Srila Sanatana Gosvami quotes the following verses from Sri Harivamsa (Vishnu-parva 11.18-22), which describe the banyan tree:

dadarsa vipulodagra-

sakhinam sakhinam varam

sthitam dharanyam meghabham

nibidam dala-sancayaih

gaganardhocchritakaram

parvatabhoga-dharinam

nila-citranga-varnais ca

sevitam bahubhih khagaih

phalaih pravalais ca ghanaih

sendracapa-ghanopamam

bhavanakara-vitapam

lata-pushpa-sumanditam

visala-mulavanatam

pavanambhoda-dharinam

adhipatyam ivanyesham

tasya desasya sakhinam

kurvanam subha-karmanam

niravarsham anatapam

nyagrodham parvatagrabham

bhandiram nama namatah

"They saw that best of all trees, which had many long branches. With its dense covering of leaves, it resembled a cloud sitting on the earth. Indeed, its form was so large that it appeared like a mountain covering half the sky. Many birds with charming blue wings frequented that great tree, whose dense fruits and leaves made it seem like a cloud accompanied by a rainbow or like a house decorated with creepers and flowers. It spread its broad roots downward and carried upon itself the sanctified clouds. That banyan tree was like the lordly master of all other trees in that vicinity, as it performed the all-auspicious functions of warding off the rain and the heat of the sun. Such was the appearance of that nyagrodha tree known as Bhandira, which seemed just like the peak of a great mountain."