Why Living Trees Suddenly Fall Down

It is quite common that in winter or after big storms we wonder why living trees suddenly fall down. In June 2021 in Victoria, savage storms swept Victoria sending trees crashing down, destroying homes and blocking roads. This article, republished here with permission of the author and The Conversation, explores the reasons behind this phenomenon  Some minor editorial changes have been made.

Under climate change, stronger winds and extreme storms will be more frequent. This will cause more trees to fall and, sadly, people may die.  These incidents are sometimes described as an act of God or Mother Nature’s fury. Such descriptions obscure the role of good management in minimising the chance a tree will fall. The fact is, much can be done to prevent these events.

Photo: James Ross AAP

Why trees fall over

Firstly, in thinking about why living trees suddenly fall down, it’s important to note that fallen trees are the exception at any time, including storms. Most trees won’t topple over or shed major limbs. I estimate fewer than three trees in 100,000 fall during a storm.

Often, fallen trees near homes, suburbs and towns were mistreated or poorly managed in preceding years. In the rare event a tree does fall over, it’s usually due to one or more of these factors:

  1. Soggy soil

In strong winds, tree roots are more likely to break free from wet soil than drier soil. In arboriculture, such events are called windthrow.

A root system may become waterlogged when landscaping alters drainage around trees, or when house foundations disrupt underground water movement. This can be overcome by improving soil drainage with pipes or surface contouring that redirects water away from trees.

Photo: Shutterstock

You can also encourage a tree’s root growth by mulching around the tree under the “dripline” – the outer edge of the canopy from which water drips to the ground. Applying a mixed-particle-size organic mulch to a depth of 75-100 millimetres will help keep the soil friable, aerated and moist. But bear in mind, mulch can be a fire risk in some conditions.

Root systems can also become waterlogged after heavy rain. So when both heavy rain and strong winds are predicted, be alert to the possibility of falling trees.


  1. Direct root damage

Human-caused damage to root systems is a common cause of tree failure. Such damage can include roots being:

  • Photo: Greg Moore

    cut when utility services are installed

  • restricted by a new road, footpath or driveway
  • compacted over time, such as when they extend under driveways.

Trees can take a long time to respond to disturbances. When a tree falls in a storm, it may be the result of damage inflicted 10-15 years ago.

  1. Wind direction

Trees anchor themselves against prevailing winds by growing roots in a particular pattern. Most of the supporting root structure of large trees grows on the windward side of the trunk.

If winds come from an uncommon direction, and with a greater-than-usual speed, trees may be vulnerable to falling. Even if the winds come from the usual direction, if the roots on the windward side are damaged, the tree may topple over.

The risk of this happening is likely to worsen under climate change, when winds are more likely to come from new directions.

  1. Dead limbs

Dead or dying tree limbs with little foliage are most at risk of falling during storms. The risk can be reduced by removing dead wood in the canopy.

Trees can also fall during strong winds when they have so-called “co-dominant” stems. These V-shaped stems are about the same diameter and emerge from the same place on the trunk.

If you think you might have such trees on your property, it’s well worth having them inspected. Arborists are trained to recognise these trees and assess their danger.

Photo: Shutterstock

Trees are worth the trouble

Even with the best tree management regime, there is no guarantee every tree will stay upright during a storm. Even a healthy, well managed tree can fall over in extremely high winds.

While falling trees are rare, there are steps we can take to minimise the damage they cause. For example, in densely populated areas, we should consider moving power and communications infrastructure underground.

You may be thinking large trees are just too unsafe to grow in urban areas, and should be removed. But we need trees to help us cope with storms and other extreme weather.

Removing all trees around a building can cause wind speeds to double, which puts roofs, buildings and lives at greater risk. Removing trees from steep slopes can cause the land to become unstable and more prone to landslides. And of course, trees keep us cooler during summer heatwaves.

It is a concern when a lot of trees fall but removing them is not the answer. Instead, we need to  learn how to better manage and live with them.

