Trends in trees have prompted some thoughts in Dave Crispin, senior arborist at Treeswest Australia and ArbWest committee member, and he outlines some advantages of tree diversity.
Diversity in planting was once desirable, but not always achievable. Now it’s almost a necessity.
I’m talking about streetscapes of course, in particular street trees.
Many Local Government Areas (LGAs) in the past 20 to 30 years have moved away from mass planting of a single species for several reasons, one of the biggest being pest and disease management.
A good example of a species used for mass planting throughout Perth CBD and the surrounding suburbs is Platanus acerifolia. With the ability to tolerate a wide range of edaphic factors, the London Plane Tree thrives in the centre of busy cities where pollution is at its highest, pavement widths are at their narrowest, and consequently, treeplanting space is at a premium. Yet still they thrive.
Ellenby Tree Farm
With the advent of the Polyphagous Shot Hole Borer (PSHB), and the avenues of closely planted, maturing trees, the ever-present threat of the spread of the borer from one tree to another is of real concern. Although regular infections of the Anthracnose fungus (Apiognomonia veneta) have been managed by good cultural practices, or in severe infections by administering a fungicide, PSHB is a different story. Diversity in planting will be another management strategy.
With these challenges in mind I spoke with Craig Woodroffe, co-owner of Ellenby Tree Farm, one of Perth’s largest tree-production nurseries.
Craig’s father David, an avid horticulturist working in Canberra at the time, started Ellenby Tree Farm back in the 1980s after deciding to relocate to Perth. Being actively involved in the horticulture industry, he realised the offering of trees in Western Australia at that time was very limited. Craig recalls most of the trees they imported were from Victoria, a far cry from today where they selectively choose what they grow from their extensive tree gene pool.
‘Growing tomorrow’s tree today!’ is the company’s tag line.
Originally in Gnangara in Perth’s north – which is now the sales nursery – Ellenby Tree Farm has a 70-acre production nursery with more than 60,000 trees in Bullsbrook, northeast of Perth. Growing over 400 species ranging in size from 45 litres to mature transplants, Craig explained Ellenby is investing more time in growing the trees in-ground (field grown) to allow natural root formation, then using a variety of mechanical tree spades to lift them. Consequently, a better-quality tree that is less prone to root girdling, or roots being rootbound after growing in the confines of a pot. I asked if there were any memorable projects. “Many!” was his enthusiastic answer.
“One that we were recently involved with was the new Karrinyup Shopping Centre,” Craig recalled. “We lifted in two Caesalpinia ferrara, Gleditsia sunburst and shademaster, Fraxinus raywoodii, Calodendrum capense, and two Delonix regia, all advanced trees, and they’re all doing really well.”
I asked Craig what challenges Ellenby was facing.
“Lack of sawdust and pinebark used as part of the growing medium for the trees is becoming a problem,” he outlined. “It’s getting harder to source, and predicting future trends in trees is always a challenge, but the Agonis varieties, Angophora costata, and Cupaniopsis anacardioides, are always a favourite with councils.
Celtis australis and Occidentalis are popular, but can become heavily infested by a variety of Psyllid insects. Trees on narrow verges that have a large canopy overhanging the road, and have residents parking their cars underneath for shade, often end up with the sticky honeydew residue splattered all over their windscreens and roofs – honeydew being the substance excreted by the Psyllid after enjoying a healthy chlorophyll meal. Depending on the extent of the infestation, the type of pest, and the size of the tree, there are several options available to the arborist.
The time of applying the product is critical in the life cycle of the offending critter. Consideration must also be given to the beneficial insects or natural predators the product may effect.
For small trees you may be able to use a foliar spray – wind and location permitting. Soil drenches are noninvasive, but require a larger area to administer. It would have minimal effect trying to soak a tree’s root system in a one-square-metre tree well.
The other option is more invasive and involves drilling into the tree and using a systemic insecticide.
Drilling a series of holes around the tree’s trunk, usually staggered, and drilled vertically down, obviously creates a wound in the tree and a potential entry point for any harmful pathogens. Any pruning cut, wound or breaking through the outer bark and into the cambium breaks through the anoxic seal.
Once the insecticide is administered, usually by syringe, the hole must be sealed. As with any type of systemic injection, it is critical to follow the manufacturer’s recommendations.
Soil health is crucial
Soil health plays an increasingly important role in helping the tree combat disease and disorders. The soil provides crucial elements to nourish the tree. Arborists are becoming more acquainted with soil biology, another part of the tree jigsaw. Identifying beneficial microbes, such as nematodes, protozoa, fungi, and bacteria – all part of the soil food web – helps identify what may be contributing to tree health and decline. An increasing number of Perth arborists are purchasing microscopes. Chemical analysis is a little more specialised, and expertise in this field is typically sought by a qualified agronomist.
A good friend who works in broadacre farming once made the statement, “Trees outsource their stomachs to the soil.” It’s an interesting concept. Several Perth municipalities have recommended various soils to be used in residential planting and attached to a development application (DA).
Good urban management
The City of Gosnells, a south-eastern region of metropolitan Perth, has a population just shy of 130,000 and covers an area of 128 square kilometres. The region has history, and was originally purchased by Charles Gosnell, back in 1903, mainly for its fertile soil. The owner was part of John Gosnell & Co, a London based cosmetic company, who saw value in planting aromatic flowers in the rich soil.
I asked the city council’s CEO, Ian Cowie, if the city was moving towards greater diversity in its tree-planting schedule.
“The city plants a wide variety of native and exotic species,” he responded, “with selection determined by location, available soil volume and proximity to infrastructure or environmentally sensitive areas.
“The species of trees available for planting are reviewed biennially and all efforts are made to plant the largest variety of appropriate trees. “Recently, the city has placed more emphasis on planting larger endemic trees to support local fauna, such as Black Cockatoo populations, particularly in areas such as the city’s parks and reserves.”
I enquired whether the city grew any of its own stock.
“The city sources its tree stock from commercial nurseries,” replied Ian. “Some of these vendors offer the ability to grow stock specifically for the city’s requirements.”
What challenges does the council face in the urban landscape, given the declining rainfall, and drying climate?
“Increasing density has resulted in smaller planting spaces in newly developed suburban streets,” outlined Ian, “often with infrastructure in close proximity, which has limited the city’s street tree-planting options.
“Another challenge has been providing water to newly planted trees in a drying climate. The city has extended its watering season in response to this.”
What are the council’s predictions for trees in Gosnells?
“The city recently adopted Greening Gosnells: Our Public Tree Strategy 2022-2030, provides the city with a clear direction for managing public trees and sets targets for canopy cover and tree planting. It includes clear action plans for protecting our natural assets and increasing the number of trees in the public realm to ensure the City of Gosnells continues to be an attractive place.”
To see more from Dave Crispin and ArbWest, log on to arbwest.com.au,