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