To be thick (trees and grass): 무성하다, 울창하다. Forest: 숲 Thick forest: 무성한 숲, 울창한 숲 ------------------------------------- Please note that 무성하다 usually has the nuance of trees growing out of control; therefore, being thick. 울창하다 has a more positive nuance.

an example) George Orwell's novel ... His men were idle and dishonest, (the fields were full of weeds,) the buildings wanted roofing, the hedges were neglected, and the animals were underfed. ... >>밭(the fields)은 잡초(weeds)로 무성했다.