* "grasses" can be replaced by "grasses" (the plural of "grass")
* "stalks" can be replaced by "stalks" (the plural of "stalk")
Since these words are already homophones of themselves, it's tricky to find a single word that sounds the same but has a different meaning.
It's important to note that "grasses-stalks" is a hyphenated noun, meaning it's a single word.