The Things People do . . . . . . . . . . to Trees!!!

Given the splendour and usefulness of trees in the urban environment, it is really amazing what otherwise sensible and apparently sane people do to them. All too often people forget the basics – that trees are alive and should be treated as living things. This is one of the most important messages that I can give about trees. If taken to heart then trees will not only be treated with the respect that all living things deserve, but also we may avoid many common and potentially dangerous problems.

How often have you see signs nailed into a tree? I have seen a whole planting of mature trees seriously damaged by people nailing signs for weekend garage sales onto their trunks.

No one would approach something that they see as a living thing with a hammer and nail into its structure. These people are probably not villains! They just see the trees as lampposts with leaves, and don’t realise that they are doing serious long term damage.

As with all living things, trees grow and respond to changes in their environments. We all know this but when a blue gum (Eucalyptus globulus) is planted as a small tree from a nursery container, placing it 20 cm from a brick wall is not a good idea! This species can easily grow to 10m in a few years, on its way to a mature height of over 30m and with a trunk in excess of 1m in diameter. When it starts to lift the foundation and crack the wall, it is the tree that is blamed not the person who planted it. They forgot that it was alive and growing.

I have seen many trees planted under the eaves of a house or on the south wall in almost constant shade. These trees may grow away from Too close to wallthe house leading to a sickly and poorly structured specimen, which is often later removed. What a waste of time, resources and opportunity! However, I have seen one specimen that grew under the eave and lifted the roof right off the wall, causing serious damage to the house. While a tree getting revenge has some appeal, who could forget that trees get taller as they grow?

PavementHow many trees have you seen planted or growing in paving, bitumen or concrete? Trees need space both above and below ground. A large tree always has a large root system, even if you can’t see it and much of it is in neighbouring properties. Most of the roots are close to the surface, so the root plate tends to be shallow and spreading. The root system develops this way because most of the roots will be where the moisture, nutrients and oxygen are located. So plant where there is sufficient space and don’t pave around or under the tree if you want to avoid trouble and expense.

The root systems of trees are often the victims of thoughtless actions. Most people still think that mature trees have a huge tap root that keeps them safe in the ground, but this is rarely the case. For many species the tap root, if it exists at all in urban areas, is a juvenile characteristic that is soon lost. So it is the extensive lateral roots with occasional descending roots that anchor the trees. So don’t cut lateral roots, especially if they are large (greater than 75mm) in diameter. This can have a serious impact on tree health and vigour as well as threatening the trees stability.

Each year, as a new tree grows over the old wood, the older roots gradually increase in diameter. Eventually they come to the surface and push up the surface soil. You can’t do anything about this – this is what trees do! So don’t cut the roots or mow over them re-wounding the roots every time you mow. This can cause the roots to decay, which again affects tree health and stability.

As often happens with trees in urban environments, the failure of trees has less to do with nature and more to do with our management practices. Too often the advice of horticulturists and arborists is either not requested or ignored by those managing our landscapes. The understanding of trees as living things and our knowledge of their root systems and response mechanisms has expanded enormously over the past decade or two. We can no longer use ignorance as an excuse for the often terrible things we do to trees in urban areas!

We have the knowledge to manage trees wisely and well to maximise both their longevity and the aesthetic contribution to our environment. By treating them as sophisticated living things, and providing appropriate growing conditions, we can avoid the high costs of the terrible things that people do to trees!


Harris, R.W., N. Matheny and J.R. Clark. 2004. Arboriculture. Integrated Management of Tree Shrubs and Vines in the Landscape, 4th Ed. Prentice Hall, New Jersey.

Anon, 2009. Protection of Trees on development sites, Australian Standard # 4970, Standards Australia, Sydney.

Article and photos copyright to Greg M. Moore, Senior Research Associate, Burnley College, University of Melbourne

Economic Value of Trees

Many people would take it as a given that parks, gardens and trees have their own aesthetic and intrinsic values. However, so often in capitalist societies, decisions are based on the economic or monetary value of systems and their components. In such systems, as soon as something performs a function or provides a service that is valued, then it too has an economic value. It worth thinking about the economic value of the benefits provided by our gardens and trees because it increases their status in decision making processes1.

