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Q+A: How to build a rain garden in Portland

Misha Ashton, owner of Revive Gardens PDX, offers some tips on channeling excess water runoff into a vibrant natural area in your yard.

A newly finished rain garden shows young plants outside of a white home in Portland. .You can see a solid drainpipe that empties the downspout into the river rock French drain.

French drains typically carry rain to the garden, where plants that like their roots wet in winter and dry in summer thrive.

We can imagine your raised eyebrows — it’s summer, why would you want to even for one second think about rain?

Well, if building a rain garden in your yard is something that’s at the top of your to-do list, it’s much easier to do so when the water isn’t actually falling from the sky. By incorporating bioretention areas, you can reduce water intrusion to your house and runoff on your property, while attracting wildlife and filtering pollutants out of the environment.

For tips on such a big project, we turned to an expert: Misha Ashton of Revive Gardens PDX.

Tell us about your business.

My main focus is helping clients turn their yards into a space that is both usable and easy to maintain, while meeting Backyard Habitat Certification. That often involves removing lawns, diverting downspouts, and planting as many native plants as possible.

I work with all levels of gardeners, from those who like to do everything themselves and just need a little guidance, to those who want the full design and install.

Misha Ashton, owner of Revive Gardens PDX, poses in her backyard with a Backyard Habitat Certification sign.

Ashton has a small-scale native plant nursery in her backyard that’s not open to the public, but offers online shopping and in-person pick up. It’s only open during the months of September through June to encourage people not to plant in the summer.

How do you plan a rain garden?

First, asses the location to ensure you have the room and that there aren’t any buried utilities where you’ll be digging (always call 811).

You do not need a permit in Portland to disconnect your downspouts, but you will need an adapter and solid drain pipe to carry water to the garden. The general rule is 10 ft away from a home with a basement and a minimum of 2 ft from one without.

The size of the rain garden is determined by the surface area of the roof that is feeding into the downspout. A quick example: If the roof’s surface area is 200 sqft, then your rain garden should be a minimum of 20 sqft (10% of the impervious surface area) and a minimum of 6 inches deep.

What other factors come into play?

If you have poor draining soil, as many of us do (Portland has high levels of clay), the rain garden has a higher chance to fill up and overflow with heavy rain fall. You’ll want to ensure that the ground is graded away from the house and that the far end of the rain garden has an area lower than the point where water flows in.

You also want a shallow bowl shape, where the edges from the top to the base are gradual, not steep, to create more surface area for planting and limiting erosion.

Alternating images show the before and after of a yard worked on by Revive Gardens PDX.

Adding a catchment system (think: rain barrels) can help repurpose rain for other parts of your yard during the dry season.

Our shovels are ready, what comes next?

For burying the drain pipe, you’ll want to trench from the downspout to your rain garden at roughly a 2% decline, so water flows all the way through. You can use any excavated soil to set the grade, then backfill over the pipe.

I then put river rock around the pipe’s exit point and the overflow area. Many rain gardens are completely filled with river rock, for the dry river bed look, but personally I don’t love doing that because it heats up the soil in the summer and makes it hard to weed.

Is it time to add some greenery?

Yep. Use the soil within to build up the soil on the edges and add a thick layer of compost. This will help with soil drainage and provide a boost in nutrients for the new plants. My go-to is Mt. Scott Fuel in the Foster-Powell neighborhood, specifically their Dirt Huggers mixture.

There are many native plants to choose from, just be sure to pick those best suited for the available light. When you have finished planting, be sure to put a 2-to-3-inch layer of mulch over all open soil. I use medium dark hemlock, as it’s considered “splinterless” and takes a bit longer to break down than the fine mulch.

It takes a few years for plants to become established, so be sure to water them in the dry months.

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