Why Beautiful Trees Can Become a Hidden Retaining Wall Risk

Tree roots and retaining walls can coexist without any obvious problems for years. I know because I’ve got exactly that situation at my own house.

It’s a 1.6 metre concrete sleeper retaining wall, and along the high side we’ve got a row of beautiful Foxtail Palms.

When they were planted, nothing seemed wrong. The wall worked exactly as it should, the palms looked great and the landscaping made sense.

Years later, those palms are now around eight metres tall.

More importantly, the retaining wall is beginning to tell us something.

A few of the sleepers are starting to bow.

I love the palms and would much rather keep them. However, we’re getting to the point where we’ll eventually need to make a decision. Either something changes with the trees, or we risk allowing a relatively manageable problem to become a much bigger one.

It’s a good reminder of something homeowners can easily overlook when planning landscaping around retaining walls.

The tree you plant today is not the tree you will have in ten or twenty years.

A retaining wall might appear permanent, but everything around it continues to change.

Trees Do Not Stay the Size You Plant Them

When you buy a tree from a nursery, naturally you think about how it fits into the property today.

Does it provide screening?

Will it soften the retaining wall?

Does it suit the garden?

Will it create some privacy?

Those are sensible questions. However, there is another one worth asking before the tree goes into the ground.

What will this tree become?

A relatively small tree sitting comfortably beside a retaining wall can eventually develop a substantial trunk, canopy and root system.

Meanwhile, the retaining wall remains in roughly the same place.

The tree does not.

That is essentially what has happened at my house. I do not think someone made a catastrophic landscaping mistake when those palms were planted. At the time, the relationship between the wall and the trees worked.

The property simply evolved.

That distinction matters because good landscaping needs to consider more than whether something fits today. It should also consider whether the landscape will continue to work as trees mature and the conditions around permanent structures change.

How Tree Roots and Retaining Walls Can Come Into Conflict

Tree roots can contribute to retaining wall movement and damage.

However, the relationship between tree roots and retaining walls is rarely as simple as imagining one giant root pushing against a wall until something breaks.

Large roots increase in diameter as trees mature. We’ve all seen established trees lift footpaths, paving or sections of concrete when the tree and the structure begin competing for the same space.

Retaining walls can experience similar pressures.

Roots are only part of the picture though.

Trees also influence the soil around them. They draw moisture from the ground, while their roots explore areas where growing conditions are favourable. As those conditions change, the surrounding soil can respond as well.

Drainage around the retaining wall may also behave differently over time.

All of this can happen slowly enough that nobody notices anything unusual for years.

That’s why I would never look at a moving retaining wall with a large tree beside it and immediately say:

“There’s your problem.”

The tree may be contributing.

It might even be a major factor.

But it is not necessarily the whole story.

Do Not Blame the Tree Just Because You Can See It

This is where diagnosis matters.

When a retaining wall that has performed well for years begins moving, one of the first questions should be:

What has changed?

Perhaps a nearby tree has grown significantly.

That’s one clue.

Then we need to look beyond the obvious.

Has drainage around the wall changed?

Is water reaching the area differently?

Has landscaping been added above the wall?

Has extra soil been placed behind it?

Is there now a fence, shed, driveway or other load near the top of the wall?

Has erosion occurred?

Could ground movement be contributing?

Was the original wall constructed appropriately for the conditions?

Often, wall movement is not caused by one dramatic event. Several smaller influences can develop over time until the wall finally begins showing visible signs of stress.

The easiest explanation is not always the correct explanation.

That principle applies to plenty of concrete and retaining wall problems.

Do not fix the symptom until you have a reasonable understanding of the cause.

Otherwise, you can spend a lot of money solving the wrong problem while the actual issue remains.

What to Watch Around Tree Roots and Retaining Walls

Homeowners do not need to become engineers to notice when something around their property is changing.

In fact, you often have an advantage because you see the property every day.

