The phrase "dense thickets" means a lot of plants or trees growing closely together, making it difficult to see through or pass through them. Full definition
The plant life is equally mysterious, with dense thickets of bushes with vicious thorns, and giant cacti and bottle - shaped trees that hold moisture like a camel's hump. (newscientist.com)
These corals grow in dense thickets, some of them 30 feet tall, off the coasts of Scotland, Norway, Alaska's Aleutian Islands, and many other places. (discovermagazine.com)
Introduced to North America by Botanical Gardens in the 1890s, Amur honeysuckle — referred to by Conover as public enemy number one — has formed dense thickets in the local forest sub-canopies, choking out native species everywhere it grows. (sciencedaily.com)