Recent heavy rains may have drawn your attention to soggy areas of the garden. Devastating floods are beyond your control, but damp spots...
Recent heavy rains may have drawn your attention to soggy areas of the garden. Devastating floods are beyond your control, but damp spots caused by topography and rain can be a challenge or an opportunity when designing a garden.
Wet areas in the garden are caused by an excess of water on your property or from neighboring property and from poor soil drainage.
Here are some ideas and basic information on how excess water can be managed in the garden. For serious water issues, consult a drainage specialist.
Your garden may be at a low spot that takes runoff from neighbors’ properties. One way to handle this situation is to intercept the water and safely drain it away with an open ditch or an underground pipe. Direct the water into a storm sewer, or develop an on-site collection system.
You can also use a French drain, a gravel-filled trench with a perforated pipe at the bottom, to intercept water. The spaces in the gravel provide a place for water from the surrounding soil to collect, and the pipe takes it way.
French drains are generally laid perpendicular to a slope and to the flow of water that they are intended to catch. If water is moving both on the surface and below it, the bottom of the trench has to be deep enough to catch it. This may not be economically or physically feasible.
For more information about creating a rain garden and natural drainage systems, see the following Web sites:
Rain Garden Network:
Seattle Public Utilities: www.seattle.gov/util/naturalsystems
Creating a diversion
Excess water can also come from your own property, flowing off impervious surfaces of your roof, patios, walkways and driveway. Gutters and downspouts collect and concentrate water from the roof. If that water is not channeled into pipes that lead to a sewer or storm drain, it may be contributing to a soggy landscape.
You can divert the water away from the house with splashguards (found at most garden and home-improvement stores). The water also could be channeled into a dry well, a hole filled with drain rock (round rocks are used so water can flow between the gaps). From here, the water can disperse slowly into the surrounding soil.
Wet spots also can be caused by compacted or clay soils. Sandy, free-draining soil is never wet, because water just runs through it.
Glaciers that covered our area left a complex pattern of soils that can vary sharply from one area to another on the same property.
Soil can become compacted from heavy equipment used in home building. Break up the compacted soil with another heavy machine such as a backhoe or excavator, or do it by hand with a pick.
In some cases, the natural topsoil may have been scraped away, exposing consolidated till or hardpan. This can be broken up and organic material such as compost dug in to keep it loose, or new topsoil can be placed on top.
Working with wetness
Instead of building a drainage system, which can be expensive and invasive, you may choose to live with a wet area and select plants that will thrive in damp conditions. If the soil stays damp through the summer, you may have the benefit of not needing to provide any water in the dry season, a big saving of time and money.
A rain garden, a new idea in on-site collection systems, is a depression made in the soil and planted with wet-tolerant plants. Runoff water diverted into a rain garden is slowed so that it can seep into the soil. Some of the water is transpired into the atmosphere by the plants. Rain gardens can also filter out pollutants that run off from buildings and driveways so that only clean water enters streams and Puget Sound.
A rain garden can be as small as 100 square feet. It should be built in an area that has good soil drainage and where runoff can be diverted into it.
To create a rain garden, dig a shallow bowl or build a berm to temporarily hold water until it seeps into the soil.
Choose plants that will thrive in wet winter and dry summer conditions. For the wettest part of the rain garden, red osier dogwood (Cornus stolonifera) will do well. Many Northwest natives will do well at the margins, including vine maple (Acer circinatum) and salal (Gaultheria shallon). Beebalm (Monarda dydima) and royal fern (Osmunda regalis) are ornamentals that like wet feet in winter.
Phil Wood has a degree in landscape architecture and designs and builds gardens. Send questions to firstname.lastname@example.org. Sorry, no personal replies.