Of dirt and roots

[Norm G]

Seriously, has all the hype about killing trees by covering the roots with more soil and mulch been overkill (pardon the pun)? We have some mature, very tall Douglas fir in our yard. Some trees over the years have died and fallen. Some erosion of soil and time has exposed some roots. But roots run very deep and very wide. Nearby to the area we want to berm, we have a flower bed around a giant western cedar. There are masses of fine roots all over, and every year we haul in more compost and topsoil (just a few inches) for the beds. The tree roots very quickly grow through it.

Can building a berm that is about 3' to 4' wide and 1' to 3' high (low end to high end) that curves through some of the Douglas Fir trees (not up against them or surrounding them like a moat - rather snaking through them) seriously kill the trees? Really?

I am being old by one arborist I will kill the trees for sure. Another is saying they should be o.k. And God help the fool (like me) trying to find non-hearsay, validated, and practical information on the internet!! I even got conflicting information from the county extension office. SO FRUSTRATED!!

Seriously, the trunks of these trees range from 9" to 18" diameters, they are 40' or more in height. Our climate is very rainy (north of Seattle). Soil is acidic, clay and rock. The total length of the berm we want to make would be about 25' long. We would plant shade tolerant shrubs and ferns on it, and cover with layer of wood chips. A few well placed two-man rocks. A wood chip path would follow the curve of the berm around one side of the berm for light and occasional (family) traffic. We have dogs and grandkids that would love to run up and down it, too, we are sure.

The reason for the berm is to create a better growing area for some appropriate plants, to cover a mass and mess of roots, rocks, and uneven walking area, and to improve aesthetic appeal.

Please: would like to hear from an arborist with experience building berms around mature trees.

 

[Raintree]

If you must, any detrimental effects should be minimal.

 

[Norm G]

Thanks, Raintree. I appreciate your reading through my post and responding. But I am trying to verify whether the berm(s) would be detrimental or not, and why or why not...and not just some cut and paste wisdom from journal articles some "author" wrote for credit or a gardener quoting wisdoms from Mother Earth. Can you tell me when/where/how of berms and trees based on your own landscaping and arborist experience? Please?

 

[Raintree]

Factoring in the size & type of landscaped berm to the root zone area adversely impacted. Also taking into consideration the species and age of the trees. It is my opinion & experience that your project will have minimal negative impact.

 

[Norm G]

Factoring in the size & type of landscaped berm to the root zone area adversely impacted. Also taking into consideration the species and age of the trees. It is my opinion & experience that your project will have minimal negative impact.

Thanks! That's exactly what I needed to know!

Mill

 

[ATH]

Just FYI (since you asked) on the "WHY" soil over roots damage trees:

Oxygen.

The roots need oxygen to come into the soil for the process of respiration. The looser the soil, the more oxygen that will get through. The more soil on top of the roots, the less oxygen. Tree root depth is limited by oxygen availability - in other words, roots will go as deep as they can until they can no longer "breath". You may kill some of the deeper roots by putting soil over the root system. If it is healthy enough, the soil is loose enough, and enough roots remain in tact to help the tree recover from those lost roots, you will be OK. If you put 2' of heavily compacted clay over the entire root system, not much fresh air makes it down there and you are in for trouble.

 

[sgreanbeans]

The roots will rise to the berm. After a few years you will be able to find them right under the surface of the new dirt. Like they said, this is only if the soil is loose and not compacted. I have seen where a builder pushed a bunch of soil 4ft above grade at a new home lot. Then compacted it via bulldozer. Trees didnt last to long. Seen another guy raise a bed 2ft around a few maples and contain the soil with wall stones. Used a mixture of mulch and soil. The beds were engulfed in roots soon after. Its all conditional.

 

[hardpan]

ATH, is a couple inches of good organic soil added over the bare roots better or worse than undisturbed bare roots? Maybe more protection but less oxygen available?

Sgreanbeans, are you saying 2' of mixed soil and mulch against the trunk is generally OK? How about covering more of the root system with the same material? I've seen many nice yard trees killed when people cover the roots BUT they almost always use junk, fill material. We have a lot of clay here.

