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