If you mouse-over or click the word "unlock" the game will tell you what you you have to accomplish before that crop is unlocked
Example: to unlock Super Strawberry you have to have mastered, all 3-stars, Strawberry.
If it is a Musical note crop -- it will tell you what the previous musical note crop needs to be mastered before it, same with the Space themed crops as well as the other specific themed crops.
Additionally, some of the crops require you to be a certain level before you unlock them.
Example: to unlock Azuki you need to be both player level 15 and jade level 2.
What do you think?