Rain Gardens for Montana Yards: Siting, Sizing and Plants

What is a rain garden and what does it do?

The Lewis and Clark Conservation District in Helena describes a rain garden as a plot of land that has been lowered to collect runoff from a home, sidewalk or other impervious surface. The Colorado Stormwater Center's guide Building a Rain Garden in Colorado explains the purpose: runoff from roofs, lawns and driveways picks up fertilizer, pesticides, pet waste and other pollutants on its way to storm drains and streams, and a rain garden holds that water briefly and lets it soak in, usually within 24 hours, while soil, compost and roots filter it.

It is not a pond. A properly built rain garden is dry most of the time and drains within a day after a storm. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide.

Where should a rain garden go?

Downhill from the water source and away from anything that should stay dry. The conservation district says to keep it away from septic systems, water wells, utilities and foundations. The guides we checked give different setbacks from buildings: the Colorado guide says at least 10 feet to keep water out of basements, while a USDA NRCS site and soil assessment card (written for Connecticut) treats less than 20 feet from a foundation as unfavorable, to prevent basement seepage or frost damage, and less than 10 feet from a septic drain field as unfavorable. With basements and frost both in play here, the larger distance is the safer starting point.

The NRCS card adds that the land should slope down from the house toward the garden, ideally between 1 and 15 percent, that the garden site itself should be fairly level (slopes over 12 percent are unfavorable), that it should be more than 10 feet from trees to avoid cutting roots, and that full shade is a poor choice. Colorado's guide asks for full or partial sun year round.

Do you need to call 811 before digging a rain garden?

Yes. Montana law requires an excavator to notify the one-call center before beginning an excavation, and the facility owner then has 2 business days to locate and mark their lines. Montana811 says the service is free and that homeowners can file a request online or by dialing 811. It also warns that private underground facilities may not be located through the system.

The Colorado guide makes the practical point: 811 will not mark irrigation lines, so those need to be found by the owner or the irrigation contractor. In most yards we work in, the sprinkler main and the landscape lighting wire are exactly where a rain garden wants to go.

How do you test whether the soil drains well enough?

With a simple infiltration test. The Colorado guide's version: dig a hole about the size of a coffee can where the garden will go, fill it with water and measure the depth, then measure again after 4 hours and divide the change by 4 to account for sideways movement. If the result is less than half an inch, the guide says the area is not suitable for a rain garden. The NRCS card uses a 6 to 10 inch hole and treats water dropping less than 1 inch an hour as unfavorable.

Also check the obvious signs. The NRCS card says areas where water already stands more than two days after heavy rain are poor candidates, because they cannot take more water, and that ponding longer than 72 hours can breed mosquitoes. It treats bedrock within a foot of the surface as unsuitable. Heavy clay and very compacted soils need amending or a different spot. If the problem is a wet yard rather than a place to send water, start with standing water in the lawn or French drains.

How big should a rain garden be?

Big enough for the area draining to it. The Lewis and Clark Conservation District suggests roughly 5 to 10 percent of the impervious area that drains into it; its example is a 3,000 square foot roof at 7 percent, giving a garden of about 210 square feet. The Colorado guide sizes to capture half an inch of runoff from the contributing area: work out the roof area served by the downspouts going to the garden, multiply by 0.5, and divide by the garden depth in inches to get the surface area in square feet.

For one downspout draining a quarter of a 2,000 square foot roof (500 square feet) into a 6 inch deep garden, that formula gives about 42 square feet. The NRCS card suggests multiple rain gardens or other measures for drainage areas over 3,000 square feet. The Colorado guide adds that a smaller garden than the formula suggests is still better than none.

How deep should it be?

Shallow. The NRCS card ties depth to the slope of the site: 3 to 5 inches where the slope is under 4 percent, 6 to 7 inches at 5 to 7 percent, and 8 inches at 8 to 12 percent. The Colorado guide sets depth from the infiltration result so that water is gone within 24 hours. A bottom that is as flat as possible spreads water evenly; the downhill side is usually built up as a low berm with the excavated soil.

How is a rain garden built?

The Colorado guide's sequence: mark the outline and confirm no utilities or irrigation lines run beneath it; dig to the chosen depth with a flat bottom; mix compost into the bottom 8 to 12 inches of soil, at about half a cubic yard per 100 square feet; direct runoff in by extending the downspout, burying a pipe 4 to 6 inches deep, or building a shallow cobble swale; and line the inlet and the overflow point with cobbles so water does not scour the bed. It recommends shredded mulch, which does not float away, and specifically says not to use weed barrier, because it interferes with filtration and causes standing water.

The overflow matters in western Montana. A rain-on-snow event or a fast spring melt can deliver more water than any garden holds, and the overflow should spill toward somewhere safe, away from the house and the neighbor. Our guides to downspout drainage, grading away from the foundation and dry creek beds cover the routes water can take.

What plants go in a Montana rain garden?

Plants that handle both a soaking and a dry spell. The Colorado guide recommends natives that are drought tolerant but can stand occasional standing water, and its list includes blanketflower, prairie coneflower, purple prairie clover, white yarrow, golden columbine, sulphur flower and grasses such as Indian ricegrass and western wheatgrass. Most of these, or close relatives, also appear in MSU's 2025 native plant list for Montana (MSU lists common yarrow, rice grass, and Colorado blue and yellow columbines).

The low center can take wetter-site natives from the MSU list, such as swordleaf rush and Torrey's rush, spotted Joe Pye weed, swamp verbena and Canadian anemone, with drier-site plants on the rim and berm. MSU's 2025 MontGuide reminds gardeners that even drought-tolerant natives need regular water until established. See hardy perennials and ornamental grasses for more choices.

How much maintenance does a rain garden need?

Not much once established, according to the Colorado guide: irrigation in the first growing season, especially if seeded, and occasional watering in long dry spells after that. Add to that the jobs every bed needs: weeding until plants fill in, topping up mulch, clearing sediment and leaves out of the inlet, and resetting cobbles that wash out. Check after the spring melt that water still drains within a day. If it starts holding water for days, the soil surface has usually sealed with fine sediment and needs to be loosened.

Does a rain garden replace gutters, grading or drains?

No. A rain garden is the last step in a drainage plan, not the first. Water has to be moved away from the foundation first by grading and downspouts, and in heavy clay or high groundwater a rain garden may simply not drain. Where grading, wall drainage or a retaining wall is involved, the design has to account for where all of that water goes. Our existing guide to yard drainage during spring melt covers the bigger picture.

Can HJ build a rain garden?

Yes, as part of our landscaping design and drainage work: siting, excavation, soil amendment, inlet and overflow, planting and mulch, with irrigation lines located and rerouted where needed. We are not engineers, and for stormwater systems that have to meet a permit or a subdivision requirement, the design should come from a licensed engineer.

Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate and we will walk the property with you.

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