Landscaping a Slope or Hillside in Western Montana

How steep is too steep for a lawn?

There is no single legal number for a home lawn, but there is a practical one: if you cannot mow it safely, it should not be lawn. Steep turf is hard to mow, sheds irrigation water before it soaks in, and dries out first in summer. A sloped lawn that looks patchy is often failing for that reason, not because of the grass seed.

Measure before deciding. Slope is the rise divided by the run, as a percentage. Stretch a string level from the high point straight out over the slope for 10 feet and measure down to the ground: a 1 foot drop over 10 feet is a 10 percent slope, a 2.5 foot drop is 25 percent, and 5 feet is 50 percent (a 2 to 1 slope). That number drives everything that follows, including whether a wall needs an engineer.

Why does water run off a slope instead of soaking in?

Because it arrives faster than the soil can absorb it. The EPA's WaterSense program notes that water can easily pool or run off landscapes with clay-rich soils or slopes if it is applied too quickly. On a slope, the excess does not pool; it runs downhill, carrying soil, mulch and fertilizer with it, and the bottom of the slope ends up soggy while the top stays dry.

Soil type changes how fast that happens. Many Bitterroot slopes are the gravelly escarpments at the edges of old outwash fans; NRCS maps soils such as Chereete on escarpments with slopes up to 45 percent and describes them as excessively drained. Those soils take water fast but hold little of it. Finer, silty or loamy soils hold more but seal over and shed water sooner. Our guide to Bitterroot Valley soils explains how to look up yours.

How should I water a sloped yard?

Split each watering into short cycles with a soak period in between. WaterSense calls this cycle-and-soak: dividing irrigation runtimes into intervals with short breaks so the water can soak into the soil. A smart or programmable controller makes it easy, and our guide to smart irrigation controllers covers the options.

Beyond scheduling, put the top and bottom of a slope on separate zones, because the bottom collects what the top sheds. Use low precipitation rate nozzles on spray zones, and drip for shrubs and beds, which puts water at the root zone without runoff. Our guides to drip irrigation, irrigation zone layout and sprinkler head types go deeper, and our irrigation systems crew designs and installs systems for sloped sites.

What is the best way to stop soil washing off a slope?

Cover the ground and slow the water. Colorado State University Extension's fact sheet on erosion control, written for burned slopes but built on the same principles, lists the basic tools: establishing plant cover, mulching, contour logs, straw wattles laid across the slope, and silt fences for broad sheet flow. It notes that silt fences are not suitable for concentrated flow in rills or gullies.

CSU recommends straw mulch at a uniform two to three inches, with 20 to 40 percent of the soil surface still visible, and certified weed-free straw so you do not import noxious weeds. Contour logs or wattles are laid on the contour, perpendicular to the slope, staggered so water meanders between them, and bedded into the soil so water cannot run underneath. On a home landscape, these are usually temporary measures that hold the soil while permanent plantings fill in. Our guide to controlling erosion on a sloped yard covers repair of slopes that are already washing.

Should I terrace a hillside or build retaining walls?

Terraces turn one steep slope into several flat steps, which makes planting, watering and access much easier. Low terraces built with boulders or stacked stone are often enough on a moderate slope. Taller walls carry real loads, especially when the soil behind them is wet or frozen, and they fail when drainage is skipped. Our retaining wall installation page explains how we build from the drainage out, and our comparison of retaining wall materials helps with choosing block, boulder or timber.

Height and what sits above the wall decide whether a permit or an engineer is needed. Our guide on when a retaining wall needs a permit or engineer covers that, and before regrading a steep lot it is worth asking your city or county planning office whether the work needs review. We will tell you plainly when a wall is beyond landscaping and needs an engineer.

What plants work best on a Montana slope?

Plants with fibrous or spreading roots that knit the soil, that survive on whatever water reaches them, and that need little foot traffic for care. That usually means a mix of native bunchgrasses, low spreading shrubs and tough perennials rather than turf. Our native plants guide is the best place to start, along with the guides to shrubs, ornamental grasses and low-maintenance landscaping.

Plant in staggered rows across the slope rather than straight lines down it, so each plant catches the runoff from the one above. Build a small level basin on the downhill side of each plant so water collects there. And plant in spring or early fall, when there is time for roots to take hold before summer drought or hard freeze; our guide to fall planting covers the fall window.

What mulch stays on a slope?

Fine bark and loose wood chips float and slide on steep ground, especially during snowmelt. On slopes we prefer angular crushed rock or a coarse, shredded wood mulch that knits together, kept to a depth that does not smother small plants. Jute or coir erosion netting under mulch helps on the steepest banks until plants cover the ground.

If your slope is near the house in fire country, think about mulch and plant choice in the first few feet carefully. Our guides on mulch and wildfire and how slope changes wildfire risk explain why.

How do I handle the water at the bottom of a slope?

Plan where it goes before it gets there. A swale or a dry creek bed at the toe of a slope can carry runoff to a place that can take it, and a planted rain garden can soak it up where soils drain. Water should never be directed toward a foundation, a neighbor's property or a septic drainfield.

Spring melt is the real test in western Montana. Our guide to yard drainage during spring melt covers what to look for when snow comes off a slope fast.

What goes wrong most often on sloped landscapes?

The same few mistakes come up again and again. Spray heads run on the same schedule as the flat lawn, so the slope sheds most of its water. Bare soil is left between new plants all summer, and the first heavy storm cuts rills through it. Bark mulch is spread on a bank and ends up in the gutter. A wall is built without drain rock or a drain pipe and starts leaning a few winters later. And lawn is kept on ground too steep to mow safely, so it gets scalped, thins out and fills with weeds.

Each of these is cheaper to avoid than to fix. Separate irrigation zones, temporary erosion control until plants fill in, mulch chosen for the slope, drainage behind every wall and plants instead of turf on the steep parts solve most slope problems before they start.

How should a slope project be phased?

Work from the top down and from the ground up. Deal with water first: where it enters the slope, where it leaves, and how irrigation will reach each part. Then build any walls or terraces, since they change the grade. Then install irrigation lines, then plant, then mulch. Doing it in that order means nothing has to be dug up later.

If budget means spreading the work over a few seasons, stabilize the whole slope with cover and temporary erosion control in year one, then finish one section at a time. Our guide to phasing a landscape project covers how to sequence the work so each season's piece stands on its own.

Can a slope be low maintenance?

Yes, once it is planted and established, a slope of native grasses and shrubs on drip irrigation can be one of the least demanding parts of a yard. The work is front-loaded: grading, rock, irrigation and planting in the first season, then weeding and some hand watering while plants establish. After that, an annual cleanup and a check of the irrigation is often all it needs.

Slopes are a regular part of our landscape design and installation work around Missoula, Lolo, Florence and up the Bitterroot. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate to walk your slope with us.

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