Question

Tree base  

Tree canopy   Hello Jim. I have a black walnut that my wife and I are extremely fond of and would like to have a cement patio poured near the tree in order to take advantage of the shade and feel the tree lends to the surrounding area. My concern is that the cement contractor tells me that they must excavate to a depth of 3" to 5" in order to install the base material for the patio to poured onto. My question is, what distance away from the base of the trunk would be a safe to excavate to a depth of 3" to 5" without risking the health of the tree. Also, ideally we would like wrap around half of the tree, while keeping the specified distance away. Will this negatively affect the tree from being unable to harvest water from the ground that will now be covered with concrete? Will the loss of the 700 sqft or so of the patio be enough to affect the tree? The dimensions of the tree are:
Trunk at base 28" dia., Approx height 40-50', Approx canopy dia. at widest point 40'.

Thanks so much for your help. Take care.

-Mark H

Answer

Yes it will damage the roots of the tree. To what extent will depend on the amount of soil that is moved when grading and the percent of the root system that is damaged. The tree can do fine with as much as 25 percent of the root system damaged without much problem to the the foliage. When roots are damaged he tree can not produce enough food and water to support the total foliage and some foliage will die back. The roots are within the first two feet of the soil surface and extend out about 1 1/2 times the width of the foliage. You can figure the percent damage by out lining the roots width and deleting area that will be damaged. IF this is more than 30 percent the tree will exhibit a good bit of foliage damage and die back.

I can already see a few surface roots and any digging will damage the root system. Pavers can work as long as you do not add more than a couple of inches of sand over the roots.Cement will with any digging will damage the trees root system and in turn the foliage will dieback. Also the cement can change the pH of the soil and cause foliage damage. I would think about a wooden patio. Place the runners on pavers and level it before making the patio. Use treated lumber and leave small gaps between the boards for drainage so the root will get the rain fall. Do not come any closer than about 3 feet from the trunk this will leave space for growth and not damage the root collar. Also cement and pavers will be lifted by the roots over time. The wooden should not have this problem. Make the wooden  patio as         closet to the ground as you can 2X4 laid flat will give only 2 inch step up.

IF you dig 3-5 inches of soil from around the tree you will damage the root system. IF more than 25 % of the root area is damaged you will see dieback of tree foliage and limbs. Not a good idea.