You know whether a wall has always had a slight lean.

You remember whether that section of paving has always sat at that level.

Perhaps water has always appeared in one particular area during heavy rain.

What becomes important is when something starts behaving differently.

A sleeper that has stayed in essentially the same position for fifteen years is not particularly interesting.

One that has recently started moving deserves attention.

Warning signs can include:

• Posts beginning to lean or shift
• Sleepers moving out of alignment
• Concrete or paving lifting nearby
• Soil beginning to mound or change level
• Large roots becoming visible
• A fence above the wall starting to move
• Water appearing somewhere it did not previously
• One section of the wall behaving differently from the rest

None of these signs proves there is a problem between tree roots and retaining walls.

Think of them as clues.

Taken together, those clues can help reveal what is changing before relatively small movement develops into a much more difficult problem.

Water Still Matters

Even when a conversation starts with tree roots, we eventually end up talking about water.

There is a good reason for that.

A retaining wall does more than hold soil in place. It also needs a strategy for managing water behind the wall.

Without adequate drainage, water can accumulate in the retained soil and increase the pressure acting against the structure.

Then the landscape begins to mature.

Roots spread through the surrounding ground searching for moisture, oxygen and suitable growing conditions.

An area that was completely clear when a retaining wall was constructed may look very different underground ten or fifteen years later.

That does not mean a root entering a drainage area automatically causes the wall to fail.

It simply tells us that the environment around the wall has changed.

When tree roots and retaining walls have existed beside each other for many years without trouble and movement suddenly appears, that change in the surrounding environment deserves attention.

The useful question is not only what you can see now.

It’s what has changed since the wall was originally built.

Do Not Immediately Reach for the Chainsaw

When a large tree sits beside a moving retaining wall, removing the tree can feel like the obvious solution.

Sometimes it may ultimately be the right decision.

That could eventually be the decision I make at my own house.

However, cutting major roots or removing a mature tree should not be the automatic response simply because a wall has moved.

Large structural roots can play an important role in the health and stability of a tree. Cutting them without understanding the consequences can create an entirely different problem.

Removing the tree also does not magically return the ground to the condition it was in twenty years ago.

The roots remain underground.

Eventually, they decay.

Meanwhile, the tree stops drawing moisture from the surrounding ground.

Once again, the underground environment changes.

Where substantial established trees sit close to retaining walls, homes or other structures, getting advice from an appropriately qualified arborist can make sense before making an irreversible decision.

The objective should not be to save one structure by creating another problem.

It should be to understand how the property is functioning as a whole.

Planning Tree Roots and Retaining Walls Before There Is a Problem

Homeowners planning a new landscape have a major advantage.

They can think about tree roots and retaining walls before the trees have matured and before there is any visible movement.

Instead of looking only at the two metre tree sitting in a nursery pot, find out what you are actually planting.

How tall is it likely to become?

How wide will the canopy grow?

What are its root characteristics?

Where is the retaining wall?

Consider the driveway, drainage, pool, fencing and underground services as well.

Neighbouring properties matter too, particularly when mature trees may eventually extend well beyond the space they originally occupied.

The useful question is not:

“Will this tree fit here?”

Ask instead:

“Will the mature version of this tree still make sense here?”

Those are very different questions.

One deals with how the property looks today.

The other deals with how well it will function ten or twenty years from now.

Sometimes the Best Landscaping Decisions Take Years to Prove Themselves

This is something we think about more as outdoor projects become increasingly integrated.

A beautiful result on handover day is one thing.

A property that becomes better as it matures is something else entirely.

Trees are a perfect example.

Good planting can completely transform a home by creating shade, privacy, scale, character and habitat.

Mature landscaping also gives a property a feeling that simply cannot be replicated when everything is new.

So this is not an argument against planting trees around retaining walls.

Quite the opposite.

The goal is to give them enough thought and appropriate space that they can mature without unnecessarily competing with everything constructed around them.

