Here's why:
* Compound words are formed by combining two or more words into a single word (e.g., "sunbeam," "rainbow," "highchair").
* Big bluestem is a two-word phrase that refers to a type of grass. The words "big" and "bluestem" are not combined to create a new meaning.
It's important to note that "bluestem" itself is a compound word, but the full phrase "big bluestem" is not.