buy the functional tree from the background objets section, then you can just click the tree in the main screen to harvest it, and yes, you need the garden to harvest trees in any way.
HEY IF YOU ARE GIVING OUT SPOILERS ON GETTING ACHIEVEMENTS AT LEAST WARN ABOUT IT! This comment board is full of spoilers, I can barely read something here!
 "Bubble Tanks Tower Defense"){kind=icon}
 "The Anti-Idle Game"){kind=icon}
 "Anti-Idle: The Game"){kind=icon}
 "SkyBuilder"){kind=icon}
 "Mousewheel Speedtest 08"){kind=icon}
 "Shoot & Avoid"){kind=icon}