Should You Cut Down Trees on a Steep Slope in Western PA?

Brinko Tree Service

3 days ago

5 min read

Only the trees that pose a real hazard, and ideally only after someone has looked at the slope itself. The U.S. Geological Survey says native vegetation on slopes is often adapted to site conditions, that its roots can help stabilize the slope, and that removing native trees and vegetation could increase the slope's susceptibility to landslides. On a steep hillside, clearing trees for a view or a lawn can trade one risk for a bigger one.

This matters more here than almost anywhere else in the state. The Pennsylvania Department of Conservation and Natural Resources (DCNR) says southwestern Pennsylvania has by far the highest concentration of landslides in Pennsylvania, that stabilizing and repairing a landslide affecting two or three properties typically costs several hundred thousand dollars, and that insurance covers landslide damage only for some business situations. This guide covers how trees affect slope stability, which hillside trees should come down, how to remove them with less disturbance, and who to call first.

Why Are Landslides Such a Concern in Western PA?

A landslide is the downslope movement of soil, rock or debris, and DCNR explains that many in Pennsylvania happen in loose soil and old landslide debris on steep slopes or along weak rock layers and fractures. Southwestern Pennsylvania, including the Pittsburgh area, has the highest concentration in the state. DCNR says mountainsides have experienced debris avalanches associated with extreme rainfall or rain-on-snow events, and that large excavations and fills for highway construction in mountainous areas create the potential for many landslides.

DCNR also notes that most slides are moderate to slow moving and damage property rather than people, and that urban and rural land development is increasing the number of landslides. It describes backyard landslides in Pittsburgh as usually repaired incompletely or not at all, and notes some properties have been abandoned when repair costs exceeded their value. Deaths and injuries from landslides are rare in Pennsylvania, according to DCNR, and almost all known deaths have happened when rockfalls or other slides along highways involved vehicles.

Do Trees Help Hold a Slope in Place?

Their roots can. The USGS says roots of native vegetation can contribute to stabilizing a slope. Research by Kazutoki Abe and Robert Ziemer, published by the U.S. Forest Service in 1991, found that root reinforcement of soil increases quickly for about the first 20 years after trees are established and then stays about constant. The researchers used the pull-out strength of roots to analyze the stability of four different types of forested slopes, and they point to root reinforcement as especially important where roots can extend into joints and fractures in bedrock.

That timeline cuts both ways. A mature tree on a hillside has had decades to build root support, and a replacement planted after removal needs years to catch up. Clearing a slope all at once removes that support from a large area at the same time. If trees do come out, slope stabilization guidance from the Federal Emergency Management Agency (FEMA) recommends surface coverings such as seeding, hydroseeding, transplanted vegetation and mulch to protect against wind and water erosion.

Trees are not a cure-all, though. Trees can still fail when a slope is saturated after heavy rain. Our post on how saturated soil affects tree stability explains that side of the problem.

Which Hillside Trees Should Come Down?

Focus on hazards, not on clearing. Trees worth removing on a slope include:

Dead or dying trees that could fall onto a house, garage, driveway or road
Trees with a new or worsening lean toward something that matters
Trees with cracked trunks, major decay or exposed, damaged roots
Trees whose failure would take out a fence, retaining wall or utility line

A new lean after heavy rain deserves quick attention on a slope, and our posts on why a tree leans after heavy rain and what causes trees to uproot in summer storms explain the warning signs.

Healthy trees that simply block a view are a different case. Thinning or raising the canopy through pruning can open a view while leaving the roots in the ground, so the tree keeps doing its part to hold the slope.

How Should Trees on a Slope Be Removed?

With as little ground disturbance as possible. Practical steps:

Remove only what is needed. The USGS warning is about removing native vegetation, so selective removal is better than clearing.
Keep heavy equipment off the slope when you can. Rigging trees down in sections or lifting them out keeps machines from rutting soft ground. For large trees near homes on steep lots, whether crane tree removal is worth the cost covers one option.
Think twice about the stump. On a slope where stability is a concern, ask whether grinding is necessary or whether the stump can stay.
Replant or let native growth return. Given the roughly 20-year buildup Abe and Ziemer describe, the sooner new roots start, the better. FEMA also notes that live plantings such as stakes and fascines can help reinforce the soil and withstand soil movement through interconnected root systems, though they can be installed only during dormant seasons, and that live plantings placed among gabion walls or crib walls can help reinforce the fill.

Before any digging for new plantings, the Pennsylvania Public Utility Commission says homeowners must call 811 at least three business days ahead.

Who Should You Call Before Cutting on a Steep Lot?

If the slope has cracks, bulges, wet spots or a history of movement, start with a slope expert. The USGS recommends hiring a professional geologist or engineer with landslide experience for an evaluation of landslide risk, noting that in regions with known landslide issues, such professionals can usually visit your home.

Two public resources help too:

DCNR's Bureau of Geological Survey takes landslide questions at 717-702-2017 and by email at RA-NRGeohazards@pa.gov.
Allegheny County's Landslide Portal provides guidance on preventive and corrective actions for slopes at risk from landslides.

Once the slope question is answered, a tree crew can handle the hazardous trees. Your insurer is the place to ask about coverage, given DCNR's note that insurance covers landslide damage only for some business situations.

FAQ

Can cutting down trees cause a landslide?

It can contribute. The USGS says removing native trees and vegetation could increase a slope's susceptibility to landslides.

Does homeowners insurance cover landslide damage in Pennsylvania?

DCNR says insurance covers landslide damage only for some business situations. Read your policy and ask your insurer.

How much does landslide repair cost?

DCNR says estimates of several hundred thousand dollars are typical to stabilize and repair a landslide affecting two or three properties.

Should I grind stumps on a hillside?

Not automatically. Ask whether the stump is in the way of anything before grinding on an unstable slope.

Is it safe to leave a dead tree on a slope?

It depends on what it could hit. A dead tree above a house, road or driveway should be evaluated and usually removed; one far from any target is a lower priority.

How long do new trees take to help hold a slope?

Abe and Ziemer's 1991 research found root reinforcement increases quickly for about the first 20 years after trees are established.

Hillside Tree Removal and Pruning From Brinko Tree Service

Brinko Tree Service, based at 1010 Sunset Blvd in Harrison City, brings nearly 20 years of experience to properties across Westmoreland and Allegheny Counties. On sloped lots, the team provides tree removal for dead, leaning, or hazardous trees and pruning and trimming to open views while leaving healthy trees in place, along with cleanup and debris hauling. Brinko is fully licensed and insured, follows OSHA and ANSI safety regulations with an OSHA safety site supervisor on every job site, and offers free estimates.