A well considered landscape makes room for both the structures that are built today and the plants that will continue growing for decades.

That is the difference between landscaping purely for today and planning for the property you hope to have years from now.

A Retaining Wall Is Part of a Living Property

Concrete feels permanent.

Steel feels permanent too.

A retaining wall can sit there for decades looking almost unchanged.

The land around it is another story.

Trees mature. Roots spread. Soil moisture changes. Water finds different paths. Gardens become established.

Homeowners change their properties as well.

A pool might go in.

Someone builds a shed.

A fence gets replaced.

New paving or a driveway alters how water travels.

Additional soil might be placed behind a wall to create a flatter garden.

Every one of those changes can alter the environment in which the retaining wall operates.

A wall that was perfectly suited to the property fifteen years ago may eventually find itself dealing with conditions that did not exist when it was built.

That does not automatically mean somebody made a mistake.

Sometimes the property simply changed.

What matters is recognising those changes early and understanding whether they are beginning to affect the wall.

At my own house, that may eventually mean saying goodbye to Foxtail Palms I would much rather keep.

I’d still prefer to make that decision deliberately than wait until the retaining wall makes it for me.

Frequently Asked Questions

Can tree roots damage a retaining wall?

Yes. Tree roots can contribute to retaining wall movement or damage, particularly when substantial roots develop close to the structure.

However, drainage problems, soil movement, erosion, additional loads and the original construction of the wall can also contribute. A nearby tree should not automatically be assumed to be the sole cause.

How can I tell if tree roots are affecting my retaining wall?

Look for changes rather than relying on a single symptom.

New leaning, displaced sleepers, lifting paving, visible large roots, changing soil levels and localised wall movement near an established tree can all justify further investigation.

None of these signs proves root damage on its own.

Should I cut roots growing near my retaining wall?

Not without understanding what you are cutting.

Significant roots can be important to the health and stability of a mature tree. If substantial root removal is being considered, advice from a suitably qualified arborist may be appropriate.

Will removing a tree fix a leaning retaining wall?

Not necessarily.

Removing the tree will not reverse movement that has already occurred, while drainage, soil conditions or other contributing factors may still need attention.

Existing roots also remain underground and gradually decay after the tree is removed.

How far should I plant a tree from a retaining wall?

There is no single distance that suits every tree or retaining wall.

Mature tree size, root behaviour, soil conditions, drainage, wall construction and surrounding structures all matter.

The better approach is to plan around the mature size and characteristics of the species rather than the size of the tree when you purchase it.

Can tree roots and retaining walls coexist safely?

Yes. Tree roots and retaining walls can coexist for many years without creating an obvious problem.

What matters is the mature size and behaviour of the tree, the space available, the condition and design of the wall, drainage and how the surrounding property changes over time.

Should I worry about a tree beside a retaining wall that has never moved?

Not necessarily.

A mature tree beside a retaining wall does not automatically mean there is a problem. What matters is whether the wall, surrounding ground, drainage or nearby structures begin showing new movement or other changes.

Monitoring those changes can help identify a developing problem earlier.

Final Thoughts

The relationship between tree roots and retaining walls deserves more thought than simply asking whether a tree is planted close to a wall.

If you have a mature tree beside a retaining wall that has started moving, do not automatically blame the tree.

Do not ignore it either.

Start by looking at what has changed.

Watch the wall and surrounding ground.

Pay attention to where water travels during heavy rain.

Look for movement in sleepers, posts, paving or fencing.

Then consider how the landscaping has evolved since the wall was originally built.

If you are designing a new landscape, take that same thinking in the opposite direction.

Do not plan only around the plants going into the ground today.

Plan around what those plants will become.

Good landscaping asks:

Will this look beautiful today?

Great landscaping also asks:

What will this decision become in ten or twenty years?

That is the property worth planning for.

Have a question or a topic you’d like us to cover? Email us at advice@conker.com.au.