taru-adjective, adverb taking the 'to' particle
flourishing; cultured; aromatic
Literary 'taru' adjective describing a state of thriving culture, refinement, or a rich, pleasant fragrance. Often used in formal or poetic contexts.
その庭園は郁郁とした文化の香りに満ちていた。
The garden was filled with the fragrance of flourishing culture.
彼の書斎には古書の郁郁たる香りが漂っている。
In his study, the rich aroma of old books hangs in the air.
盛ん is a common na-adjective for thriving or prosperous, but lacks the aromatic nuance of 郁郁.
The exact derivation is uncertain; the kanji 郁 carries meanings of 'fragrant' and 'cultured', and the reduplicated form 郁郁 is a literary Sino-Japanese expression.