That's simple. Depending of the dummy spell you use, you create a tree either at the (Target Point of Ability Being Cast) if it is targetable or (Position of (Casting Unit)) if it is untargetable like Fan of Knives.
And the action is:
Destructable - Create 1 Summer Tree Wall at (Target Point of Aiblity Being Cast) facing (Random Angle) with scale 1.00 and variation 0