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I have a very large Cotton Wood tree in a low area (3-4 feet lower than surrounding ground)behind my house. I want to fill in that area but keep the tree. How far from the tree trunk should a retaining wall be? I want to build a box retaining wall around the tree to raise ground level. Someone told me that if the wall is too close it will kill the tree.

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  • add pictures please
    – black thumb
    Jul 7, 2018 at 19:01

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Trees like the soil level just the way it is. Contrary to some thoughts most of the "breathing" goes on within six inches of the surface out to the drip line or farther for most trees.

The most non invasive approach that will do the minimum harm to the tree is to build a deck or platform on deck blocks over the low area. Do not:

  • raise the soil level with soil, mulch or rocks
  • excavate footings anywhere under the drip line
  • change the drainage patterns
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    That 'breathing' is why 95% of all roots are found within 4 to 6 inches of the surface. I like that word, Kevinsky. I've seen people bury the roots of trees and insert pvc pipe into the soil to get air and water to the poor roots but I can't imagine it worked very well.
    – stormy
    Jul 7, 2018 at 20:55
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    To reiterate what Kevinsky said: Do Not Mess With The Tree. I was taught that the feeder roots go 50% beyond the dripline, which makes his points even more restrictive - no trenching, digging, or raising the soil level under the dripline + 50%. If you were to change the grade/increase the soil level, the tree would die slowly, and then, being a cottonwood, fall down in a storm.
    – Jurp
    Jul 7, 2018 at 23:04
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You really need to send a picture. You need the wall to extend all the way out to the drip line of your tree. 3-4 feet of soil will kill the roots below.

Cottonwood is also a tree with weak wood and if you bury and kill too many of the roots it would weaken the integrity of your tree.

If you send some pictures we might be able to give you a better idea to protect the tree and landscape around it. Landscapes that are different levels are far more desirable than one flat level. Draw a sketch of your ideas and send with pictures?

Check your covenants or regulations. Retaining walls can be no more than 4 feet high (without engineering) and come with specifications such as footing drains behind the wall, surcharge slope. Include a rough sketch of this box retaining wall? Type of construction.

Where is it that you live? Zone? Is there a stream nearby? Drainage would have to be addressed so your tree doesn't end up sitting in a swimming pool.

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  • Not worried about zoning, we live in the country. With the situation the way it is, the tree sits in a "swimming pool" now, when we get a lot of rain. The tree is surrounded by higher ground on all four sides. We're also talking about a tree that has a 4 ft trunk up to a height of 25 ft and is 70-80 feet tall, with a drip line of 50 ft.
    – Nate Eller
    Jul 9, 2018 at 1:21
  • Is there a river nearby, a canal? High water table? A 4 foot in diameter trunk? Another point is Cottonwood is short lived. This guy is huge and probably very old. And I know the county ordinance cops are alive and well. If you would please send a picture we could at least feel we led you in the right direction. I would do everything to save that tree. Perhaps there are temporary measures, a more stepped program to keep the tree yet keep moving in the direction you want. This tree must be very healthy and very old...this tree has survived all those years...a wise tree, grins!
    – stormy
    Jul 9, 2018 at 6:08

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