Many gardens in the PNW are plagued with poor drainage, soggy soil, and muck for much of the year. Improving the drainage in your garden is not always a simple solution and often needs to be addressed using multiple tools/solutions.
Most of the time we find that improving soil conditions, modifying slopes, and incorporating a drainage element to channel water when used in combination, make gardens that were once inaccessible in the winter into usable functioning spaces.
Useful Drainage Solutions
The existing soil conditions will influence what the best amendment will be. Adding sand and aggregates can be used to increase pore space and reduce compaction but if used in the wrong native soil conditions can turn your soil into concrete so be cautions. Adding organic matter will increase water holding capacity of the soil and encourage soil life to aid in breaking up clay soils. Most often a combination of sand or aggregate and compost are the best solution. We prefer to use a mix higher in compost for planting beds and higher in sand for lawn areas that will see more foot traffic. Soil amendments need to be mixed into the native soil rather than layered on top which can create a water barrier. The slope of the entire site is also important to plan to ensure a positive slope away from all structures and is planned and executed to eliminate any low spots or barriers to allow water to flow in the desired direction.
French drains are used to channel water from one area to another. They are installed using a perforated pipe surrounded by cobble and filter fabric. French drains installed with the cobble all the way to the surface will capture surface water and sub surface runoff. If covered and buried, they will only capture sub-surface drainage. The effective depth of the trench will depend on soil conditions. Hard and compacted clay soils will necessitate a shallower installation. Black corrugated pipe is often used for French drains, but we recommend using perforated PVC hard pipe which is less susceptible to filling with debris and can effectively maintain a reliable slope without settling overtime. French drain systems can also be used to disperse drainage along a larger surface for infiltration. When larger boulders and varied size cobble is added in a meandering line following the line of the French drain it can be transformed into a beautiful dry-stream feature in the garden.
Trench drains are used to capture surface runoff. These are most effective when installed inside or adjacent to patios, walkways, or driveways. The end of the trench drain will be connected to a pipe to route to an effective outlet (rain garden, municipal drainage line, catchment system, or dry well.)
Catch basins are used in much the same way as French drains but can also serve as a hub point connecting pipes from French drains and/or trench drains. Decorative drain lids for both catch basins and trench drains can make these into beautiful garden elements rather than just a functional utility feature.
Dry wells are large pits filled with cobble to create a holding capacity for rainwater until the surrounding soils have the ability to allow it to infiltrate. These can be used at the end of French or trench drains when another outlet to route drainage is not available. This also allows rainwater to be dealt with on-site rather than removing it and sending it elsewhere. If the site conditions condone handling of all stormwater on-site it is the preferred and most sustainable option.
Serving the same purpose as a drywell, rain gardens create an on-site water holding capacity for runoff. Rain gardens are very dynamic elements that take more time to establish but in time can often outperform many other options. To create a raingarden the native soil is excavated in an area to create a depression or pond like feature, then it is mostly filled with a special bio-retention soil mix which has a very high water holding capacity. Then the feature is planted. The bottom of the rain garden is planted with species that are tolerant of periodic emersion (my favorite is our native Carex obnupta). The sides of the rain garden are planted with plants that are suitable to the amount of water they will receive by how close to the ponding mark they will be (progressively less tolerant of soggy soils as they reach the outskirts of the feature). It is important that the soils in the rain garden are not compacted during installation and maintenance to keep it draining properly. As the plant material establishes a healthy and robust root system, they will uptake more water to reduce the amount of water that is needing to infiltrate into the soil. The size of the rain garden needs to be carefully considered when planning to ensure it is large enough to contain the water being routed to it and is best if it also has an overflow drain for larger storms that may overwhelm the holding capacity of the rain garden. Rain gardens can be used to both slow down and reduce or eliminate the amount of stormwater being directed off-site and can be designed to be beautiful garden features.
Drainage issues may not come with a quick fix solution but the right combination of elements for your property can allow your garden to be a thriving functional space that can be enjoyed even in the soggiest times of the year. Let us help you plan your garden.