Walking along the streets of downtown Seattle, you may run into awesome stormwater management projects such as this one. On the block between Harrison and Republican Street, the east side of Potinus Avenue features a roadside bioswale spanning the entire block. Who needs street parking?
The design is a simple wave, expressed alternately by the concrete curb and aluminum planting rail. Gaps between curb modules allow water to flow from a high point at the building down into the swale, where is can be cooled and filtered before entering the storm sewer. Information boards are posted on either end of the swale, outlining the water filtration process.
I noticed very few weeds, if any, on this planting. Why is that?
We all know that there is little maintenance provided to public street plantings, so something else it at work here. One possibility: the planting density. Densely planted beds cast a shade canopy over the soil, inhibiting germination for most weed seeds. But that can't be the full story--these grasses cast more of a dappled shade over the ground. So what's the trick? A hint: check out the soil.
The second picture shows exposed soil next to some river rock. Check out the color. It does not look like a rich, fluffy, dark loam now, does it? Unproductive soils are a great way to control weeds. By specifying a soil mix with less organic matter, and planting species native to unproductive soils, you drastically reduce the number of weeds due to nutrient stress. Species that perform well in bioswales must be tolerant of drought stress as well as occasional flooding, which conveniently matches the profile of some less productive soils, such as those derived from limestone (calcicoles). Something to consider!