Knotgrass is a mundane plant used in potions.
- Knotgrass is an ingredient in Polyjuice Potion (CS10). It grows in the Forest and can be difficult to walk through (OP30).
The reference in OP -- "Harry and Hermione were struggling through a patch of thick knotgrass" -- suggests that their may be a magical version of knotgrass which actually binds or entangles a person who walks through it. If so, Rowling is playing on the word "knot" in the name of this common plant. And indeed, the author wrote she chose knotgrass for Polyjuice potion because both people are "tied" together (Pm).