taru-adjective, adverb taking the 'to' particle
Describes vegetation, forests, or undergrowth that is thick, overgrown, and luxuriant. Often used with と (e.g., 鬱蒼と茂る) or as a たる adjective (e.g., 鬱蒼たる森). Conveys a sense of wild, impenetrable greenery.
鬱蒼と茂った森の中を歩いた。
I walked through the densely overgrown forest.
その庭は鬱蒼たる緑に覆われていた。
The garden was covered in lush, thick greenery.
Rarely used kanji form. 葱 means 'green onion' but here contributes to the sense of lush greenery.
密 is a more general term for 'dense' or 'close', used for population, texture, etc., while 鬱蒼 specifically describes thick, wild vegetation.
生い茂る is a verb meaning 'to grow thickly' (of plants), often used in a more neutral or descriptive way, whereas 鬱蒼 carries a literary, slightly ominous nuance of impenetrable lushness.
鬱 (うつ) means 'dense' or 'gloomy', and 蒼 (そう) means 'blue' or 'green' (often of vegetation). Together they form a compound describing thick, dark greenery. The reading うっそう comes from the kanji readings うつ + そう with gemination. The variant 欝 is a simplified form of 鬱.