Tree maintenance, pruning and disease management

Autumn is an important time in the management calendar of any garden, and it’s especially significant for the Royal Botanic Gardens Cranbourne in Victoria.

For the Royal Botanic Garden’s horticulture and arboriculture teams, this time of year calls for an ‘autumn reboot’ to revive plants after the hot summer. “In Australia, we have summer dormancy due to heat and dryness,” says John Arnott, Horticulture Manager at the Gardens. “Autumn is the trigger for things to start to regrow and rehydrate,” he adds.

From March through to May, moderate rainfall ensures plenty of moisture in the soil. This makes it a great time for tree maintenance, as STIHL’s resident garden expert Charlie Albone found out recently.

TREE PRUNING 

There are a number of good reasons to prune in autumn. It’s a balance of safety, aesthetics and trying not to fight a tree’s natural instinct to grow. It comes down to removing the three D’s – dead, damaged and diseased. 

“For Australian trees, a lot of their active growing season takes place throughout autumn. It’s an ideal time to prune,” says Charlie Carroll, Arboriculture Manager at the Gardens.

Canopy and weight reduction  

The Gardens undertake frequent thinning of eucalypts in autumn. This reduces fire risk, whilst canopy separation is a great way to manage possums by ‘breaking the bridges’ they use to scurry about.

Weight reduction of limbs is also a major focus for the team, who take a preventative approach to tree management. Weight reduction of larger limbs can improve the structure of a tree and reduce the likelihood of limbs failing. 

Structural pruning 

With the dropping of leaves and debris in autumn, arborists and gardeners can get a good look at the structure of a tree. 

“We start by looking on a ground level and then our inspection level goes up as we ascend,” Mr Carroll says. “Or, we use different types of machinery to do other assessments depending on what we see on ground level.”

Formative pruning 

With plenty of sunlight and soil moisture, autumn is a good time to plant and pot. With this comes the process of formative pruning. It’s a great way to set young trees up for future success.

Plant selection is also key. “A sensible practice is establishing tree stock. Picking the right healthy stock from the get-go is key to developing good trees,” says Mr Arnott.

The Bonsai Technique 

Royal Botanic Gardens Cranbourne has a world-class reputation and this extends to their pruning methods. A prime example is the Seaside Garden. Lining the large body of still water are melaleuca trees, pruned using the Bonsai Technique.  

“An iconic element of the coast is the breeze and that prevailing wind that shapes and leans plants over because of the salt spray,” explains Mr Arnott. The technique begins as a formative prune, overseen by Nursery Team Leader Mandy Thomson. Mandy and the team plant seedlings at a 45 degree angle, then use secateurs to prune them to a couple of leaders, which encourages lateral growth.

Dead-wooding 

When dead-wooding, it’s tempting to get rid of everything, but that can give you a staged look. The trick is to find a happy medium, in line with the objectives of your garden.

Mr Carroll says, “We try to address a balance between people’s safety, fire loading, ecological concerns, and then the aesthetic look of the tree.” 

DISEASE MANAGEMENT  

The management of pests and disease is key part of the Gardens’ operations. “We have quite stringent bio-security protocols that try to minimise the incursion of new pests and diseases,” says Mr Arnott. “Prevention is better than the cure. We focus on good hygiene practices. We have an acquisition policy that means we only buy from accredited nurseries. So it’s about the quality of plant material coming in, and monitoring.”

Myrtle rust 

Outbreaks of Myrtle rust is an ongoing threat to plants in the Myrtle family. As such the gardens have a stringent monitoring program where sentinel plants are regularly inspected. The gardeners are quick to jump on any incursions – especially when the weather is warm or humid. If it’s a small incursion, they’ll remove and bag it, then spray all myrtaceae in and around as a preventative. Monitoring is the key to ensuring any outbreaks are addressed swiftly, before they become a bigger issue.  

Armillaria 

Armillaria is a fungi that is very difficult to control because it moves from root system to root system. It can move around a metre a year if the conditions are right. In fact, armillaria is documented as the world’s largest living organism – making it the biggest threat Royal Botanic Gardens’ collections.  

“It’s a wicked dilemma because it rots wood but it’s also a parasite,” says Mr Arnott. “It has the capacity to kill woody plants and trees. We have incursions in and around the site which we’ve documented and mapped, so we know where it is. We’re running an interesting research project and trial to see if improving the general health of plant material can keep armillaria at bay. If the plant is healthy, it may have improved capacity to be resilient against armillaria infestation – the armillaria will be present in its system, but it won’t take over and kill it.” 

This process of building resilience into the tree stock is a revolutionary one, as many gardens around the world still rely on chemicals to treat armillaria. These are counter intuitive when you’re trying to build up soil biota, because they tend to kill all the organisms in the soil including the good ones. 

“You can also dig out root systems and remove all infected material, but for us that’s not practical at this scale. We’re hanging a lot on building resilience in plants for a way of living with armillaria,” says Mr Arnott. 

GARDEN SYSTEMS AND PROCESSES   

Maintenance calendar  

The management calendar is key to the care of any garden collection. “There’s an overall program we run with an inventory of all the trees and tasks that need to be done. That’s the programmed work,” says Mr Carroll. “Then there’s always unprogrammed work – especially after storm events, there’s clean-up,” he adds.

Tree management tools  

The horticulture, arboriculture and nursery teams at the Gardens use everything from secateurs, loppers and hand saws through to chainsaws of all varieties and sizes. The tools used by these professionals is shifting with the evolution of new technologies.  

“We’ve gone battery at the gardens almost exclusively, until we absolutely need something bigger in cases where we are doing removals or larger work,” says Mr Carroll.

Not only is it easier on the body (reducing fatigue), but battery chainsaws don’t require a drop start, which is not ideal when they’re up a tree. “The STIHL MSA 161 T has changed my climbing life,” he says. 

Another key benefit is that they don’t spill fuel, which can erode climbing ropes. It is why, when a petrol saw is required, Mr Carroll likes to use STIHL BioPlus Chain and Bar Oil. “It’s biodegradable. So, if it spills, it’s a little gentler on the environment,” he says. 

When working from the ground, the team use STIHL pole pruners which allow for longer reach.  

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