Key points for watering trees

Watering is perhaps the best service you can provide your trees. Drought is one of the leading stress factors that predispose trees to a huge variety of problems, including pests, decline and death.

Water stress occurs when trees lose moisture through their leaves and cannot adequately replace it. Loss of water through the leaves, called transpiration, is a normal part of a tree’s many functions, but can become a problem if natural rainfall is insufficient to constantly replenish a tree’s water supply. While rainfall is normally sufficient for the needs of trees throughout most of the year, the summer months typically bring a few weeks where rainfall is not sufficient and supplemental watering may be needed.

While trees are able to tolerate various degrees of drought, a good maintenance program should aim to either avoid drought altogether, or at least to limit the degree and duration of a period of drought.

Water can be applied to trees in various ways, including buckets for smaller trees, and soaker hoses or oscillating sprinklers for larger trees. Several key points to remember when watering your trees are:

• Ensure water covers the entire root zone. Water absorbing roots usually extend several meters beyond the tree’s canopy.
• Most water absorbing roots are in the top 12” of soil, so a thorough deep watering is the best way of ensuring the tree receives sufficient water.
• Trees should receive approximately 2” of water per week. If total rainfall for a week is less than 2”, supplementary watering may be needed.
• Trees are best watered overnight, or alternately in late afternoon.
• Newly established trees should be manually watered for three years following planting, and should never be allowed to suffer from drought.
• Watering periods for evergreen trees should extend into the fall. A final watering of evergreen trees should occur just before the ground freezes and should be double a normal watering.

Avoid overwatering as too much water can easily kill trees. Areas where there is clay or poorly drained soils should be monitored for signs of overwatering. Soil should be moist but not soggy. There are ways to measure the amount of water a tree is receiving. One method is to place several small containers under a tree’s canopy. When the average depth in the containers is 2”, enough water has been supplied. Another method is to probe the soil to a depth of 6-8” to make sure the soil is saturated.

Watering trees is often the best form of protection against decline, pest attacks and death. By providing a sufficient supply of water for your tree, you are well on the way to growing a long lived, healthy and vigorous plant that will provide many aesthetic and environmental benefits.