1

Location is Portland, Oregon.

TL;DR I want to plant shrubs and shade tolerant ornamentals under trees in one corner of my lot. The soil is clayey and very compacted, and due to long-term slow erosion the tops of some of the tree roots are partially exposed.

Question: How to amend the soil around the trees to allow planting, is compost or mulch the right way to go? What should I be considering?

Longer Version

One corner of my lot has several trees, as shown here. The triangular area is about 45x55 feet, about 1240sf.

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There are two 80-100' tall evergreens that I think are some form of cypress (the large trunks in the middle). The two on the left and right are maples, and then there are the two double trunks that I believe are firs.

That corner of the lot is higher than the rest by maybe 2 feet, and it looks like over the years erosion has exposed quite a few roots. The soil is clayey and very compacted and hard even after rain.

Here are some closeups of the exposed roots.

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Question: What needs to be done to the soil to

  1. protect the exposed roots; and
  2. make it easier to work the soil to plant other plants
2

This can happen over time with mature trees - some varieties are more prone than others to this, and it's not always due to soil erosion. You describe the soil as clay and compacted, so it's possible these roots are visible above ground in this way to get more oxygen, meaning it's not a good idea to thickly cover them up.

If you want to cover them, start with a thin layer of good topsoil and organic material such as composted garden material, then adding another thin layer a year later if necessary. This inevitably means you won't be able to plant most of that area - the visible roots are not the only ones there, there will be many more beneath the soil, making digging and planting difficult; this will also make it less likely any plants you do put in won't survive, since they will be competing with the trees for root room, nutrients and water,and are unlikely to win that uneven contest.

Further information here https://www.thespruce.com/will-soil-over-tree-roots-hurt-tree-2132816

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First of all, you should order a few bags of wine cap, then spread it around the trees (under the wood chips).

Then I would lay wood chips about a foot thick around the trees with either a cardboard barrier, or plastic barrier about 1 foot away from the base of the tree as it will damage the tree if it's close to the tree (kill them slowly).

  • Could you explain your reasoning further? Why winecap mushrooms? Why wood chips as opposed to, say, compost? Also, 1 foot over 1240sf is 46 cubic yards, a rather expensive proposition. Do I really need 1 foot? – Jim Garrison May 20 at 22:33
  • wine cap mushrooms break down the wood chips well, are mycorrhizal, and provide you some great food. – black thumb May 21 at 3:58
  • 2
    A layer of chips a foot thick around the tree will kill the trees slowly by preventing air exchange from the air to the roots. – kevinsky May 21 at 12:37
  • that's why I said near, not on – black thumb May 21 at 15:08

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