"For a 5 gallon plant, how large should the planting hole be? For the Arbequina variety, are there any other special instructions for planting and care?"
A good rule of thumb for planting holes is to make them one-and-a-half to two times the diameter of the container. Pay close attention to the soil level when you plant an olive tree; you want the final soil level of the planted tree to be at the same place as the ground level was in the container. When you dig your hole, make it one-and-a-half to two times the diameter of the pot, but don't dig it too deep or you can have problems with things sinking. Score the sides of the hole--you want a rough surface, not a smooth one. Put a mound of soil into the bottom of the hole for the tree to sit on. When you pull the tree from the pot, loosen the roots and unwind any that are circling--if they are troublesome go ahead and cut them. You want the roots to be untangled and draped down around the mound of earth to encourage them to grow outwards. Plant the tree a little bit high to allow for settling.
Arbequina olives don't have any extraordinary needs. They do tend to be very early and generous producers of olives, so it is a good idea to provide them with ample resources. Regular irrigation and fertilization will help the trees look their best while producing lots of olives. You will also want to control foliar disease by spraying a fixed copper fungicide as soon as the olives are harvested. The UC IPM site is a great information source for pest management: www.ipm.ucdavis.edu/PMG/r583100311.html
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