Trees and shrubs: I know nothing except this: if you dig a hole below grade and back-fill with improved soil, make sure that the lowest part of that hole has somewhere for water to drain DOWN and out through. If there is no porous soil under and around it, and no trench leading down and away from it, it will fill with water, turn your nice newly improved soil to mud, and drown every root, which kills every plant.
One way to irrigate without wasting water is a dripline or drip tape. They have drippers ("emitters", like 1/2 GPH or 1 GPH) every 6", 9" or 12", and rely on water spreading horizontally into a cone as it passes down through the root zone. They can often be run at very low pressures, especially if you pay for "pressure-compensating" emitters.
They are more efficient than sprayers because wind doesn't blow the water away, and it doesn't evaporate in the air and on the surface of the soil.
They discourage weeds because only a tiny spot on the surface of the soil gets wet. Any weed seeds more than an inch away from a drip emitter stay dry and don't sprout.
If you lay the dripline UNDER some mulch, UV doesn't corrode it, and NO water sits on the surface. It ALL goes down into the root zone.
Also, drippers UNDER mulch are invisible.
With containers, maybe 12" spacing would let you position the containers under the dripline's emitters. Or you could run 1/2 irrigation mainline around your containers, then run 1/4" tubing to each bucket, with individual drippers manually grafted into that 1/4" line. Then you can have drippers exactly where you want, and e4aisly put more than one dripper into some containers - or mix 1/2 GPH with 1 GPH and 2 GPH drippers.
Typically if I have a zone on mini-sprayers, I only run the water for 10-20 minutes. With dripline, I would have to run that zone for more than an hour. And I was always afraid that if I only used drippers, I would forget some day and waste a LOT of water. With sprayers, you can see and hear them.
