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TREES & plants

Watering trees and shrubs

Watering is a simple yet essential part of caring for trees and shrubs.  Keep your trees and shrubs adequately watered by following the guidelines listed below.

When and how to water your trees

  • 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, use either a hand trowel,  a soil probe, or place 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. 
  • Water the roots. There is no reason to water the leaves of a plant. Water the soil, where the roots are. The Arboretum recommends watering within the drip line of a tree, from the trunk out to the end of the branches, to reach the roots most effectively. The water-absorbing roots are within the top two feet of soil; you want to keep these roots moist but not wet.
  • Avoid frequent, light watering. Instead, water deeply at wider intervals such as once a week.

Let a garden hose run slowly at the drip line of the tree, moving it around occasionally. At medium pressure it will take about five minutes to produce 10 gallons of water.

If using a sprinkler system, place a straight-sided container under the sprinkler and water until it has filled one to two inches. If you deliver the equivalent of one to two inches of rain, the water will percolate into the soil about six 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. So continue to water as long as you can.

Newly planted trees

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.  The tree draws most of its moisture from the root ball. The root ball can dry out in only a day or two, even while surrounding soil remains moist.
  • Plants should receive up to one inch of water weekly. Water both the root ball (right around the trunk), and the surrounding area. Water deeply and 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 2-3 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 to insulates soil against extremes of temperature fluctuations and holds in soil moisture. Apply no more than three to four inches deep of mulch 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.


Established Trees

  • The top 8-12 inches of soil should be kept moist around trees during periods of drought, at least as far as the branches spread (dripline). It is impossible to give a formula on how much or how often to water a tree to keep the soil moist 8-12 inches deep. 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 10-14 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 is chosen, it is important that you don’t saturate the trunk and that you keep the top 8-12 inches of soil evenly moist throughout dry periods.