Storm Water, Rain Gardens, and Native Plants
"Storm water" sounds pretty technical, but it can be simply defined as water that runs off of hard surfaces like roads, driveways, and rooftops. Along the way, this water picks up and carries pollutants like nutrients, heavy metals, oils, and other compounds. Eventually, this storm water finds its way into storm sewers that drain into our favorite urban creeks, wetlands, and lakes. Over time, these pollutants can accumulate and cause adverse effects on aquatic systems.
In Minnesota, cities, counties, and watershed districts are in charge of managing storm water. One of the most popular ways to reduce and treat storm water is by creating rain gardens. Rain gardens are just engineered low spots in the landscape that trap and infiltrate storm water.
Some consider rain gardens as natural filters that help to clean water. Every rain garden helps to reduce pollutant loads and preserve and improve our aquatic systems. Cumulatively, they can make a big difference.
Some of the most effective rain gardens are designed to treat road runoff. A common method involves strategically cutting a section of curb out of the roadside, and allowing water to flow off the road and into constructed garden areas. An average "curb cut" rain garden is able to treat thousands of gallons of water each year.
To live up to their name, a majority of the rain "gardens" in Minnesota use plants to help in the filtration process. So how do plants play a key role? And what happens to the pollutants that end up in the rain gardens?
A large percentage of the nutrients in storm water are absorbed and retained by the plants. For instance, native plants are able to absorb dissolved phosphorus, ammonium, and nitrates. Research has shown that some plant species are better than others at using nutrients in the storm water. For example, dogwood , Joe-Pye weed, sunflower, and Blue flag iris are great at absorbing nutrients. Plus, they are able to handle both wet and dry periods. Of course, plants only absorb the nutrients during the growing season, making nutrient uptake during winter less effective.
Also, once the plants die back each year those nutrients can be released as they decay. It is a good strategy to remove the old vegetation before it decays so the nutrients are better managed.
Heavy metals come from industrial, manufacturing, and transportation activities. These metals can be attached to soil particles or dissolved into the storm water. Plants growing in rain gardens can actually accumulate metals into their tissues (LeFevre et al., 2014). Some plants are more efficient at doing this than others. Species that are especially good at absorbing metals are called hyper-accumulators (Salt et al. 1998).
Plants can also uptake organic pollutants like hydrocarbons (from oil leaks, emissions, grease, and tire particles). Plants house microorganisms in their roots that also help break down contaminants. Their roots also aerate the soil, increasing levels of oxygen that encourages aerobic chemical breakdown.
Rain gardens can greatly benefit Minnesota's waters. They can be especially important in urban landscapes where we try to blend development with environmental responsibility. Their main purpose is to intercept the storm water before it runs into our creeks and lakes; however, well designed rain gardens can also look great and provide food and habitat for pollinators. Native plants are essential for our pollinators, and play an important role in water quality.
Interested in learning more or about installing your own raingarden? Contact us and we can help get you started!