Pronoun problem

pronoun problem

The passage below is from a book, ‘The social conquest of earth’ by Edward Wilson. In this passage I’m not sure what ‘they’ represents. It seems to stand for ‘the woodland undergrowth’ in a sense of context. But as you can see in ‘thickens,’ the number of the quoted phrase is singular. How can I accept this conflict?

The next step taken on the road to eusociality was the control of fire. Ground fires spreading from lightning strikes are a commonplace in African grasslands and forests today. When they are suppressed, as by the moist soil in forest patches around streams and in easily flooded swales, the woodland undergrowth thickens until it becomes tinder.