taru-adjective, adverb taking the 'to' particle
lush; overgrown; exuberant (of vegetation)
Archaic literary word describing thick, flourishing plant growth. Used as a taru-adjective or adverb with と. Not used in modern everyday Japanese.
古い漢詩では、山が「蓊鬱として」茂っている様子が詠まれる。
In old Chinese-style poetry, mountains are described as being lush and overgrown with 蓊鬱として.
鬱蒼 is a more common literary word for dense, gloomy vegetation, while 蓊鬱 is rarer and more archaic.
The exact derivation is uncertain; the kanji 蓊 and 鬱 both relate to thick growth, and the word is a classical Chinese borrowing.