Drought and Tree Watering

The direct impact of drought on trees is characterized by slowing or eliminating growth, a serious health threat causing injury or death. Drought also impact trees indirectly, by increasing their susceptibility to wildfire, insect pests, disease, and overall long-term decline. Severe droughts cause widespread tree mortality across all landscapes, be it urban, acreage, or farm, with profound effects on the function of tree and forestry ecosystems and the overall environment.

Alberta native plant communities, including grass, shrubs, and trees, are well adapted for dry summers and fall, as well as for a period of prolonged drought. Despite this, the effect and impact of drought on trees is devastating and long lasting. Trees that were already stressed by other issues, like harsh winters, winterkill, poor soils, salt, herbicides and mechanical damages, or insect infestation, are likely to decline even more following a drought. Watering trees is extremely important for the well-being and survival of your trees during drought conditions.

Watering Considerations

Water is scarce in the prairies, and water requirements for trees can be substantial, particularly for large trees. Understanding how much water trees need during drought is difficult to determine. Some large trees can use nearly 200-500 gallons of water on a hot, summer day. To understand the water requirements of your trees, there are several key factors to consider prior to watering.

• Soil Type – clay vs sandy soil, poor vs rich, compacted vs natural
• Age of Trees - newly planted, a few years old, or mature trees
• Tree Species Types– water loving vs drought resistant, shallow vs deep rooted, wide vs narrow roots
• Water Quality and Quantity – overall availability of water
• Water Delivery System – irrigation, hand watering, or bags
• Time of Day – morning, afternoon, or evening
• Cost of Watering
• Mulching  - using arborist wood chips is the best way to keep moisture in long-term

Each of these considerations will determine how much watering you need to provide to your trees. Based on these considerations, the amount of water required for each tree can be determined.

Proper Watering During a Drought

Unfortunately, lots of people do not perform proper watering during a drought. There are several steps to consider:

1. Test your water for sodium before watering your trees. If it contains high levels of sodium, it will kill your trees fast and not provide a chance for them to survive
2. Check the moisture in soil by using a garden trowel or knife to pierce to a depth of 4-6 inches. If you can, easily push or insert a 6-inch screwdriver into the soil, there is enough water.
3. Even today, science does not provide the exact amount of water required for each tree, but there are some rules of thumb. During drought, trees grown in sites without lawn irrigation need approximately 10 gallons (38 litres) of water per week, per inch (2.5 cm) of trunk diameter measured. For example, for a tree that is 4 inches (10 cm) in diameter, it will require 40 gallons (152 litres) of water. Some trees can handle drought better than others. For example, pine trees are more drought tolerant than poplars or elm trees.
4. The optimal time to water trees is early in the morning. Try to avoid watering late at night, as there is the potential of developing fungus. It is also extremely important to water trees when the temperature is scorching during the day. If your trees are showing signs of water stress in the middle of the day, you should water them.
5. A very common mistake people make is to put the water hose right next to the trunk. The fine mesh of roots that is responsible for nutrients and water absorption is the furthest away from the trunk. Trees should be watered at what an arborist calls the 'drip line' - an imaginary line extending from the outermost branch tips straight down to the ground. Most of the roots are spread beyond the drip line and usually are equal to tree heights.
6. Drip irrigation is the best way to water trees, as you can control the amount of water delivered, as well as the speed of water droplets. If you don’t have drip irrigation and are using a hose, sprinklers, water gator bags, or buckets, it is extremely important to perform long and slow soaking at the outer edges of the drip line, avoid any water run-offs and water hitting the trunk.
7. Water trees once a week, with slow soaking water. Avoid overwatering, even during a drought, as you can 'drown' your root system if you have heavy clay in your soil.
8. Mulch by adding 4-6 inch arborist wood chips.
9. Do not forget to water trees in the fall!