Mr. Hedge-Head really wants you to understand this!
A plant's success depends greatly on how you plant it. There's not much more to it than digging a hole about the size of the pot the plant arrived in, and gently removing the plant and the soil from the pot, and gently lowering it into the hole. It's when we get fancy that we mess up.
Planting so low that that part of the stem or trunk is under the surface does not protect the plant.
Excess watering can kill a newly planted tree, shrub, or annual in hard clas soil just as surely as it can when the plant is in a pot
The most import thing to be aware of when planting is a part of the plant called the crown. It's the transition point between the stem or trunk of a plant or tree, and the roots. It must end up just at the surface of the soil, or any plant, with minor exceptions, will be compromised and even die at a young age.** Trees can live for hundreds of years, but they'll die in 5 to 10 years if planted too deeply. Keep an eye of for trees that rise up from the soil like phone poles. They are at risk. A healthy tree flares at the bottom.
Essential: When the plant and its soil are sitting in their new hole, the top of the soil that came with the plant should be a but higher than the surrounding soil. This allows for some settling once it's all watered in and recovered from the jostling that went on while you were taking it from its pot.
If the plant came from a pot that it had been in too long, its roots will be all you can see. It's helpful to most plants if you do some loosening. Some don't mind aggressive techniques like cutting with a knife. Others, like Bougainvillea, can't tolerate any disturbance of their roots.