The phrase "thick undergrowth" refers to a dense and tangled mass of plants, shrubs, or vegetation that makes it difficult to see or move through an area. Full definition
Ecologist Linda Geiser works her way through thick undergrowth on the steep hills of the Bull Run Watershed just outside of Portland, Ore.. (sciencenews.org)
The Grand Basset was bred to accommodate low - lying, thick undergrowth during hunting in the countryside. (bil-jac.com)
Moving through thick undergrowth and in burrows is one of the things rabbits do in the wild to keep safe and they are comfortable crawling into tight spaces. (bunnyproof.com)