Covering roots...how much, if any, is acceptable?

[Darrel]

We're like to put up a retaining wall in our front yard. We have a 20' Maple tree growing on the top part of our yard. The canopy's edge overhangs the steep, eroding hill that we want to put the retaining wall on.

If we put the retaining wall there, we'll end up covering about 20% of the root zone, and about 15% of the dripline area with soild. The coverage would go from nothing (closer to the tree) to about 2' (towards the wall).

From what I've read online, the general rule is that, at least with Maples, 'coving part of the roots may cause damage'...ie, nothing too specific.

We're in a bit of a bind, as we can either cover the roots and take the risk, or leave the hill, which will just continue to erode away towards the tree.

So, my question, are we risking the tree too much by covering that portion of the roots?

 

[pinus]

The picture can help understanding from the situation. IMO, the partial covering (20%) of roots, if tree is health and the covered area is not close to the tree trunk, probably will not very big danger

 

[treeseer]

pinus is right, the worst danger will be from digging a footing above the wall. Keep the wall above ground, use loose soil and you should be ok covering 20%. post pictures and we can tell you a lot more.

I respect the opinions of posters here at AS who caution against fill, but I respect more what a guy with the initials AS said in A New Tree Biology, p. 93: "Most trees can tolerate some fill up to a foot about their base IF the roots have not been injured first."

 

[Darrel]

Thanks for the comments thus far!

As for a picture...he's a semi-legible one I took this afternoon:

(Hopefully that shows up...if it doesn't, let me know.)

Orange = rough outline of the canopy. It's actually a bit larger than that, as there are a few long branches shooting out over the hill a few more feet (that will likely be pruned a bit).

Purple = crest of the hill

Blue = bad rendering of the wall (solid line) and fill (dashed line).

Where I'm digging is about 15 ft from the tree (about 6' outside the drip line) and I've already hit a few smaller roots if that means anything. The wall will go straight about another foot forward from where I've dug, then arch out in a large radious following the tree, hence that area will be under dirt.

If I do bury that, does it make sense to break up the turf a bit first? Will the roots eventually try to grow towards the surface more?

 

[treeseer]

If I do bury that, does it make sense to break up the turf a bit first? (No I don't see why. best to spray first so it doesn't grow thru.)Will the roots eventually try to grow towards the surface more?

Yes. what will you be planting in the fill? why put the wall midlsope and still be stuck with that mowing chore by the sidewalk? just an idea here, but I would consider just laying down 3 rows of timbers after killing grass and planting flowers and vines above those.

your plan however seems very tree-friendly.

 

[Darrel]

This hill has been a headache. It's about 12' total with a 3' retainig wall at the sidewalk. It faces south, so ends up being a patchwork of burnt grass, dandelions and eroded soil by August, and is not only a pain, but also dangerous to mow.

So, what we're doing is putting another wall midway up the hill, with this 'turret' surrounding the tree. The 'turret' will be level, and likely grass with a border of flowers. The lower level will be mulched/rocked and planted with various non-mowing shrubs/grasses/plants/etc.

So, it sounds like the wall won't do much harm to the tree. Good news!

The bad news...I still gotta put the wall together. ;o)

 

[pinus]

If the blue triangle on pic is the working area then tree should not get serious harm.
Tree itself looks a young and strong, so all probably shold be OK.

 

[Darrel]

I just wanted to thank everyone for the advice. I really appreciate it!

Pinus...the tree is fairly young, and shooting out at a rapid pace (I'm going to have to have it pruned this winter). It has a forked trunk, though, which I wish the previous owners would have taken care of, but it's a good shade tree for the front of our house...and the only one on our property, so I want it to pull through. ;o)

 

[treeseer]

yeah that fork does look bad; post a closer pic of it and the guys here will shoot you some ideas on pruning etc.

 

[fpyontek]

I respect the opinions of posters here at AS who caution against fill, but I respect more what a guy with the initials AS said in A New Tree Biology, p. 93: "Most trees can tolerate some fill up to a foot about their base IF the roots have not been injured first."

From my own experience I know that some trees may tolerate some fill. It depends upon the species, the general health of the tree, and how much fill.

That aside, P.P Pirone in his book "Tree Maintenance" devotes a chapter to non-parasitic injuries. In the first section of chapter 10, "Damage Caused by Grade Changes around Trees" , he details the delicate relationship between the root system and the soil.

I would like to quote Mr Pirone from the fourth paragraph, Why Trees Suffer from Soil Fills.
"... Air (primarily oxygen) and water are essential for normal functioning of roots. Soil microorganisms, necessary to break down organic matter that serves as food for the roots, also need air. In addition, when air is lacking, certain gases and chemicals increase and become toxic to roots. Toxins produced by anaerobic bacteria may be more harmful than the damage resulting from asphyxiation due to lack of oxygen. ..."

I agree with you that some arborists can be over cautious about soil fill. However, another expert opinion (Pirone) couldn't hurt.

Fred

 

[Darrel]

Well, here's a (crazy?) thougt. To be on the safe side, what if, before laying down the additional fill, I lay in a few of the perforated drain tiles (wrapped in landscape fabric) on the top of the current turf, vented out the wall. Would that allow some venting/air? Worth the trouble?

Treeser...thanks for the offer. I'll try and get a pic tomorrow that shows the main fork. You can't actually see it at this angle. I've done some pruning each of the past two years...namely 5 or so bigger branches, but I can't keep up with it's growth, nor the height of a couple of the large shooters, so I'll be bringing in someone more than likely this winter.

 

[treeseer]

what if, before laying down the additional fill, I lay in a few of the perforated drain tiles (wrapped in landscape fabric) on the top of the current turf, vented out the wall. Would that allow some venting/air? Worth the trouble?

Not crazy at all, done all the time. perhaps not necessary here but it could not hurt, and the materials are cheap.

fred, the point Pirone raises are good to consider. i don't see them conflicting with Shigo's; perhaps porous fill allows gas exchange to keep everyone happy down there.

 

[TheTreeSpyder]

i think more properly porous/ larger aggregate cover. i think that mulch feeds etc., but also is a loose breathable layer, that absorbs dynamic motions/shock values and spreads forces from above out to. For less compaction and more breath-ability.

These are the mechanics that the surviving trees that have not been brought to extinction in millions of years accommodate/ expect. These are the mechanics to imitate/duplicate. Also, try not to change drainage, unless it has been bad. Trees are adaptive creatures, but have been adapting to the world as they know it for millions of years, and that tree to that location and drainage etc. for all for it's life. Every second trying to maximize to the conditions. That is why they are the largest, heaviest, longest lived etc. ever!

See also: The Soil Sea & Soil Protection

 

[Kneejerk Bombas]

The tree is young, the area to be filled is only on one side, the fill area is a good distance from the tree, all lead me to believe this tree will be fine with the project.
Going to extraordinary measures to try to keep the roots under the fill alive seems like a waste of time and effort.
If this were a mature tree, then we could talk about ways to maintain the roots and to help the health of the tree with watering, mulching, cambistat, maximizing soil fertility, water, aeration under the fill, moving the wall, and other goodies, but like I say, there isn't a problem as I see it.

 

[treeseer]

Mike I don't think that perforated pipe would be extraordinary; it's cheap and i see no minuses to it.

Here's a link on this topic; pretty conservative approach imo.
http://www.isa-arbor.com/mediasource/resources/3-9-05 LAM Article PDF - Planting Under Trees.pdf