Creating a Rain Garden

Creating a Rain Garden

There are countless reasons to grow a garden — to delight your senses, to connect with nature, and to reap a wide variety of psychological and health benefits are just a few — and the wonderful thing is that these goals aren't mutually exclusive. In fact, a garden can and should serve various functions for those that use it, and that isn't limited to just people! We share our gardens with all sorts of plants and wildlife, and the decisions we make in our individual gardens can have a collective impact that ripples outward.

At Cornell Farm, we think environmental responsibility and sustainability are tenants that should factor into your gardening practice. This can look like planting a water-wise garden, reducing or eliminating pesticides and inorganic fertilizers, providing food and habitat for pollinators, and looking for ways to responsibly manage our precious natural resources. One major resource that we often fail to consider in urban and suburban areas is stormwater, and rain gardens and bioswales provide an effective and aesthetically-pleasing way to manage this runoff for the benefit of the environment.

Illustration courtesy of City of Portland Environmental Services.

What is a Rain Garden?

  • At its core, a rain garden is a planted depressions that receive runoff from impervious surfaces like roofs and driveways and helps it filter into the ground slowly.
  • A bioswale is similar to a rain garden, but more linear, directing runoff along a channel that helps it filter into the ground as it flows.

Both of these types of green infrastructure can be implemented in home landscapes, and help to filter out some impurities before the water drains into sewers, groundwater, and bodies of water like streams and rivers. And plants play a large role in this!

Unlike a water garden, rain gardens and bioswales aren't designed to hold water for extended periods of time — or even stay wet-year round. Rather, they give precipitation somewhere to go when it falls, which can vary greatly with the seasons. Portland is known for our rainy winters, but in the summer, it's warm and sunny. That's why, when selecting a palette of plants, it's important that they can tolerate both extremes of wet and dry. Thankfully, many plant species native to the Willamette Valley (and the Pacific Northwest more broadly) are already suited to these types of growing conditions. In addition to being climate-adapted, they have evolved alongside other native flora and fauna in unique and resilient ecological communities, so planting a rain garden full of natives also helps to support these ecological relationships and bolster native biodiversity in our backyards.

Constructing a Rain Garden

When siting a rain garden or bioswale in your back yard, you will want to find a location that is at least 10 feet from the foundation of your home, and near a downspout or two. From there, you can excavate a depression into which these downspouts can be redirected. (You want to make sure that, in the event of a sudden influx of water, any overflow drains away from your home, rather than towards it.)

A rain garden can be planted and mulched to retain moisture — which is especially important in the first few years. Keep in mind that plants closest to the bottom of the basin will need to be able to withstand more persistent flooding and moisture than plants towards the edges. Your light conditions will also determine which plants will thrive in your rain garden. In general, many of our native rushes and sedges make for great rain garden candidates that can effectively filter out many pollutants. For blooms, our native camassia will provide a springtime show, and scarlet monkeyflowers will bloom throughout summer. For structure, look to our native shrub dogwoods and other shrubs, many of which provide berries for birds, as well.

Below, you will find a couple of vetted free resources that will walk you through how to create a rain garden in our region in more detail, as well as a collection of native plants we've hand-picked to grow in the conditions found in these types of gardens. If you have questions about selecting appropriate plants, feel free to ask our knowledgeable team at Cornell Farm.