If you can’t plant a tree, plant a rain garden!

As our world is warming, more and more water evaporates. And, as we all learned in physics class, what goes up must sooner or later come down. As Portland is bracing for at least a week of more or less torrential rains, I find myself wishing that I had some large-scale way to collect all this goodness coming down, so I could use it during our increasingly hot and dry summers. After all, over the past 12 years, the price of water in around 25% of US municipalities have doubled, so it’s against my frugal nature to see it go to waste. I find myself wondering if the underground tanks of the Living Building Challenge I worked on a couple of years ago, are large enough to store the increasing volumes of water falling on its metal roof.

A couple of weeks ago, Boulder, CO experienced highly unusual rains in proportions that media termed “biblical”, which flooded the city to the tune of some $49 million. “Biblical” is a really funny choice of word in this setting, as those most likely to subscribe to a literal interpretation of the Bible are the ones that most vehemently object to the idea that climate change is caused by us humans, or even to the mere notion of global warming, for that matter. Anyway, water has been on my mind a lot lately, so when the opportunity arose to take a workshop on rain gardens this past Saturday, I pounced. It was hosted by the NE Coalition of Neighborhoods, and taught by the East Multnomah Soil and Water Conservation District. Rarely has a class felt more timely. As we were  busily working on our hypothetical projects, the rain thundered against the roof, and through the windows, we saw gallons of water literally explode through the scuppers, only to forcefully hit the asphalt outside. Case in point – the setting was perfect!

This very narrow rain garden is only a couple of years old, but already very effective in diverting storm water run-off from going to our overloaded city sewers.

The idea of a rain garden is beautifully simple. It provides a place for storm water to collect, and escape down through our mostly impermeable urban landscape. Because it is allowed to slowly percolate through the ground, it filters out impurities, chemicals and pollutants before the water re-joins the ground water en route to rivers, lakes and other waterways. Rain gardens mimic the natural flow of water, and are a crucial component in maintaining healthy streams. They reduce flooding, absorb toxins, and recharge aquifers. And, since most of the plants suitable for the actual planting of the garden are natives, it also ensures beneficial support of wildlife. What’s not to love?

The plants you select will range from moisture loving to drought tolerant. In the case of those that go in the ponding area (as opposed to up the sides and atop the berm) they need to thrive in both wet and dry conditions. Plants native to Oregon and the Pacific Northwest work best here, but regardless of where you live, there will doubtlessly be site-specific natives that will do the trick. Rushes and sedges work well here. I learned a fun rhyme too, about how to tell them apart: “Rushes are round, sedges have edges, grasses have nodes, and willows abound.” Pretty fun, huh?

The making of a rain garden makes complete and total sense. Not to reiterate the entire class (and spoil your enjoyment if you decide to take it) there are a few safety guidelines worth mentioning. First and foremost – go with the flow. Obviously it needs to be located at the lowest point of your lot, and as close to your largest water-shedding surface as possible. If need be, you can adjust gutters and move downspouts to maximize the effect. The size of the rain garden should be 10% of the total area of impermeable areas draining into it. You need to make sure that your chosen spot has sufficient drainage. “Sufficient” translates into between 1″ – 2″ per hour. You never build a rain garden on a slope that is steeper than 10%  – especially not if you have neighbors down below. Water and constructs are rarely a good match, so make sure your rain garden is at least:

– 10′ from a building or retaining wall

– 2′ from a slab or crawlspace

– 3′ from a sidewalk

– 5′ from the property line

The ponding area of the rain garden should be 6″ – 24″ deep. The inflow obviously needs to be located higher up than the overflow, and the overflow should direct any extra water to the nearest storm drain. You build up a berm around and above the pooling area to prevent overflow where you don’t want it, and to direct the water toward the outlet. For the most part, an optimally functioning rain garden will eliminate any need for a storm drain, but you always have to have an overflow – just in case.

Mahonia (Oregon Grape) and Salal on the berm, and sedges and rushes growing in the ponding area.

When the plants are young, the rain garden won’t be as effective as it will once the plants mature. In fact, in Oregon you have to make sure they are watered regularly throughout the first summer, until the rains begin, or they are properly established. At the end of the workshop, we visited a two-year old rain garden. Our visit coincided with the sun breaking through the clouds again, soon after that massive rain fall mentioned earlier. I expected to see standing water, and was amazed to see that the garden already was dry! It was tremendously effective. After learning all this, I was almost a little bummed to realize that there really is no good spot in our yard to build a rain garden. Considering distances from buildings and trees, clearances and available space, there simply isn’t room. But, according to our instructor, the drinking capacity of the giant Magnolia in our front yard more than makes up for any rain garden I possibly could have installed in the same amount of space. Go figure – not surprisingly, surrounding yourself with trees is a good alternative to a rain garden. Well, duh – if we hadn’t cut down all our trees, and turned the forest floor into parking lots and roofs, we wouldn’t need rain gardens at all! But, as things stand, they are a pretty good solution to a self-made problem.

Nothing like a big tree to make things right. This is our equivalent of a rain garden.

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