Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized. For example, "Florida," "John," or "Amazon River."
* Common nouns are general names for people, places, or things. They are not capitalized unless they are at the beginning of a sentence. For example, "tree," "state," or "river."
"Palmetto tree" is a common noun because it refers to any tree belonging to the genus Sabal. It's not a specific name like "Washington Monument" or "Mount Everest."