Uprooting urban tree myths in the climate emergency

Australia faces a shrinking window of opportunity to plant enough trees to help cool our cities and protect our farms, writes sustainability expert Michael Mobbs.

Tree planting can stabilise and restore the weather patterns that sustain the seasons but we need to plant millions more trees, using the most up-to-date planting practices, and we only have a small window of time in which to do it.

My guess is that Australia has two to four years of tree-growing weather to plant nine million to 15 million trees needed to cool our cities, and many millions more to return our farms to productivity. 

Other countries are making a start. For example, Ireland recently announced it would plant 22 million trees a year for the next 20 years.

But, with a few exceptions, such a verdant vision seems unlikely to appear in Australia’s barren political landscape any time soon. And when we do plant trees, we are often relying on out-dated or impractical planting practices.

For example, new street trees planted by local government are often doomed to die, as you can see from the photographs below.

It takes at least five years for a newly-planted street tree to cool the footpath and road beyond the nature strip, and about 10 years for its canopy to interlock with adjoining vegetation and trees.

Saplings need to be hand-watered because when it rains on an average suburban street most of the water runs off the hard surfaces – paths, verges and roads – that surround the tree, and flows down gutters, wasted.

Even the rain that falls on grass and sandy nature strips mostly evaporates and runs off, sped on its way by pedestrian-compacted soil that has become hydrophobic, that is, unable to absorb the very water it needs.

Even mature trees struggle to get the water they need because they are surrounded by hard surfaces, as you can see in this photo below, taken in Sydney’s Bondi Beach.

Despite being planted in sandy soil, with an aquifer below, this tree root has grown down into the road drainage pit in search of water.

“Business as usual” where I live in the City of Sydney council area has resulted in more than 13,000 trees planted over the past 13 years. On average, that’s 1000 trees a year. Council says it plants the street trees in the cooler months, from March to October, “to ensure they don’t suffer heat stress, and establish quickly”.

But how many trees do we need to cool Australia’s cities and towns?

There are over 9 million houses in Australia, and while no one is exactly sure how many trees there are, a fair guess is that there is a tree for every 10 houses. 

Let’s explore those numbers.

If we want to shade the paths and roads outside most houses, we will need about nine million to 15 million new trees. Top that number up with trees that need to be replaced at the end of their life.

Planting up to 15 million trees in two years equates to roughly between 20,000 to 40,000 trees every day for the next two years.

But that’s not all. These new trees need other plants to support their growth.  As Melbourne City Council explains: “A lack of species diversity leaves the urban forest vulnerable to threats from pests, disease, and stress due to climate change.”

Can local councils plant this many trees and the plants needed to support them, in the next two to four years? Not if they use business-as-usual practices. 

Tree-growing conditions vary widely in Australia, depending on local soils, rainfall, climate, and mulching and compositing practices.

For example, in the Perth and adjoining coastal areas, it is common to limit public tree and vegetation planting to the months of July and August to match rainfall. However, those efforts are now being undermined by less reliable rainfall in those months. In eastern Australia, there is more rain across the year and temperature and evaporation rates are lower than in South Australia and Western Australia.

These narrow windows for optimum tree planting make it difficult for councils to change their tree planting practices and plant the millions of trees we need in the next few years. 

It means that instead of having to plant between 20,000 to 40,000 trees each day for the next two years, we will need to plant five to 10 times that many in the few months when there is enough rain to support early growth.

It’s a Herculean task but it seems to me to be the only viable strategy.

Here are eight ways residents and local governments can plant a tree and give it a good chance of surviving in today’s changed seasons and survive the increasingly hot weather coming our way:

1. Create subsurface irrigation to irrigate tree roots when it rains
2. Use leaky drains or slots in kerbs wherever possible
3. Plant a diversity of vegetation between trees and plant diverse tree types
4. Compost and mulch as much as possible where the soil is poor, exposed to sun and heat, where it is compacted, or level with the pavement and unable to hold rain
5. Set nature strips at least 30 millimetres below footpaths and kerbs to hold rainwater where it falls or so it can drain to that lower level, and help stop mulch and compost being washed away
6. Let residents plant trees on nature strips, using pre-approved planting tips
7. Let local councils reward residents with rate reductions if they plant trees and install leaky drains
8. Let these works be classed in council budgets as maintenance not capital works (because the latter requires at least a year-long bureaucratic process)

Keeping rain water where it falls is key to success in these eight solutions.

Let’s test one of these solutions: setting the level of a nature strip below the level of the footpath and kerb.

The photo on the left, taken at Bondi Beach, is of new grass turf planted by the local council with surface irrigation and installed in September 2018. The work took about a month and probably cost over $30,000. On the right, you can see the same site a year later. The turf has mostly died, and the surface irrigation has failed. Older grass is doing better in some spots than the new turf. 

