The Upside Down Peepal Tree or inverted Peepal Tree is also described in Katha Upanishd. Adhyaya 2 -Valli 3-Mantra 1
ऊर्ध्वमूलोऽवाक्शाख एषोऽश्वत्थः सनातनः ।
तदेव शुक्रं तद्ब्रह्म
तदेवामृतमुच्यते ।
तस्मिंल्लोकाः श्रिताः सर्वे तदु नात्येति कश्चन
। एतद्वैतत् ॥ Katha UP. 2.3.1 ॥
ūrdhvamūlo'vākśākha eṣo'śvatthaḥ sanātanaḥ |
tadeva śukraṃ
tadbrahma tadevāmṛtamucyate |
tasmiṃllokāḥ śritāḥ sarve tadu
nātyeti kaścana | etadvaitat || 1 ||
Root up and branches down is this ancient asvattha tree, that (its
source) is pure. That is Brahman and that alone is called immortal. On
that, do all worlds depend and none passes beyond that. This verily is
that.
The Upside Down Peepal Tree or inverted Peepal Tree is actually a metaphor of "Tree of Samsara"
Shankara’s Commentary:
this tree consisting in manifold miseries of birth, decay, death and
grief, etc.,
The location of this tree's roots is said to be Param Pada of Lord Vishnu.
Shankara’s Commentary:
the source (of the samsâra tree,) by ascertaining the nature of the
effect, the tree of samsâra. Root up, having its root
up, i.e., ‘that highest place of Vishnu’is its root, this tree
of samsâra, extending from the avyakta to the immoveable, has its root
up, i.e., in Brahman. It is vriksha (tree), so called, because it is
felled (cut down).