Check the soil. There is no way to look at the soil from above and tell how much moisture is in it. Dry soils can cause the death of small roots and reduce a tree’s capacity to absorb water, even after the soil is re-moistened. Drought stress can increase a tree’s susceptibility to certain diseases and insects.

To check the soil’s moisture, insert a hand trowel, a soil probe, or your finger into the soil (low-cost soil moisture meters are not very accurate). Very dry soil will resist penetration and indicate the need for watering.

Avoid frequent, light watering. Watering lightly on a frequent basis does not allow water to go deeply into the root system. Instead, water deeply at wider intervals such as once a week. Let a garden hose run slowly at the dripline of the tree, moving it around occasionally. At medium pressure, it will take about five minutes to produce ten gallons of water. That provides a benchmark that can be applied to both small and large trees.  Small trees may be well watered by running the hose run for five minutes on each side of the rootball.  A large tree may need to have the hose run for 20 to 30 minutes at three to four locations along the dripline.

If using a sprinkler system, place a straight-sided container under the sprinkler and water until it has filled 1 to 2 inches. If you deliver the equivalent of 1 to 2 inches of rain, the water will percolate into the soil about 6 inches, reaching the fine water-absorbing roots.

Use watering bags for trees. When filled with water and placed by or around a tree trunk, these vessels will allow water to drip directly into the soil around the roots. The bags let you deliver a set amount of water each time. They are a good way to keep your younger tree well-watered. Versions are available under several brand names.

Don’t forget the trees on your parkway. During droughts, street trees need water too. Keep checking in the fall. Trees and shrubs, especially evergreens and newly planted trees, need ample water in their root systems as they go into winter. Continue to water as long as you can.

Proper watering is the single most important maintenance factor in the care of transplanted trees. Too much or too little water can result in tree injury. More trees are killed by too much water than by too little.

Check new plants and trees often. Newly planted trees, shrubs, and perennials are still establishing their root systems. Check the soil around their roots often to see if it has dried out.  A tree draws most of its moisture from the root ball. The root ball can dry out in only a day or two, even while the surrounding soil remains moist.

Plants should receive up to 1 inch of water weekly. Water both the root ball (right around the trunk) and the surrounding area. Water deeply, then let the water soak in to encourage new roots to grow deeper into the soil.

Continue regular watering for the first few years. Newly planted trees and shrubs may need to be watered regularly for two to three years, until their root systems become established. Large transplanted trees may take longer.

Water trees in containers more frequently. Because there is little soil to hold water around their roots, container plants can dry out and wilt fairly easily. If container plants are in full sun, they will likely require more frequent watering than those in shade.

Check on sensitive trees and shrubs. Drought-sensitive trees and plants that are likely to show the effects of reduced moisture include magnolias, Japanese maples, dogwoods, beeches, larches, tulip trees, and birches. Hydrangeas also suffer during dry weather because they’re shallow-rooted and therefore drought-sensitive.

Spread mulch. A layer of organic mulch, such as shredded bark or leaves, insulates the soil against extremes of temperature fluctuations and holds in soil moisture. Apply no more than 3 to 4 inches deep in a circle around trees and plants and in an even layer over garden beds. Do not let it touch the trunk or stems of the tree or plants.

The top 8 to 12 inches of soil should be kept moist around trees during periods of drought, at least as far as the branches spread (drip line). It is impossible to give a formula on how much or how often to water a tree to keep the soil moist to that depth. The amount of water required will vary with local site conditions, but without adequate rainfall, established trees may need to be watered as often as every 10 to 14 days.

Don’t wait until your plants show signs of stress, such as wilting or yellowing. Any of several methods of watering work well. Remember, you are not watering plants, you are watering their roots.

If the ground is level, simply let an open hose run on the ground and move it around occasionally to get good distribution. If the ground slopes a little, water may easily run off the surface, and a sprinkler or soaker hose would distribute the water more slowly.

If the ground slopes severely, a root-watering needle may be necessary. Insert the needle no more than 6 inches into the ground, and move it around frequently since it moistens a small area around the insertion point. No matter which watering method you choose, it is important that you don’t saturate the trunk and that you keep the top 8 to 12 inches of soil evenly moist throughout dry periods.