In Florida, citrus trees grow an important crop. would the subject be Florida?
No.
Florida is in a prepostional phrase (in Florida) so it cannot be the subject.
Start with the verb. Then ask what is doing that action?
grow an important crop?
Isn't grow an important crop part of the predicate?
citrus trees
Yes, "citrus trees" is the complete subject.
The best way to analyze a sentence is to follow these steps:
1. Find the prepositional phrases and put parentheses around them. This eliminates them from the subject, verb, or direct or indirect objects.
(In Florida,) citrus trees grow an important crop.
2. Underline the verb.
(In Florida,) citrus trees grow an important crop.
3. Find the subject, the word or words that tell who or what is doing the action.
(In Florida,) citrus trees grow an important crop.