To grow thickly; to be in full leaf — used for plants becoming dense or luxuriant.
夏になると庭の草が茂って手入れが大変だ。
In summer the garden grass grows thick and it's hard to maintain.
川沿いの藪は多年で茂り、道が見えなくなった。
The bushes along the river have grown over the years and the path became invisible.
木の葉が茂って木陰ができた。
The tree's leaves grew thick and created shade.