Question
QUESTION: Hi Jim,
I have a gully next to my yard and it are about six mature Douglas Fir trees - I want to fill in the gully  level to my existing lawn extending my yard and save the trees.  I know I can't fill in around them with the dirt or I will kill them.  I am wondering if I fill in around them with large rocks and then fill on the outside of the rocks with the dirt if they would be okay?  It is expensive to do it this way so I hate to spend that kind of money and end up losing my trees - they are probably fifty years plus as they were mature when I moved there thirty years ago.
I am looking to fill in on the one side of the trees probably minimum of six feet - not as high on the lower ones on the down side but the closest to the existing yard are definately a lot of fill.
Any suggestions would be appreciated.
thanks

ANSWER: Fill dirt frequently is added around existing mature trees so that a level or more visually desirable lawn can be established. Fill dirt changes the ratio of oxygen to carbon dioxide around tree roots and the roots may subsequently die if too much is added.

In general, roots grow where the resources of life (water, oxygen, and mineral nutrients) are available. They usually will not grow where there is no oxygen or where the soil is compacted and hard to penetrate. This need for oxygen explains why a majority of tree roots are located in the top 12 to 18 inches of soil. Root systems are also extensive. They often extend outward from the tree trunk to occupy an irregularly shaped area 1 1/2 to two times larger than the crown (branch) spread. It is easy to see why any type of soil disturbance near trees can, and usually does cause damage. As trees mature in the landscape they attain a rather delicate balance with their surrounding environment. In fact, trees grow best in an environment of minimal change.

Soil additions reduce the oxygen supply to roots, compact the soil, and often raise the water table. Soil additions six inches or less will probably not harm "fill-tolerant" trees  especially if the fill material is good topsoil, high in organic matter and loamy in texture. But, irreparable damage will result if as little as two inches of clay soils are used as fill, particularly around "fill-intolerant" trees (oaks fit into this category of fill-intolerant trees).  Never pile the fill up on the trunk of the tree.

Now you can add up to two inches of sand over the roots without causing any damage to the tree. Do not compact the sand and DO NOT fill within a foot of the trunk.

That is a long answer but the short one would be no the fill will damage the roots and kill the trees. Rocks will do the same. Sorry!! Not a good idea.

---------- FOLLOW-UP ----------

QUESTION: Thank you for your answer - I am wondering if I change my plan and do a large rock retaining wall.  It would be only on the backside of the trees and about a foot from the tree - would there be enough distance and then maybe about six inches of gravel over the front side so I could level the ground to the gravel level around the front?  I could keep that a foot to a foot and a half from the tree's base.  Maybe on the edge of the gravel around the front, I could then do large rocks to be able to fill into the rocks from the front to level as much as can for pasture and parking? Does that sound safe enough or am I pushing it?  Because the project is expensive, when equiment is involved, I hate to spend the extra money saving the trees only to end up killing them
thank you

Answer
If 1/3 or more of the roots of a tree are damaged then you will get dieback at the least and possibly death of the tree. The roots spread out about 1 1/2 times the width of the foliage and building a wall a foot or so from the tree will damage about 1/2 the roots. Not to mention the machinery driven over the other root area. I would say if you did what you are suggesting the trees will die form the compaction of the roots either from the weight of the rooks and/or the machinery damage. Parking on the rocks under the trees will add to the compaction damage. Based on what I am thinking you are saying it is not a good idea if you want the trees to survive. Sorry!!