Reducing Temperature

schooltrees (640x426)The shade from trees can lower temperatures by up to 8°C, which can reduce air conditioner use by 12-15% and also decreases carbon emissions from our largely brown coal generated electricity2. Two medium sized trees, planted in the right position (usually to the north or west) can save up to $180 per year off a domestic electricity bill through lower air conditioner use. The removal of large trees from school grounds after the Black Saturday fires removed shade and required the installation of large shade sails to protect students from excessive summer sun.

Moderating Wind

It is difficult to place a value on the role trees play in reducing wind speeds of up to 10% during storms and protecting property from hail damage. However, we do know that under climate change winds will be stronger and that severe storms will be more frequent, and that trees filter and deflect wind in they are planted in appropriate places. Indeed, it is often forgotten that properly planted and positioned trees can reduce the swirling of the wind during major fire events, making fire fighting safer.

Reducing Water Runoff

Trees are very good at holding and absorbing water during heavy rainfall events. This is now being recognized as an important part of storm water run-off management as it can reduce localized flooding. In many parts of Australia, while annual rainfall is likely to fall under climate change scenarios, the rain that does fall is likely to occur in heavier downpours. In many places, the storm water pipes are not large enough to cope with the extra volume of water and so water is now being diverted to open space and tree pits to ease the flow, which will reduce local flooding and save the cost of retro-fitting bigger pipes.

Increasing real estate value

street (640x491)Determining a monetary value that appropriate trees add to real estate can be difficult, but a conservative estimate of 5% of the total value has been used in the USA and a figure of about 5.4% has merged from research undertaken in Brisbane. Real estate agents have recognized the value of a good tree in a front garden at about $5,000 for a suburban property and earlier work had estimated that a tree-lined nature strip added 30% to properties. There is also an indirect but significant financial benefit for a local council in increased house prices, which are reflected in higher council property rates.

Excessive tree removal in part of a council’s area of control could impact on its income when properties are next valued for rating purposes. Given that these are significant financial considerations, it is not difficult to envisage a situation where the unnecessary removal of a safe and healthy tree from a streetscape could precipitate legal action by a resident for the loss of property value. I am often amazed at how seldom the effects of tree removal on real estate value are considered until after the tree is gone and the loss is realized.

Improving Health

Treed open spaces improve human heath, extend life spans, reduce violence and vandalism, and lower blood pressure. It is not for aesthetic reasons that hospitals now have gardens, even if they are many floors up. Their presence and visits by patients speed recover and shortens time until discharge, which reduce the costs of hospitalization. The vegetation also humidifies the air which can ease breathing and reduce the need for medication in those with respiratory difficulties. It is often forgotten that the fires of Black Saturday killed 172 people, but the heat wave surrounding it was responsible for 374 deaths. In reducing the urban heat island effect the shade from trees can substantially reduce excess deaths that occur3, predominantly among the elderly, during heat waves.

There is ample evidence that treed landscapes also foster both active and passive recreation, which in return significantly reduces annual medical expenses for each 1% increase in the population recreating1. Green and leafy environments are vital tools in dealing with children lacking exercise and becoming obese, encouraging an ageing population to exercise and curbing ever-increasing health costs. Indeed there are several studies indicating that interaction with trees is a necessary part of childhood development as well as being a foundation of the human condition.

In many Australian cities, new house blocks are becoming smaller and smaller and in the green and leafy suburbs where house blocks were larger land has been subdivided for townhouses, multi-unit developments and on a larger scale dense modern housing. While developers are maximizing their yields, at a time of climate change, it is worrying that the diminution of both private and public open spaces by urban development puts at risk long term sustainability. In many of these situations there is insufficient open space - public or private - for the planting of trees and the opportunities for the role of vegetation in ameliorating the heat island effect, lowering wind speed, providing shade and reducing energy use are lost. Even the roads are so narrow that nature strip trees often don’t survive the quest by residents for parking. This raises questions about the economic viability of such development, as well as their environmental sustainability.

