After three to five years, fruit grows on the new trees. would the subject be grows on the new trees?
The subject is A NOUN (usually) and it identifies who or what is doing the action of the verb.
First, find the simple verb; then ask yourself, "Who or what is doing this?"
fruit
Right. The subject is "fruit."
Yes, the subject in the sentence "After three to five years, fruit grows on the new trees" is "fruit."
To determine the subject of a sentence, you typically identify the main action or what/who is being talked about. In this case, the main action is the growing of fruit. Therefore, "fruit" is the subject of the sentence.