Tree roots above ground: cover them, cut them, or leave them?

Why tree roots grow above ground

Most of a tree’s roots live near the surface anyway. Oregon State University Extension says most are in the top 18 inches of soil, and ISA puts the fine, water-absorbing roots of both trees and grass mostly in the top 6 inches. Where the soil over them is thin, part of that system shows.

Soil decides how high they ride. ISA says surface rooting is most common where topsoil is shallow or the soil is clay; Alabama Extension calls exposed roots a common problem for mature trees in compacted urban soil; and the USDA Forest Service lists shallow soil, compacted clay and saturated soil as conditions that restrict root systems.

Species matters too. Several of the region’s common big trees root shallowly, especially on wet ground:

Source: USDA Forest Service, Silvics of North America species pages for Acer macrophyllum, Tsuga heterophylla, Thuja plicata, Pseudotsuga menziesii and Alnus rubra, checked 2026-10-02.

The site’s species page covers bigleaf maple in more detail.

Can you cover exposed tree roots with soil?

Only a little, and mulch is better. OSU’s construction guide says significant damage may be avoided if no more than 2 to 4 inches of fill goes around an existing tree and the fill is coarser than the soil already there. It names maple, oak and evergreens as the trees most sensitive to added soil, and elm, ash, willow, sycamore and locust as the least.

The reason is air. ISA warns that fill soil can cut oxygen to the roots and suffocate them, and says to consult a certified arborist before adding soil over roots. Mississippi State Extension notes that too much mulch piled at the base can cause the same rot problems as a tree planted too deep. Portland’s tree code counts smothering inside a tree’s root protection zone, from compaction, paving or any impermeable layer, as injury to the tree.

How to build a mulch ring over surface roots

A ring of mulch protects exposed roots from mower and trimmer wounds, which Mississippi State Extension says land on the structural roots above the soil line. Alabama Extension recommends the same fix: cover exposed roots with mulch.

1. Mark the ringAt least a 3-foot radius (ISA’s minimum), and out to the drip line if the yard allows.
2. Clear the grass at the surfacePull or scrape it by hand. Don’t till or dig; ISA says earthmoving in the root zone can do more harm than good.
3. Leave the roots as they areNo soil layer underneath the mulch.
4. Spread wood chips 2–4 inches deepISA’s range; OSU says results with chips are usually best at 3–4 inches. On wet, poorly drained ground, ISA cautions that more than 2 inches can keep the root zone too wet and lead to root rot. Fresh arborist chips are an excellent material, ISA says.
5. Keep it off the trunkLeave a hand-width gap at the base (Trees for Seattle) and never build a mulch volcano.
6. Top it upChips break down faster than bark nuggets (OSU), so add more as the layer thins. In dry summers, water deeply: ISA puts a tree’s need at about 1 inch of rain every 7–10 days.

Where to find chips is covered in free wood chips.

Tree roots killing the lawn? What to grow instead

Grass and tree roots fight over the same few inches of soil. ISA’s turf guidance lists the results: thin grass in shade, trunks and roots nicked by mowers, and fertilizer meant for one plant taken up by the other (more in fertilizing trees). OSU says young trees in lawns establish and grow faster with a mulched circle around the trunk, because turf roots compete less.

West of the Cascades, OSU’s native-plant guide lists groundcovers and low shrubs that take shade. Plant them small, in pockets dug between roots, rather than tilling a bed:

Source: OSU Extension EC 1577, Gardening with Oregon native plants west of the Cascades (revised October 2022), checked 2026-10-02.

Where you want grass anyway, ISA notes that shade-tolerant varieties keep coming onto the market; let the turf grow to the top of its recommended mowing height, which deepens its roots.

Will cutting tree roots kill the tree?

It can, and it can also leave a live tree that falls over. ISA’s construction brochure explains why: a root zone can reach well past the trunk and the branch spread, and the part essential for health and stability is the critical root zone. Grading, trenching, paving, and adding or removing soil inside it all damage roots, and how much a tree tolerates depends on its age, species and condition.

The numbers come in two sizes. OSU, citing ISA, defines the critical root zone as 1 foot of radius for every inch of trunk diameter at 4.5 feet, and says it holds 85 percent of the root mass; Portland’s root protection zone uses the same 1 foot per inch. The Forest Service’s risk manual uses a larger critical root radius, 1.5 feet per inch, usually beyond the drip line. For a 20-inch trunk, that is 20 feet and 30 feet.

The Forest Service says up to 40 percent of that area can be damaged before anchoring is seriously impaired, but some species tolerate less, and for a tree hemmed in by sidewalks or buildings it treats any loss of main roots inside the radius as a likely high risk. OSU adds the rule that matters most at the trunk: most roots over 4 inches thick are likely structural, and cutting them can make the tree fall over. Root pruning, OSU says, is best left to an experienced professional using sharp, clean cuts.

Pruning tree roots: what’s safe and what isn’t

Sources: ISA (Trees and Turf, Proper Mulching Techniques, Avoiding Tree Damage During Construction); OSU EM 8994; Mississippi State Extension P3717; Portland City Code 11.80.020; checked 2026-10-02.

Two cities put rules on it. Portland’s code counts removing 25 percent or more of a tree’s functioning root area in one growing season as excessive pruning, and requires root cutting to follow proper arboricultural practice with clean, sharp tools (PCC 11.80.020; 11.60.040). Seattle treats removing live roots 2 inches or larger as reportable work: if you hire it out, the company must be on SDCI’s tree service provider registry, and the work must be posted online 3 full business days ahead.

Surface roots and Northwest windstorms

The roots people want gone can be the ones holding the tree up, and winter is when that shows. The Forest Service’s silvics manual records occasional heavy blowdown losses in the Pacific Northwest when high winds follow heavy rain, says stands of western hemlock are subject to severe windthrow because of their shallow roots, and says western redcedar is often windthrown in wet environments. NOAA’s storm records show how the year is weighted:

A tree that loses anchoring roots in summer meets those November winds within months. The warning signs of a root plate starting to fail, a new lean with soil heaving or cracking on the far side, are in leaning tree; the Forest Service calls a new lean a possible high risk of failure.

Roots lifting a path, driveway or foundation

A root lifting a path close to the trunk may be one of the tree’s main anchoring roots, which is why the cut needs a second opinion. Who pays for sidewalk and sewer-line repairs in Portland and Seattle is covered in tree root damage on your property, and whether a tree is too close to the house is covered in is that tree too close to your house.

Before anyone grinds a root flat, have it looked at. An arborist can tell you which roots can go, and ISA says roots in the way of construction can be pruned cleanly before the digging starts, rather than torn by it.