by Ian Shears.
Senior Tree Planner. City of Melbourne

City of Melbourne Experience

The City of Melbourne is internationally recognised for its tree-lined boulevards, parks and gardens. Trees beautify, define and soften landscapes and give scale to buildings in addition to providing shade and wildlife habitat. Trees are the most life enriching of all the types of vegetation used in the urban environment. They also contribute significantly to the maintenance of a healthy urban environment by trapping airborne pollutants and absorbing carbon dioxide.

The City of Melbourne manages approximately 60,000 trees including approximately 18,000 street trees.  Using the City’s ‘tree amenity valuation formula’ the total value of Melbourne’s trees is estimated to be over $600 million.  This asset is irreplaceable in the short term and the tree population requires close monitoring and management to ensure its continued good health.  Street trees also increase values of adjoining properties.

Melbourne has some of the most significant stands of mature elm trees remaining in the world following the destruction of many of the elm populations in the Northern Hemisphere by Dutch Elm Disease.  The elms lining the major boulevards of Victoria Parade and Royal Parade, along with the avenues of trees in the Fitzroy Gardens are registered as significant by the National Trust.

Of the City’s tree stock approximately 15,000 trees have grown in turf areas with regular irrigation. These include park trees and those grown in turf medians such as Royal Parade. Irrigation systems in the past have generally been designed to water the park surface, median or nature strip grass using manual or automatic surface sprinklers.  Although this method of watering keeps the grass green it is not efficient in watering trees as it encourages them to develop surface root systems.  Regardless of the species of trees and because of historical horticultural practices and the perception that water is a limitless commodity, trees have become dependent on regular surface watering and are less drought tolerant.  Many of the trees in the City of Melbourne have been stressed over recent years as a result of low soil moisture.

The severity of the problem has increased over the last couple of years. There are a number of factors that have contributed to this situation. Reduced rainfall in recent years with Melbourne experiencing 10 years of drought. Reduced application of supplementary water through changes to irrigation management initiated by water restrictions.  Reduced uptake of rainfall through increased hydrophobicity of soils.

In response to the drought and a move away from using turf sprays to irrigate trees the City of Melbourne has used a range of alternative ways to deliver water effectively to tree root systems and maintain soil moisture at levels to maintain trees in a healthy condition.

Soil moisture readings are taken in the City’s main gardens and boulevards in order to inform water application by monitoring the available water for the trees.  The City’s irrigation systems are being changed in order to ensure that the trees are provided with adequate water.

In a major move away from turf sprays over 160km of sub-surface drip line has been installed.  These systems are hooked up to existing infrastructure.  A fleet of water tankers and water-filled barriers have been brought in to supply water to drought stressed trees that cannot be adequately watered using the irrigation systems.  The water tankers are taking reclaimed water from the Royal Park Wetlands.

Recycled mulch has been placed under a large number of trees in parks that may be more susceptible to the dry conditions.

The sub-surface drip lines are considered to be a temporary measure and a more permanent and robust system has been developed to deliver water efficiently in a sustainable way. Street trees present particular challenges in terms of irrigation. These challenges include:

• Tree roots contained within median and street structures – limited water storage volume and limited catchment opportunity
• Tree root distribution highly variable and non symmetrical
• Access to root systems often limited by hard surfaces
• Tree roots in competition with turf roots for irrigation water
• Significant roots located deep within the soil profile – water needs to be delivered at depth
• Canopy interception of rainfall can be significant
• Compacted soils (low infiltration rates) - particularly on nature strips
• High peak daily water requirement
• Street trees are often high traffic and high maintenance areas
• Root disturbance and damage e.g. excavation reduce the effectiveness of parts of tree root systems
• The street trees may already be stressed due to disease or damage

A key consideration for the future watering of the trees will be the development of tree watering systems that will be permanently installed and supplied from a sustainable water source.

The following criteria were developed to establish the context within which a suitable tree watering technique could be identified.

