Landscaping in Soggy Spots
Low-lying sites are problematic for many plants, but the right landscaping can absorb excess water and turn waterlogged trouble spots into assets.
Plants, trees, shrubs and groundcover all need water, but not too much. Therein lies the problem. Oxygen levels decrease in saturated soils, and the slower respiration rates can cause harmful gases to build up around roots. Soggy conditions also favor soil organisms that attack plant roots.
“If your yard or landscaping has poor-draining soil, it’s important to ask why,” says David Barmon, co-owner of Fiddlehead Landscape and Design in Portland, Oregon. “Often, wet areas are prevalent from fall until late spring, when there’s colder weather and more precipitation.” Observing the soggy spot over several seasons should help you determine whether you have a problem, especially if you live in a region with wet and dry cycles.
What is the native soil in your area like? Different soil types have different drainage characteristics. In general, sandy soils drain the fastest, silty soils drain moderately well, and clay soils retain the most water. However, development can compromise any soil’s drainage capacity. If your house is newer, was your soil recently compacted by heavy equipment? In compacted soils, the structure has collapsed to the point where roots and water can’t move through it.
“Sometimes soil that is compacted and poorly draining will turn a grey color,” says Barmon. A strong odor indicates the presence of anaerobic bacteria, which thrive in soil with poor oxygen levels.
It could be that development plans weren’t a match for the existing soil, and have increased the volume of water flowing into an area.
“When we build houses, we create a lot of impermeable surfaces, and then water piles and pools up and runs off our roofs or down our driveways and then races across the lawn,” says Erin Muir, landscape architect at The Figure Ground Studio, also in Portland. “Water pools in places where it can’t percolate down to the water table.”
An area with slow-draining soil is typically the low spot on the property. This might be at the bottom of a hill or just a depression in the ground. There could also be a natural underground spring charging the area. Water draining from an adjacent property could be part of the issue. “Properties on hillsides often have water moving through the landscape,” says Barmon. Ponding—the unwanted pooling of water—is more likely to happen on properties in flat areas.
Observe the plants that may already be growing in a wet spot; they can be indicators of soil type. If those plants are thriving, you don’t have a wet spot. If they’re failing to thrive, you have a problem.
Here are some telltale signs of a poorly draining spot in the landscape:
The ground is consistently squishy.
Lawn in the wet spots grows sparse, with muddy patches, and mowing is impossible.
Plants look wilted or sick and may eventually die.
Less well-draining areas in a garden aren’t always noticeable to the eye. A percolation test can tell you if you have slow-draining soil. Simply dig a hole in the earth and fill it with water, then time how long it takes for the water to drain. If water drains at a rate of less than one inch per hour, drainage is poor.
Mitigating the Water
Now that you’ve assessed the problem, you can decide on your approach. One of the more traditional ways of dealing with wet spots is to incorporate four to six inches of compost into the soil. Adding sand will also help dry out the area. In severe cases of soggy landscape, a drainage system can be installed to intercept and convey water away from the wet area. Dry wells and French drains can also be incorporated. Just remember, water pulled away from a wet area has to go somewhere else. Sending water to the property next door may not be the best good neighbor policy. Instead, consider routing water to a swale or rain garden.
Gardens for Waterlogged Soil
In the last few decades, there has been a growing interest in rain gardens and bioswales as tools for managing water on-site. A bioswale is a landscape element consisting of a shallow drainage course with gently sloped sides, meant to slow the movement of water and filter out pollutants. The swale can be planted with vegetation, often grasses and groundcovers, or with a combination of plants, rocks and compost. A rain garden is a shallow ground depression planted with native and regionally appropriate flowering plants and grasses that trap runoff water. Rain gardens direct water into the ground instead of allowing it to flow into nearby streets, gutters and sewer systems; they also support wildlife, including pollinators like bees and butterflies.
For rain gardens and soggy areas, Muir recommends going with native plants. “You are guaranteed that those plants will deal with the hydrologic cycle—the movement of water on, above and below the earth’s surface—in your area, as they have adapted to the climate already,” she explains. “Native plants will require the least amount of care.”
When choosing plants, it’s important to know whether you’re talking about an area that is always wet, meaning roots are under water all of the time, or intermittently so, meaning roots are submerged occasionally. Seasoned nursery staff will be able to direct you to the perfect plants for your regional zone.
“There is a saying, ‘right plant, right place,’” says Barmon. “It’s often easier to work with nature rather than against it.” Which brings up another point: We often choose landscaping plants and designs based on what we’ve seen in someone else’s garden, or a magazine. Taking a more integrated approach can save money, headaches and extra trips to the nursery.
“There is a bigger and broader perspective about the purpose of landscaping, and that can be in a small backyard or acres and acres in a rural setting,” says Barmon. “It comes down to understanding and managing resources in a way that’s smart.”
Once you’ve learned to work with your site’s topography and the water flowing in and out of it, you can expect your garden to thrive for many years to come.