The key is to make the connection reasonably obvious.
Simple example. "Bob and Fred entered the office. He sat behind the desk." Who sat behind the desk, Bob or Fred? We don't know. You'd have to change something to make it clear.
But, "Bob and Sally entered the office. He sat behind the desk." Now there's no ambiguity. If it was Sally who sat behind the desk we would have written "she", not "he".
Often we can pretty well guess which is meant from our general knowledge of the nature of things. "The man and his cat entered the office. The doorway was so low that he bumped his head as they entered." I think we'd assume that it was the man who bumped his head, because people are normally taller than cats. If your intent is that this is a very large cat, then you need to spell it out.
Writers sometimes get tripped up on what they think is obvious versus what the reader thinks is obvious. My mind went blank on real examples, but I know I've stumbled across these now and then. If there's any possible ambiguity, I'd spell it out.
In your example, it's hard to imagine how someone's eyes could look like a cluster of ferns, so I don't see this as being much of a problem. But ... maybe someone would think that your intent was to say that her eyes were green like ferns.
When in doubt, reword to make it clear.