Full list of all 15 The UnderGarden achievements worth 250 gamerscore. It takes around 8-10 hours to unlock all of the achievements in the base game.
The base game contains 12 achievements worth 200 Gamerscore, and there is 1 DLC pack containing 3 achievements worth 50 Gamerscore.
Grow your first fruit tree
Grow all the flowers on every level
Collect all the bonus flowers in the game
Pickup up every musician in the game at least once
Grow 50% of plants in "Wind Surfing", "Kaboom!", "Blown Away" and "Handle With Care"
Drop a bomb on a musician
Re-bloom some flowers carrying 3 musicians at once
Hit every pollen sack in "Batteries Not Included", "Zap!" and "Into the Machine"
Complete "Grab Hold", "Watch Out, They Spit", and "Hold on Tight" in under 20 minutes
Drop a bomb on the second player in a co-op game
Clear most of the smog in the smoggy levels
Hold a musician, heavy fruit, lantern fruit, and explosive fruit while wearing red horns
Successfully complete “Horn of Plenty” either alone or with a friend.
Successfully complete “The Sum of all Things”, either alone or with a friend.
Collect 17 crystals in the game