The Fir Tree requires a Large (purple outlined) pot in order to plant it. 

To plant the Fir Tree:

• Visit the Outskirts Greenhouse
• Tap the chair icon in the bottom left of the screen
• Scroll to the right until you see a purple outlined pot (pictured below)
• Tap on the purple outlined pot and you'll see your Fir Tree in the OWNED inventory- tap it to plant
  • Note: If you tap on a medium (pink outlined) pot, you will not see the Fir Tree in your inventory as tapping on a small pot will only show small plants.