Answer: Trees typically have root systems that grow slightly beyond the canopy of the tree. I don't expect your dwarf trees will cause problems in your raised planter or the retaining wall because the trees themselves will remain small. Your citrus trees will not cross-pollinate so growing them in close proximity won't change the types or flavors of the fruits they produce. Give them enough room to grow without crowding, by planting them 5-6 feet apart.
Best wishes with your new trees!
Q&A Library Searching Tips