Tree cover in many Australian cities is declining, particularly with the loss of private open space. While the damage and nuisance values attributed to trees are well-known, the benefits they provide are often under-appreciated. Cities are biodiversity hot spots due to the variety of habitats available in front and back yards. For most people planting the right tree in the right place at the right time for the benefit of future generations is a significant legacy. What else delivers so many benefits immediately: benefits that last centuries into the future, which prolong healthy lives and make cities both sustainable and livable?

Dr. Gregory Moore is a Senior Research Associate, School of Ecosystem and Forest Sciences, Burnley College, University of Melbourne.


1. McPherson E G (2007) Benefit based tree evaluation. Journal of Arboriculture & Urban
Forestry, 33 1–11.
2. Moore G M (2012) The Importance and Value of Urban Forests as Climate Changes
The Victorian Naturalist. 129 (5) 167-174.
3. Nicholls N, Skinner C, Loughnan M, Tapper T. (2008) A simple heat alert system for Melbourne, Australia. International Journal of Biometeorology 52, (5) 375-384.

Can’t see the Trees for the Lawns?

Each spring and early summer it is amazing how fast grass, weeds and lawns can grow. The combination of longer, warmer days and the occasional shower makes for ideal growing conditions, and there is always something to do in the garden.

For most of us, there will be the need to mow our lawns (if we have them) fortnightly, if not weekly, and if things have been a bit out of hand, we may even need to use a trimmer to cut back the weeds. You might begin to wonder if there is anything we can do to improve the situation?

Australians as a whole seem to be a group of lawn shavers rather than of lawn mowers. Many of us cut our lawns far too short! Grasses are living things and grass leaves grow from the base, which is just above the soil surface. So we should be cutting lawns higher, and although the height of the cut varies with the species of grass, soil type and weather conditions, in most instances we should not cut closer than 50mm.

Picture 019 (1024x768) (3)Cutting lawns too short has a number of consequences, which are often unknown to home gardeners:

  • Very short lawns use more water than longer lawns. With the water rating systems moving to charge more for water and based on water usage this could see substantial savings on water bills for those who leave lawns to grow longer! And, of course, the extra water used depletes water storages
  • Cutting too short often damages the grass, and can result in dead patches developing in a once lovely lawn
  • Dead patches in the lawn allow weeds to invade, which leads to unsightly lawns and the need for tiresome weeding.

So try to remember not to cut your lawns too short.

While on the subject of lawns, I would remind everyone of the need to protect trees growing in lawns from trunk problems due to motor mower damage, as people try to cut too close to the trunk. Trees are major assets in any garden so don’t cut lawns around tree trunks!

Tree damageOver the past few years the most common problem I have seen with trees is trunk damage from mowers and weed trimmers. Never cut closer than about 150mm, and remember that young, thin-barked trees are particularly vulnerable to weed trimmer injury.

The best way of dealing with grass or weeds growing around the trunks of trees is to hand remove material, use a suitable mixed particle size mulch.at a depth of 75mm or use an appropriate herbicide at the right concentration. In many instances organic mulches that you can produce yourself will prove ideal.

When you work in your garden remember that it is the trees that make a major contribution to its ambience and beauty and which shelter your garden and house from weather extremes. So make sure you don’t harm them when you are mowing your lawns and weeding. See trees as the assets they are and do all you can to protect them!

Your garden not only gives you pleasure, but adds real value to your home and the quality of your life. It is worth doing things properly in the garden as you will be amply rewarded for the time and money invested!


Harris, R.W., N. Matheny and J.R. Clark. 2004. Arboriculture. Integrated Management of Tree Shrubs and Vines in the Landscape, 4th Ed. Prentice Hall, New Jersey.

