Can you plant trees next to retaining walls?
August 13, 2022 11:00 PM   Subscribe

We have a wooden fence in our backyard with a small garden bed next to it which is held up with a ~25 cm thick stone brick and mortar retaining wall. The garden bed is about 1 m measured from the retaining wall to the fence. We want to plant a couple of trees in the garden bed to hopefully provide some privacy, as neighbors are currently able to see over the fence into our backyard. But we want to make sure that the trees don't damage the retaining wall.

I’ve read that trees and retaining walls should not be placed together. On the other hand, someone also told me it’s not a problem, so long as you pick the right tree that does not have invasive roots. We are thinking of putting in an olive tree and a tristaniopsis laurina, "water gum" tree, which apparently do not have invasive roots. I'm hoping this would work. I guess I’m hoping that if there is limited space for the roots to expand, the tree will just stop growing, rather than try to break open the retaining wall, kind of like how potted trees don't damage the pot.
posted by strekker to Home & Garden (12 answers total)
The root system closely duplicates the canopy of a tree, therefore both the roots and the limbs and leaves (and flowers, if any) become the problems of your neighbors.
Either add height to your fence or add a trellis and vines on your property.

A summer option in your zone might be tall annuals like sunflowers or canes.
Be wary of plants that grow out of control or are poisonous to children and animals.
posted by TrishaU at 3:43 AM on August 14 [1 favorite]

Not to threadsit, but --
Check with the city planning division or similar department about the status of this location. It is probably the side yard/backyard setback.
This means that water and sewer lines may be buried there. Utilities can be routed through there. There may be restrictions on what you can build in that area.
Services like internet may bury lines through there.
We've had some city equipment in our backyard working on water lines and tunneling holes under the fences. Luckily we have a gate so no one had to dismantle part of the fences to get the equipment in.
Trees are difficult to work around.
posted by TrishaU at 4:05 AM on August 14 [1 favorite]

Check with the city planning division or similar department about the status of this location. It is probably the side yard/backyard setback.
This means that water and sewer lines may be buried there. Utilities can be routed through there. There may be restrictions on what you can build in that area.
Services like internet may bury lines through there.

In a lot of places, there would be an easement contained within the legal description of the property to allow for utilities to permanently access these features, not just zoning setbacks.
posted by LionIndex at 8:32 AM on August 14 [2 favorites]

My question would be Who's the expert? Check to see if your town has an arborist, and call a good landscaper. I'd consider lilacs, which get tall, but I'm in New England, where they thrive. (unless the %^&?%!! mice eat the roots of your seedlings, grar.)
posted by theora55 at 8:55 AM on August 14 [1 favorite]

I have no expertise about tree roots. However, arborvitae make a nice screen. They don't get too big, low maintenance, no dropped leaves. There are short and tall varieties.
posted by H21 at 9:57 AM on August 14

I have planted trees near walls like this--some are okay and don't have especially vigorous roots, some I really wish I hadn't planted there and knowing more about gardening now, regret my choices, especially a Russian olive, which I thought was far enough away but which turned into a beast. There are a lot of Japanese maples that can stay smallish, depending on where you live.

What zone are you in, though? There might be some plants that are kind of on the trees/shrubs line, where they get tall and can be shaped like trees (like my camellias, which were pruned into kind of lollipop shapes when I first moved in, or strawberry bushes) or the aforementioned arbor vitae. Also, if you're good with digging, you can install bamboo barrier (this requires some digging down a bit, so it's work) and plant some bamboo--I made a screen for my fence with black bamboo, and it's really lovely in a breeze and gives off a soft scent that you can smell once in a while.
posted by kitten kaboodle at 12:35 PM on August 14

What are your soils like? If clayey/wet, especially at depth plants are going to surface root and increase chance of interacting with objects. How deep is the wall foundation?

Where in the world are you as that would help answer this. The taproot thing is a bit of a myth as all trees / large shrubs also make side roots, it's a matter of finding the best one for where you are.

Is there lawn next to the garden?, if yes then the tree will be able to root under that.

I sometimes use a root barrier (but only if there's no other way). But they do require digging a deep trench. Do not let anyone sell you a chemical root barrier if you go this route.
posted by unearthed at 2:19 PM on August 14

H21 has the best option. Arborvitae.

A tree that close to the wall will eventually wreck the wall. Especially with a species whose roots run close to the surface. Arborvitae, otoh, make wonderful privacy walls. We planted a long row of them in the backyard along the side where a busy street ran. In a year of two, you would be hard-pressed to know the street was there. You couldn’t see it and you barely heard it.
posted by Thorzdad at 2:30 PM on August 14

You want a tree with a tap root, i.e. goes down, doesn't spread. Think carrot. ( a carrot. Not a carrot tree. )

You probably know to avoid things like willows and maples (shallow, spreading roots). What you do get is dependent on where you live and what you like.

Grasses are also an option if you're in an area with winter -- you can leave them up all winter long and they're pretty and many have fibrous roots that grab soil like it's held in fists.

Avoid anything with spreading rhizomes like spreading bamboo.

Dense stuff that looks good in winter but gets coppiced in spring (cut down to knee level) like red twig dogwoods might be nice, depending on the density of screening you would like.

If you give more info about where you live (max and min temp) you might get more specific suggestions.
posted by A Terrible Llama at 2:57 PM on August 14

Try rectangular containers with bamboo?
posted by Hypatia at 5:41 PM on August 14

Response by poster: Wow, thanks for all the feedback.

Apologies, probably should’ve mentioned the location, but we are in Sydney, Australia, which I looked up and is in zone 10.

This is a very good point about the utilities. The other side of the fence is a road, and I know for fact that the Internet enters the property at some point underneath the fence (because the contractors who put up the retaining wall accidentally damaged the Internet connection). So, that’s probably a vote against putting in proper trees.

However, maybe some of the tree alternatives would work. I like the idea of the sunflowers or non-spreading bamboo. I also like camellias (something with a bit of colour would be nice), although I still wonder with all the things whether roots will be an issue.

@unearthed, on the side with the retaining wall, there are pavers. I’m not sure how deep the retaining wall goes, although it is about 1 m high. I don't really know my soil types, but I imagine we could swap it if needed.
posted by strekker at 6:30 PM on August 14

How about palm trees? They have really tight root footprints.
posted by The_Vegetables at 7:28 AM on August 15

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