This is the 11910th most frequent Russian word.
заросшей
Overgrown.
In this sentence, "заросшую" is used as an adjective modifying "равнину" to describe its state as being overgrown with grass.
Тропа пролегала через заросшую травой равнину.
The trail went through the grass-overgrown plain.
Here, "заросшей" is used to describe the thicket, emphasizing its dense coverage with trees.
Старый дом стоял среди заросшей деревьями чащи.
The old house stood among the thicket overgrown with trees.
In this context, "заросшей" specifies the wall's state of being overgrown with bushes.