Signs of water related stress
Mature trees can have large water requirements and may lose up to hundreds of litres of water daily through their leaves. Trees will experience stress if insufficient soil water is available to replace the water lost through the leaves. Continued stress can lead to serious health problems. Supplementary watering during dry periods can assist in maintaining tree health.
Signs of water stress can include wilting foliage, marginal leaf scorch, lack of new growth in spring, and dieback of leaves, twigs and branches. The premature shedding of leaves without the appearance of the wilting or leaf scorch is another response to water stress.
These signs, while indicative of water stress may also be indicative of other tree health problems. If unsure, consult a knowledgeable, certified arborist.
Before watering, check the requirements of the current water restrictions.
Before you decide to water your tree, check to make sure that water really is needed by assessing soil moisture. Don’t rely on the appearance of the soil surface or signs of water related stress in lawn or other shallow rooted plants near the tree.
The majority of a tree's fine water absorbing roots are located in the top 10cm to 30cm of soil. Checking soil moisture in the tree’s root zone is the most appropriate means of determining relative soil moisture. Undertake a basic soil moisture test by digging a small hole or probing the soil to a depth 40cm at the dripline of the tree. If the soil is moist at this depth, water is not needed.
Deep watering of mature trees every week or two during dry periods, throughout the growing season (September to April) can be beneficial to tree health. This is particularly important for stressed trees and may assist to keep trees alive during times of drought.
It is equally important to not over-water trees, as excess water is as bad for trees as too little water.
Slow watering that provides an even coverage and targets the absorbing roots is the key to successful watering and encourages a deep tree root system. Frequent, light applications encourage shallow root development that is more susceptible to summer heat and drought stress.
Watering is best undertaken using a low pressure system such as drip irrigation or soaker hose. Apply sufficient water to moisten the soil to a depth of 40cm. Depending on tree size and soil type, this may take several hours. Aerating the soil with a garden fork can help water soak into the soil. After watering, check that water has penetrated by undertaking a soil moisture test as outlined above. Avoid creating water runoff as this wastes water and does not allow the water to reach the targeted area.
Diagram 1. Watering near the trunk is unnecessary as for most trees there is generally few water absorbing roots in this area. Irrigating the soil from half-way between the trunk and the dripline to 3m-5m beyond the dripline will provide water where it will be most effectively used.
Preferably, water trees during the cooler evening and early morning period when temperatures are lower, humidity is higher and the air is calmer thereby reducing water evaporation from the soil surface. Watering in the middle of the day is not harmful to most trees but it is less efficient.
Water before the soil dries out. In some instances the soil may have become so dry as to repel water (hydrophobic). The application of a wetting agent under these circumstances may improve water infiltration by reducing surface tension allowing the soil to rewet.
Using mulch to conserve soil moisture
Using mulch is the most effective means of water conservation. Maintaining a 7cm to 10cm layer of mulch over the root zone can reduce the amount of water that is lost from the soil through evaporation. Mulch can conserve water by up to 70%, reducing the need for irrigation. Some organic mulch also has the benefit of adding nutrients to the soil as they break down, improving soil structure and assisting water and air penetration. Careful mulch selection is required as the properties of some organic mulch can actually inhibit water penetration to the soil.
Keep mulch a minimum of 20cm away from the trunk. This space will allow for air circulation around the base of the tree and helps to avoid potential disease problems. The greater the area of root zone that is mulched and free of other plants, the less competition for water, air and nutrients and the more benefit to your tree.
Fertilise lightly or not at all. Over fertilisation can stimulate foliar growth which increases tree water demand. If insufficient water is available to meet this demand the tree will suffer drought stress.
Some information within this fact sheet has been obtained from published sources. For further reading, refer to the Trees – Further Reading Fact Sheet.