Irrigation water effectively delivered to the tree root soil volume so that healthy growth can be maintained. Watering throughout the depth of the soil profile and lateral distribution are required.

• No overflow or surface flooding
• Minimum damage to existing roots through installation
• Installation technique not to impede root development
• Technique to work effectively in a range of soil types
• Plumbing of water delivery to allow regulated, low flow rate, delivery with minimum risk of blockage
• Installation technique to be flexible should large roots, services and/or rocks be encountered
• Installed technique to allow ground footprint area to be safely trafficked and not subject to subsidence. It should withstand loading that is expected from maintenance machinery and vehicles including trucks
• Robust construction of water delivery system
• Water delivery hardware to be accessible and protected using appropriate valve box and secure cover
• Technique to remain functional, without need for major restoration works, for a period of ten years
• Installation to be environmentally sensitive and responsible e.g. not waste water, any soil waste used responsibly
• Technique can be readily and safely installed and cost effective
• Water delivery program to be able to be accommodated within existing irrigation scheduling capability
• Technique to be repairable should tree root and soil conditions interfere with the functioning of the system.

The City of Melbourne decided, in early 2007, to investigate watering techniques that could be used to maintain trees, located in high profile streets and boulevards, in a healthy condition. The trial investigated a range of drip watering and tree watering well products. Restricted root systems, highly variable soils, high traffic and high exposure characterize these trees.

The trial was carried out in Royal Parade, Parkville, where elm trees are positioned in both medians and nature strip areas. The medians are typically raised concrete structures, approximately 500 mm high and 4 metres wide.

A range of water well products and a watering trench were trialled with the watering trench found to perform best.

The key evaluation criteria included the (a) distribution of water (vertical and lateral), (b) presence of overflow and (c) installation requirements.

The trench was considered to potentially have the advantage of providing a wider distribution of water, allow a relatively large volume of water to be delivered rapidly and, if necessary, allow grass to be grown over the surface.

The basic dimensions of the trench was approximately 1.2 metres long, 300 mm wide and 300 mm deep. The total volume of the trench cavity is approximately 110 litres. Both sand and graded gravel (7 mm), referred to as quarter minus, was used as the trench medium.

The trench version which performed best consisted of quarter minus gravel. The quarter minus provided ample void water space for water storage (approximately 30%) was stable when saturated. The washed sand material was found to become soggy or slushy and provided virtually no top loading support. This is an important consideration for a watering system in these areas is that that present no undue risk to the public.

Water distribution from the trench was found to be variable however typically in the range of 500 mm laterally, beyond the edge of the trench, at a depth of 500 mm.

The water jet technique was considered to be the most effective in terms of constructing this type of trench. However the watering jet technique is potentially expensive and requires considerable support in terms of roadway traffic management restrictions to accommodate the truck.  The trench watering system has been installed in the majority of Royal Parade, sections of St Kilda Road and in sections of Birrarung Marr

The change to sub-surface delivery of water has improved the health of trees. Generally trees have taken one summer season to adapt to the new source of water and by the following summer are displaying greater health with fuller canopies and minimal indications of drought stress.

