Tree watering is a key part of tree care and it is difficult to recommend an exact amount due to the varieties of climates. But a few rules of thumb will help guide you to water your trees properly.
Watering Newly Planted Trees: For new trees, water immediately after you plant a tree.
Watering Trees During First Two Years: During the first couple growing seasons, your newly planted tree is expanding a lot of energy trying to get its roots established in the soil. Especially during the first few summers of your new trees life, it will have a difficult time dealing with heat and drought. You can make this easier by providing water and covering the soil with wood-chip mulch. Deep watering can help speed the root establishment. Deep water consists of keeping the soil moist to a depth that includes all the roots.
How Much Water and When: Not enough water is harmful for the tree but too much water is bad as well. Over-watering is a common tree care mistake. Please note that moist is different than soggy, and you can judge this by feel. A damp soil that dries for a short period will allow adequate oxygen to permeate the soil.
As a rule of thumb your soil should be moist. Usually 30 seconds with a steady stream of water from a garden hose w/ a diffuser nozzle per tree seedlings is sufficient. Mulching is also key in retaining moisture in the soil.
You can check soil moisture by using a garden trowel and inserting it into the ground to a depth of 2”, and then move the blade of the trowel back and forth to create a small narrow trench. Then use your finger to touch the soil. If it is most to the touch, then they do not need water.
Watering Trees After the First Two Years: After your tree has been established in your yard for two years the roots will be established. This will allow your tree to withstand a wider range of water conditions including on its own because it has a proper root structure.
If your area constantly deals with drought you will want to consider trees listed as drought-tolerant. These trees are adapted to sites in their native habitat that regularly experience prolonged dry spells. Although they are native to drought and are more tolerant than others the first few years of life is critical to the survival of the any tree and follow the steps above will help your trees grow.
Some Drought-Tolerant Species Include
Thornless Honeylocust (Zones 3 to 9)
Arizona Cypress (Zones 7 to 9)
Japanese Zelkova (Zones 5 to 8)
Mugo Pine (Zones 3 to 7)
High Soil Moisture-Tolerant Species
On the opposite side of the spectrum if your area deals with a large amount of moisture or wet conditions here are a few trees that will do better in wet conditions.
Swamp White Oak
Baldcypress (Zones 4 to 10)
Shellbark Hickory (Zones 5 to 8)
Red Maple (Zones 3 to 9)
Silver Maple (Zones 3 to 9)
Paper Birch (Zones 2 to 7)
River Birch (Zones 4 to 9)
Weeping Willow (Zones 6 to 8)