This sandy area has high volumes of foot traffic that compresses the turf and soil so that it repels rather than absorbs water. 

A far better design would have included setting the level of the turf about 50 mm below the surrounding footpaths so water could pond rather than run off. A surrounding grate between the paths and the turf with a leaky drain below it would have allowed rain water to feed the turf from below the surface where its roots “drink” – that is, below the ground surface level and where foot traffic can’t compress the soil to impede the availability of the water.

You can learn more about these and other simple, proven designs in my book, Sustainable Food or at online sites such as Street Coolers. 

Treeless streets dominate our cities.

The street in the photograph above gets little shade. It’s typical of our cities and towns. The few trees that have been planted are stunted. In summer and winter, the soil is hot, rainwater runs off to the road because the “nature” strip is higher than the footpath and the kerb, and there is no supporting vegetation. It all adds up to hotter streets, hotter homes and higher energy bills.

But its actually easy to retain rain where it falls.

In Sydney’s Chippendale community, the local City of Sydney council has been doing just that. 

In the photograph above taken in 2017, a child is helping lay a leaky drainpipe. The City of Sydney and the community built a road garden and made it self-irrigating using a “leaky” agricultural drainpipe that directs rain from roofs and footpaths to below the soil to the roots of the plants and trees.

Despite a two-year drought, here is a healthy nature strip of diverse plants. Rain from roof tops and footpaths is directed below the road verge where it is absorbed to feed the roots of the plants and trees there.

But the Chippendale example is not applied council-wide, as you can see in the photograph below. Works now underway by the same council about 10 blocks away, is wasting rain water thanks to a “water insensitive” design.

These new works have failed to retain any rainwater, with most of it flowing away to pollute waterways at the bottom of the catchment at Botany Bay. There are no kerbs to divert road water to the nature strip or the garden beds.

But elsewhere, in the same council area, there is an example of clever design that has allowed trees and plants to flourish without hand watering, as you can see in the photograph below.

Here, water drains downhill from the left of the photo, across the footpath and the bike path, then across the road to where it finally enters the low point of the planted area where the garden bed is about 50mm below the level of the road and able to hold water until it is absorbed by the soil.

These examples of different designs built around the same time by the same council illustrate the lack of co-ordinated planning in local governments.

Let’s return to our “must-do” list for growing trees.

Plants and mulch are vitally important to support newly planted trees, which also need bees, insects and birds to help them grow, and which, in turn, help cool the adjoining soil, and provide nutrients down below the soil where trees “eat”.

Since 2008, residents in Chippendale, in partnership with the City of Sydney, have been planting diverse plants – like in the photo above – and continually composting and mulching in between the trees in the streets where they live. It is an example of an easy, cheap, citizen-led solution. There are hundreds more examples like it across Australia and other countries.

Back in 2008, the same Chippendale residents living in about 20 terraces installed leaky drains to send rainwater subsurface to irrigate the tree roots outside their terraces.

For a one-off cost of $300 the terrace owners bought an agricultural drainage pipe and some gravel. For the past 13 years, the drains have retained about two million litres of rain water, directing it underground to tree and plant roots. This is, “put the pipe in, get-on-with-our-lives, anyone-can-do-it” plumbing.

Acting alone, councils can’t plant the millions of trees we need in the few years we have left to take meaningful action. They must enrol residents to help.

They could do that by designing user-friendly templates that describe how, where and who may plant under an approved and funded scheme. These templates should include: 

1. options for passively directing rainwater run-off underground
2. options for mulch and compost systems to prevent compaction, reduce soil temperatures and moisture evaporation
3. a menu of recommended plantings which, when combined, enhance local plant and tree growth
4. council rate reductions for any property owner planting a tree(s) and plants according to one or more of the templates.

Most people don’t know what to do to cool their house and street but if they were offered lower rates to take this kind of approved action – low-tech, low-cost actions such as tree planting and composting – they would soon learn how to do it! The quickest way to cut bills and to cool cities is to offer rate rebates and other financial incentives to residents and local businesses.

The West Australian government, which funds local councils to help residents plant nature strips, is leading the way.

But what if if a local council won’t directly empower its residents to plant trees?

We should just go ahead and do it anyway, planting trees where we live and work.

Plant your tree now while it’s still possible in many parts of Australia

In 1996, Michael Mobbs converted his Sydney terrace into a sustainable house, disconnecting it from town water and sewerage, and installing solar panels.  The house is powered by the sun, and supplied with rain water. No sewage or stormwater leaves the site. You can see a model of Michael’s home, Ecologic, on permanent exhibition, in the Sydney Powerhouse Museum.

For the past 22 years, the house and its four residents, have enjoyed energy and water bills of less than $300 a year. You can read about its design and see the data  in two books, Sustainable House, and Sustainable Food. 

Michael designs and obtains approvals for projects that are off-grid for water, energy, materials and food.

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