Articles in Gardening Australia Magazine

Article and photographs copyright to Greg M. Moore, Senior Research Associate, Burnley College, University of Melbourne

The value of trees in urban areas

It is a great pity that so many of our citizens think of our parks, gardens, streetscapes and urban landscapes only in terms of their aesthetics. While they are beautiful and decorative, these attributes often mask the many functions that they serve in our cities to the point where their economic and environmental benefits are often overlooked.

Urban landscapes and trees have been wonderfully silent assets in our cities for decades and even centuries. They are major urban infrastructure assets. Cities are biodiversity hot spots due to the variety of habitats available in public and private open space, including front and back yards. However as assets we may need to expend resources – labour, energy, and even water - on their proper management.

While I often hear it said that; “There are better things to use the water on than plants and gardens” I challenge you to name them. What else delivers so many benefits immediately, benefits that last centuries into the future, which prolong healthy lives and make cities both sustainable and livable?

At a time of climate change, it is worrying that both private and public open spaces are threatened by urban renewal and development that puts at risk long term sustainability. In many of these situations there is insufficient open space - public or private - for the planting of large trees and so there are less opportunities for the role of vegetation in ameliorating the heat island effect, reducing wind speed, providing shade and reducing energy. This outcome raises questions about the economic viability of such developments, as well as their long term environmental sustainability.

There is real economic value in the shade provided by trees that drop temperatures by up to 8°C. They can reduce air conditioner use by 12-15% which also decreases the carbon emissions from our largely brown coal generated electricity. While it is more difficult to place a value on reduced wind speeds of up to 10% due to the presence of vegetation, we do know that under climate change winds will be stronger and that Victoria has suffered the effects of three major wind storm events over the past few years.

Urban vegetation also has significance in removing atmospheric pollutants. In New York in 1994 it was calculated that the city’s vegetation provided US$10million of benefit in pollution removal. The planting of 11 million trees in the Los Angeles basin saves US$50million per annum on air conditioning bills. Sadly there are few similar studies for Australian cities, but in the only study of its kind, economists noted that an Adelaide street tree provides a minimum annual benefit of $200 per year and that it was a gross under-estimate of the real value.

There is also the role of vegetation under a changed climate in holding and absorbing water during more intense rainfall events. The economic value of reducing localized flooding could be substantial. So if we lose our urban trees and landscapes because we don’t think they are worthy of some of our resources we could pay a very high price indeed. We won’t know what we have lost till it’s gone.

I could go on for pages about the benefits of urban trees and landscapes. I haven’t even mentioned how gardens improve human heath, extend life spans, reduce violence and vandalism, lower blood pressure and save society a truck load on medical and social infrastructure costs. Isn’t it lucky that as we let all the turf in our parks and ovals die that we don’t have a problem with kids lacking exercise and becoming obese. If we did, we might be paying an even higher price for the loss of public and private space than we ever dreamed possible.

Natural turf is quite a complex ecosystem and has a significant effect on temperature and the heat island effect, and if properly managed also sequesters a considerable amount of carbon. Perhaps it is not the villain that many think when they consider only the water component of a more complex equation.

Picture 028Melbourne is one of Victoria’s biodiversity hot spots. The parks, gardens, streets and front and backyards provide a very diverse range of plant species that generate a myriad of habitats and niches for wildlife. High density urban developments and inner city renewal make it virtually impossible to grow trees in places that were once green and leafy. We rarely ever see the real costs of such developments.

In the past decade tree populations in many Australian cities have declined, particularly with the loss of private open space. While costs, and the damage and nuisance values attributed to trees are widely known, the benefits they provide are often subtle and under-appreciated.

Urban vegetation provides economic and ecological services to society. They are assets which warrant the expenditure of resources such as labour, energy and water. Such expenditure is not wasted as trees and urban landscapes provide far more economically and ecologically than they use. In any comprehensive and fair calculation urban trees and landscapes are worth more than they cost.

Article and photos copyright to Greg M. Moore, Senior Research Associate, Burnley College, University of Melbourne