Water is Key
The most important aspect in caring for new trees is water. Newly-planted trees do not yet have a fully-grown root system, making it harder to take in the levels of water needed to survive. Because of their premature, lacking root system, new trees require a lot more watering. It is better to water deeper, and less often, as opposed to watering often, but not deep enough. Sprinkler systems do not provide the adequate amount of water a new tree needs, so be sure to always use a garden hose and nozzle attachment.
How to Water
Before you set out to water your new tree, check the moisture level of the surrounding soil. You will need to check the soil closest to the root ball of the tree (4 to 6 inches deep), so you will need a shovel to move around mulch and dirt. If the soil around the root ball is quite moist, wait to water the tree. Check back the next day or the day after. If the soil is cool and slightly moist, it is time to water. You do not want to wait until the soil is dry. Water deeply every few days, checking the soil moisture levels before you start. Deep water can keep soil moist for a week or longer, so always check the soil before you water.
If it is wintertime, the natural precipitation levels are adequate enough to provide a tree with the proper amounts of water. That is because trees are dormant in the winter, and require less sunlight, nutrients, and water to survive.