 

[ATH]

When you say "bare roots" - has some washed away to expose the roots? Or the roots are at the surface? If the have been exposed, you do need to cover them. If they roots are at the surface, that is because (in most cases, with most species...there are always exceptions) there is not O2 available for them to go deeper...so be careful about making that problem worse. To answer your question "is a couple inches of good organic soil added over the bare roots better or worse than undisturbed bare roots?" Adding a couple of inches of organic matter is probably a good thing. That will break down and improve the soil structure and chemistry over time. Refresh that periodically, depending on what you use. If you use mulch, that probably means a couple of inches added every 3 years. If you use compost you can probably add 2 inches every year or two. There are really 2 reasons why this is better than adding soil: first, it breaks down, so it is not a net addition of 2 inches every couple of years. Secondly, very slowing building up the soil means the tree has time to adapt. I would always avoid putting anything against the trunk - especially if it is a younger tree. Search "stem girdling roots" to read about one problem associated with soil or mulch piled against the trunk. I always tell people "think donut, not volcano" when picture mulch around the base of the tree.

 

[sgreanbeans]

Sgreanbeans, are you saying 2' of mixed soil and mulch against the trunk is generally OK? How about covering more of the root system with the same material? I've seen many nice yard trees killed when people cover the roots BUT they almost always use junk, fill material. We have a lot of clay here.

No, not at all. I was saying how some trees react to this. Silver maples often do this, where as a oak, most of the time, will not. The one where the roots engulfed the planter bed was a removal. In that situation, too much moisture against the trunk lead to extensive decay. The root reaction was a survival tactic. People often try to improve their curb appeal by installing a retaining wall stone circle around the tree, covering the unsightly roots and bare areas of grass. They fill it a couple ft deep with poor soil, pack it in and plant some flowers around it. Soon after, the tree is in decline, you may not see it at first, but it is hurting the tree in that spot. Little to no O2 and H20 available, keeps the trunk wet and primes the area for a assault from the bad guys. Sometimes, after a few years, you will see more root growth coming up outside the circle, pushing the stones around. I get called to root prune. Instead I tell them to remove the planter and leave the root alone. Like ATH said, just a few inches, no more than 4 over the area. Keep it away from the trunk. Mulch and compost are great. I like to put compost or Iowa grade top soil down first, just a inch or so, then top dress with a natural mulch. The bag mulch that ya get at the big box stores has a lot of preservatives in it and prevents it from breaking down. Ya end up with "waffles" and all kinds of molds and fungi growing out of it. Ya want to make sure that your not getting rail road ties and pallets that have been ground up and dyed.

 

[hardpan]

Thanks guys. Your contribution to mine, and I hope others, understanding of the delicate balance that exists with root depth and oxygen and water and different soil types. Where I live most of the high ground typically has 6 to 8 inches of topsoil, then hard yellow or orange clay, then rashy rock, then sandstone, etc. The tree roots often can't penetrate the clay so they stay shallow. Even if they could break through the clay they would be isolated from the oxygen and water and nutrients needed. With a little erosion the roots are bare so people add dirt, poor dirt, too much clay. I thought the roots were mostly cut off from water (of course water is one part oxygen) so for many years I cautioned people from adding a lift of dirt over the roots. One in particular was a neighbor building a new house about 10 years ago. To build his driveway he added about a foot and a half of dirt over the roots of a big, beautiful maple tree. I stopped by and told him he would probably kill the tree. He said he sure hoped not as he was relying on it to shade the whole side of his house and parking area. 3 years later he payed big to have the dead tree removed. What a waste.
Thanks again.

 

[treeseer]

Great advice so far. Hardpan can be broken with a blunt probe and pressurized water and/or air.

 

[hardpan]

I'm guessing this procedure is repeated within the dripline. What pattern/spacing?

 

[treeseer]

Away from scaffold roots, spacing 1-2' depending, often done beyond dripline too, wherever drainage is poor.

 

[imagineero]

Oxygen and water is one thing, but there's a whole lotta stuff going on in the top soil besides that, and they call it top soil because it's near the top. Generally the top 2"-6" in most cases. There's a reason why the roots keep going back to the top (assuming it's not erosion) and that's because it's the place they want to be. There's all kinda things going on in the top soil, it's a complex interaction between oxygen, minerals, insects and bacteria/fungi that still hasn't been fully sorted by top scientists in terms of the synergistic relationships. You can try and keep on burying them and it'll work for a while and maybe even a medium while, but you'll eventually end up with root rot, decay, and eventual death of the tree.

I'd suggest you work with what you got. There are plenty of planting options for planting around what you've got, be it ground cover, low spreaders, shrubs, or even moss. If you're chasing a specific visual outcome, talk to some local nurseries and gardeners, look around at what others are doing and get some ideas. Sure beats pushing **** uphill.