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.
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
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.