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to grow thickly; to be in full leaf; to be rampant; to luxuriate; to be luxurious
夏になると庭の木が青々と繁る。
In summer the garden trees grow lush and green.
手入れしないと雑草がすぐに繁る。
Weeds will quickly overgrow if left untended.
森の木々が繁って日差しが届かない。
The forest trees have grown so thick that sunlight cannot reach through.