The key to making sure that your newly planted trees have a long, healthy life is to dig a good hole. There are two common misconceptions about planting holes. The first is that they need to be really deep. The second is that the native soil or the dirt in your garden must be amended before the planting hole is backfilled.
Planting holes should be dug at least twice as wide as the width of the container, but no deeper than the height of the root ball in the container. Root systems mostly spread sideways, not down. After you've dug the hole, remove the plant from the container. If you haven't already checked before buying, look for girdled roots — larger, stronger roots that wind around the root ball and that seem stuck in that position. Girdled roots will never straighten out well and do their job properly. Return plants with girdled roots to the nursery or garden center.
Longer, more flexible roots can be straightened out to lie flat on the bottom of the hole. If necessary, widen the hole to make room for longer roots. Sometimes the root ball is so compacted within the container that removing the plant is difficult. Cut plastic containers apart with heavy shears and bang the root ball on the ground to loosen it and slice truly compacted root balls with a sharp knife to separate roots. (It seems brutal but, if not separated, the roots will continue to grow into a ball.)
Place the bottom of the root ball on the flat bottom of the hole so that the top of the root ball is about an inch above the soil surface. The soil and the plant will settle over time; planting a little high will help prevent root rot from water settling near the trunk and root flare.
Before you backfill the hole, fill it halfway with water and time how long it takes to drain. In sandy loam soil (the ideal soil), water will drain in five to 10 minutes. In sandy soil, the water will disappear within just a few minutes. In heavy clay soil, water might take 20 to 30 minutes to drain. Heavy clay soil is often found in areas with an impermeable layer of hardpan underground. Thinner hardpan layers can be broken up with a pickax or an augur. If water doesn't drain and you can't break up the hardpan layer, find another planting spot for the tree or bush.
Soil types can differ in different spots within your garden; identifying the soil type can help you adjust watering times.
UC Davis no longer recommends amending soil when planting trees. Research has shown that roots will spread to the limits of the amended soil and then stop growing. Backfill the hole halfway with the native soil and fill the hole with water again. Let the water drain and finish backfilling. Firm the soil around the trunk with your foot and keep the soil consistently moist, not soggy, for several weeks.