Hi j1mm3rs! When you took the trees out of their pots to plant them, did you happen to notice whether the roots were winding round and round inside the pots? If the tree now in trouble was "pot bound" you may need to unwind some roots. I also think that the trees look as if they are planted too close together and are competing for water and nutrients. Trees planted for 3 years should not need watering so frequently, they should be able to survive on their own by now! Can you space them further apart? What kind of soil are the trees planted in, and how deep is it? I'm wondering if you have just a shallow layer of good soil, and something else beneath.
