Principal Lecturer, Burnley Campus, The University of Melbourne
Successful tree establishment and continued growth is the result of sound planning and ongoing careful maintenance. It should not be a matter of luck or chance. There are numerous factors that can damage or kill newly planted trees. These include climatic conditions, mechanical damage (above and below ground), pests and diseases, compaction, water logging and lack of water.
Access to adequate soil moisture is the single most important factor that will determine the successful establishment of many urban trees.
It is critical that new tree plantings have a watering program that will give the tree the greatest chance of success. Key issues including how much water to apply, how often to apply water and where to apply it all need to be considered.
Encouragement of the development of the root system so that maximum utilisation of the surrounding soil volume should be the key strategy in tree establishment. This approach, which maximises access to soil water, will provide the greatest chance of success.
Tree Water Use
In order to establish and successfully manage a tree, it is important to have an appreciation of the amount of water required by the tree.
The rate at which trees use water depends on the water use characteristics of the plant, the stage of development and the prevailing environmental conditions. When considering the water use rate for a particular species, it is the changing climatic conditions that will have greatest influence on the water use rates. Extreme climate conditions of high ambient air temperature, low relative humidity, high wind speed and high levels of solar radiation are all associated with high water consumption by trees.
The rate of tree water use can be estimated by taking into account:
- Total leaf area of the tree – water can be evaporated/transpired from all of the leaves (Represented by Leaf Area Index value)
- Evaporation capacity of the atmosphere
- Water use characteristics of the particular species (high or low water user – Crop Factor value)
- Available water in the soil
A guide to water requirements of trees in Melbourne of selected dimensions and properties is presented:
Table 1: Weekly water use rates of selected trees in Melbourne .
|500 mm||10 L||5.5 L||2 L|
|1.0 m||40 L||22 L||8.5 L|
|2.0 m||158 L||87 L||34 L|
|3.0 m||366 L||201 L||78 L|
|6.0 m||1426 L||784 L||305 L|
- It has been assumed that the tree has a Leaf Area Index of 2.0
- A Crop Factor of 0.6 has been assumed.
- Average evaporation data for Melbourne has been used.
The total foliage area (total surface area of all of the leaves, one side only) needs to be taken into account as this reflects the total potential transpirational area of the tree. The Leaf Area Index (LAI), which is the ratio of the total leaf area of a plant to the projected plant area (CA), is used to calculate the total leaf area. LAI values are typically in the range of 2.0 to 5.0 for trees. A Crop Factor value for the tree can be used in conjunction with an evaporation value to determine the water use rate. For established trees, values of crop factor are generally in the range of 0.3 to
How Much Water To Apply
The amount of water that needs to be applied through irrigation is dependent on the size of the deficit between the water available in the soil through rainfall and the total water used by the tree.
When a new tree is planted the amount of water available from the soil/media is relatively small and hence frequent applications of water is required to ensure the root system, which often has been damaged, has access to water on a continuing basis.
Table 2: Guide to water available in various size soil volumes
|Soil/Root System Diameter||Soil Depth||Water Available (20% of total volume)|
|200 mm||200 mm||1.2 L|
|300 mm||300 mm||14.2 L|
|500 mm||300 mm||47.1 L|
|1.0 m||500 mm||314 L|
|2.0 m||500 mm||629 L|
|3.0 m||500 mm||1414 L|
- The total soil volume has been determined using a cylindrical shape.
- It has been assumed that the available water is 20% of the total soil volume.
Reference to Table 2 shows that only very limited amounts of water are available from soil storage with newly planted trees (For example, a soil root system 300 mm diameter, 300 mm deep, available water is 14.2 Litres)
It is therefore very important that:
- Water be supplied regularly to newly planted trees to ensure survival maintain growth and
- Development and expansion of the root system is encouraged so that the tree gains access to increasing volume of water in the surrounding soil.
Water Management Strategies for Newly-Planted Trees
- Maintain adequate soil moisture – match supplemental irrigation water needs to climate conditions and available soil water
- Water effectively – apply water so that it reaches the tree root system with minimal losses
- Encourage extension of the root system – apply water to extremity of root system and beyond
- Remove competition for water – mulch around the tree
Implementing the Water Management Strategies
Maintain soil moisture
The care of trees in the early stages of transplanting is critical. The tree canopy continues to demand water and if adequate amounts are not available, from the root ball, then the tree will become stressed.
In addition to the relatively small volume of water available, the tree often has diminished capacity to absorb water due to root damage and transplant shock.
Daily monitoring of soil moisture is recommended for the first couple of weeks.
