There is a basic question that only the electrical inspector (Authority Having Jurisdiction) can answer.

Is the green house a feeder circuit or a service entrance?

The answer will determine what kind of underground wire is permissible and how many conductors you will need. As to your feeder breaker sizing, that is dependent on the wire size or vice versa, the feeder breaker size is dependent on the wire size. As has been mentioned, determine what the current rating of the greenhouse entrance box and size the wiring and breaker accordingly. It is a common misconception that a distribution panel has to be fed at the panel rating. You can feed a 60 ampere panel with 20 amp breakers but obviously that would be a questionable thing to do.