeling stone" (Negative Mitzvah 12), so, too, the "Asherah" is not to be used, even if our intention is for the service of HaShem.

These are not allowed because they are connected with idol worship.