The variation in demand by the tree for water is large. A period of windy conditions with hot dry air can very quickly increase water demand by a factor of several times. Soil water reserves can be quickly depleted.
Water supplied through irrigation should be matched to the weather conditions and stage of growth of the tree. It should not be done on a purely fixed period (eg. once every two months) basis throughout the whole year.
Delivering water to the tree root zone, with minimal losses, should be the aim of irrigation.
There are a number of characteristics and properties of trees that present challenges in achieving high irrigation efficiency. These include:
- Roots located within a relatively confined ground area
- Roots located deep within the soil profile – water needs to be delivered at depth
- Compacted soils (low infiltration rates) at some sites
The choice of irrigation method and the operation of the system is very important.
A sound recommendation for the watering of trees is that the watering should be slow and deep. Water should be applied slowly so that runoff does not occur and deep so that the soil volume, where the bulk of the roots are located, is watered rather than just the top layer of soil.
It is advisable that some form of piped irrigation system be installed to aid the establishment of newly planted trees. The system may only be used for the first two or three years and then discarded.
Manually based watering systems are often prone to failure due to missed waterings, inappropriate and ineffective applications. If manual watering is all that is available, then planting must include some provision for water direction; either an earth berm should be formed around the planting hole with the backfill soil, or one of the proprietary root collars should be installed. Use of aggie pipe to “channel water to the rootball” does not fulfil the slow, deep water requirement.
Encourage root extension
Watering of a newly planted tree really has two purposes. One is to satisfty the immediate water needs of the tree and the second is to provide a moist soil environment into which the roots will develop and extend.
Both the irrigation technique and the control of the application of water need to be considered. The applied water needs to penetrate the soil profile to an appropriate depth. Slow application rates over long periods of time are generally best. Drip irrigation is well suited to this situation.
Water needs to be applied beyond the tree dripline to encourage root into this soil volume.
As a guide water can be applied to a circular area defined by a radius of 50% of the dripline and 150% of the dripline.
Irrigation drip emitters should be positioned to encourage roots to develop away from the trunk. In the first season of growth the emitter may be placed between the trunk and dripline and then, following establishment, moved beyond the dripline to the surrounding soil. This may involve installing drip emitters, using flexible tubing, to allow the emitter to be moved as the tree root system develops.
The ground area around the tree should be mulched. A depth of 75 mm or greater depth is generally recommended. By reducing the water evaporation from the soil and eliminating water wastage, through prevention of weed growth and grass intrusion, the mulch provides several water efficiency benefits.
An area of mulch around the tree also provides a protection zone, which allows maintenance, both mechanical (mowers and trimmers) and chemical to be carried out with minimum risk to the tree.
Another benefit of the mulch is the modification of the soil temperature so that the tree roots experience more stable temperatures and hence provide greater opportunity for healthy tree growth.
Due to the water absorbent properties of organic mulch, it is important to recognise that both rainfall and irrigation water can be absorbed within the mulch and may not penetrate to the soil.
Selecting an irrigation technique – Critical factors
Each irrigation method has its own characteristics and features.
The key performance characteristics to consider are:
- Wetted area
- Precipitation rate
- Efficiency of application
- Flexibility in application
- Robustness and sensitivity to vandalism
The following are key or critical features that are relevant to the selection or exclusion of a particular irrigation method for urban trees.
- wetted radius large – large area covered
- top soil layer wetting only
- system precipitation rate suitable to most soils
- wetted radius small
- top soil layer wetting only
- system precipitation rate very high
- need to ensure water stream is free of obstruction eg. turf
- high flow rate, high precipitation rate
- surface installation – impractical for site use and maintenance
- medium to high precipitation rate
- small drops, wind drift, evaporation – excessive losses
- surface installation – vulnerable to interference or damage
- low precipitation rates – suitable for soils of low infiltration rates
- multiple point source application – many drippers required for large areas
- surface installation – impractical (unless covered in mulch)
- low precipitation rates – suitable for soil of low infiltration
- multiple point source application
- damage to root system during installation of established trees
- risk of emitter blocking and pipe constriction by roots
- no visual assessment possible to check operation
Some watering don’ts
- Don’t apply water to the trunk of the tree.
- Don’t overwater a tree so that the soil remains waterlogged for long periods and the soil is depleted of oxygen.
- If runoff occurs when irrigating, stop watering. Cycle the application of water.
- Don’t apply shallow applications that only add moisture to the top soil layer (eg. 50 mm)
- Don’t continue to apply water inside the dripline once the tree has become established.