Cover roots of Norway Spruce?

[mrmackie305]

I have some large Norway Spruces (pictures here) on the property we recently purchased. We're adding an addition and have tons of clean top soil coming out of the ground. I was hoping when we are finished with the project to regrade around the house and in part add a couple of inches of dirt over the existing lawn to cover all the roots and plant with new grass. Is this okay to do? Thanks!

 

[Harmon]

What you want to avoid is piling up soil or dirt around the trunks of the trees. Where the trunk meets the roots is something called the root collar - and if you bury it in dirt and it cannot get air the tree will gradually decline in health. Cover up roots sticking out of the ground - probably ok. Keep in mind spruce trees tend to generate their own environment and if they are in a group and well established the cast off needles tend to acidify the soil which is good for baby spruce trees but makes it tough for many other plants, such as grass. On the up side it can minimize the chore of mowing the go***mn grass.

 

[ATH]

About 90% of tree roots are in the top 18" of soil.

Tree roots will go as deep as they can go and still find oxygen in the soil (coming from the air above).

So if you put 4-5" of soil over the roots, you are probably killing the bottom 4-5" of the roots...IF they were evenly distributed in the soil profile (they are not), you'd be killing about 20% of the roots.

Assuming the soil that you are putting on top will be worked over pretty well with equipment to make it smooth, you are compacting the new soil more than the existing soil. That means air and water will have a more difficult time getting through.

Also, the soil you are digging out probably isn't really top soil - is it? Sure, the top few inches is, but below that is no longer top soil ... just dirt with a lot less micro-biotic activity.

Those last 2 lines mean you are probably killing well more than 20% of the trees' roots.

Will the trees recover? Maybe. Maybe not. How healthy are they to start with. Will you see a drought in the next couple of years - because that can be another major factor leading to a more rapid decline? How about a period of extended saturation? - that is as bad, or worse, than drought. These are things a healthy tree may recover from. A tree that just lost 40% of its roots probably won't.

If there are a few roots sticking above ground and you just level the ground around those - not covering wide areas, that is different than 4" of soil over the entire root system.