Water newly transplanted trees well for the first year or two, until they become established. It's best to water thoroughly but less frequently. Instead of giving the trees a light sprinkle every other day, soak them deeply once or twice a week if rainfall is inadequate. Apply enough water so that it soaks into the base of the root ball. A 2-inch-diameter tree needs about 15 gallons of water per week; a 4-inch-diameter tree, about 25 gallons. Use drip irrigation, or place a hose running at a slow trickle over the root zone. If in doubt about how much water you're applying, run the hose at a trickle into a bucket to measure the flow. If it takes 10 minutes to fill a 5-gallon bucket, you'll need to leave the hose at the root zone for 30 minutes for a 2-inch tree or 50 minutes for a 4-inch tree.