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Recently published research in the application of carbohydrates (sucrose) in root drenches has shown considerable promise in improving general plant health whilst limiting the effects of environmental stresses and even herbicide damage. Peer reviewed and published research on field trials conducted on containerised plants, young transplanted and established mature trees demonstrated that carbohydrate treatments applied at specific rates resulted in a decrease in time taken for the initiation of new roots, an increase in the number of roots initiated and improved root elongation rates.
More traditional forms of soil improvement treatments generally contain macro and micronutrients and encourage beneficial microbial activity which in turn helps release nutrients in the soil for plant uptake. These assimilates are then transported to the crown of the plant and used in the photosynthetic process to generate carbohydrates required for root growth. By providing sucrose at specific rates in and around the root zone of the plant, the carbohydrates required for root growth are immediately available thus limiting the plants reliance on the crown to provide these assimilates. This can be particularly useful in circumstances where the plants crown may have decreased foliage levels, small and/or chlorotic leaves and which may not have the capacity to produce sufficient carbohydrates for root growth.
Carbohydrates are also known to stimulate beneficial soil microbial and fungal rhizosphere that assist in plant nutrient uptake. In addition, the use of surfactants in carbohydrate drenches can improve irrigation performance and reduce water requirements. Surfactants aid in the wetting and movement of water in media and landscape soils. These are biodegradable detergent-like products that work by lowering the surface tension of water so that wetting is uniform and effective. If media wetting is not uniform, irrigation water tends to move through the larger pores in the media where it is subject to downward gravitational forces. This in turn leads to increase leaching and reduced irrigation efficiency. By improving penetration, wetting, and water movement, media surfactants can increase the effective wetting area thereby increasing irrigation efficiency.
Carbohydrate and surfactant drenches are applied through soil injection techniques that deliver the material at a depth of approximately 150 - 200mm. This is done to reduce the loss of material through natural evaporation and to better target the root system of the subject plant. Treatments shallower than this could result in other plants (particularly turf) drawing up the sucrose before the target plant has a chance to absorb it.
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Fertilising trees refers to the practice of adding supplemental nutrients (chemical elements) required for normal growth and development.
You can’t ” feed” a tree, since trees produce their own sugar via photosynthesis, for which they use nutrients as part of the process.
A reasonably fertile soil will have enough nutrients to satisfy the requirements of growth on most established trees. Trees adjust their growth and development rates to the level of nutrients present in the soil, and will usually grow vigourously as long as the roots can continue to grow.
In nature, trees get nutrients from air, recycled organic matter, beneficial microbes, and soil minerals. In urban settings, the recycling of organic matter is often reduced, beneficial microbes may be minimal, and some minerals can be unavailable be- cause of the soil pH.
Lack of water and organic matter, and soil compaction often limit growth of urban trees much more than nutrient levels.
Infertile soils are most common in new building areas, both commercial and residential, where the ‘natural’ soil has been altered. Altered soils display poor physical and chemical properties for tree and shrub growth. Soil modifications, where required, along with proper fertilisation may improve nutrient uptake of plants.
Fertilising a tree can improve growth; however, if fertiliser is not applied wisely, it may adversely affect the tree. Mature trees making satisfactory growth may not require fertilisation.
Adding fertiliser, especially nitrogen, around stressed or root damaged trees can be harmful unless you have determined that the stress is due to nutrient deficiency.
Harm can occur when a tree in decline with low energy reserves attempts to incorporate nitrogen into cell components.
Since this process requires energy expenditure, reserves can be lowered further. This could lead to a further decline in health as trees with low energy reserves have reduced ability to deal with the effects of injury and pests.

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Peoples awareness of the potential benefits to gardens of using mulch has increased markedly over the past few decades. With the recent limitations imposed on domestic water resources and the implications for water use in the garden, plant moisture stress caused by drought is becoming an increased cause of tree decline. Mulch is gaining attention as one strategy that can assist gardens to cope with hot dry conditions.
Mulch is a covering placed over the ground. Numerous materials are used as mulch and these can be broadly divided into organic products, such as wood and bark chips, shredded leaf and branch matter, recycled pallets, leaf litter, saw dust, and straw, and inorganic products, such as crushed rock, gravel, plastic, and shredded rubber to name a few. Organic wood based mulch is the focus of this article.

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Mature trees can have large water requirements and may lose up to hundreds of litres of water daily through their leaves, especially in hot and/or windy weather. Trees will experience stress if insufficient soil water is available for use by the tree to replace the water lost through the leaves. Continued stress can lead to serious problems for your trees. Supplementary watering when required can assist in maintaining your tree’s health.
Landscape trees commonly grown in Australia originate from many different climate zones and have different water needs. Knowing what trees you have and what their water requirements are will help you understand their watering needs. If in doubt, hire a knowledgeable, certified arborist for advice.

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