Here is the full list of all 11 Cloud Gardens achievements.
Complete the credits scene
Complete every scene on the Overworld
Grow 120 plants in a single Overworld scene
Build a very tall contraption
Plant one of each seed in a single scene
Finish the Rooftops Chapter
Use all of the toys in one level
Place 30 gnomes in one scene
Go to the the Overworld after completing the first chapter
Unlock a new type of seed
Grow 10000 plants in total