Feature


Rain Gardens: The “Natural” Way to Remove Storm Water Pollution


Author: David Kuack, HRI Freelance Author dkuack@gmail.com


Horticultural Research Institute is funding research to identify alternative substrate components and additional plants for use in rain gardens.


Te U.S. Geological Survey reports that only 2.5 percent of all Earth’s water is fresh water. Almost all fresh water is locked up in ice (68.7 percent) and in the ground (30.1 percent). Just over 1.2 percent of all fresh water is surface water.


With concerns about the availability and quality of water, more municipalities are looking to protect this limited natural resource. Government officials are increasingly passing legislation and regulations to protect fresh water sources.


With funding from the Horticultural Research Institute, North Carolina State University horticulture professor Helen Kraus is studying how rain gardens can be used to protect fresh water sources from storm water runoff. She said all segments of the horticulture industry, from growers to end users, face the same problems with water.


“We all have to have water to grow plants well and it has to be good quality water,” Kraus said. “Rain gardens are a device that can help to remove pollutants from storm water runoff. As rainfall runs across the hardscapes of the urban environment it picks up nutrient pollution, including nitrogen, phosphorus and metals like iron, manganese and copper. It also picks up soil sediment that carries nutrients with it. Storm water usually goes into a storm drain which carries that water directly into an open water source and pollutes that water source.”


Kraus said regardless of how much rainfall a region receives, rain gardens are designed to intercept the water runoff generated from rainfall and remove the pollutants in that water.


“Te purpose of the rain garden is to have that water filter down through the rain garden into underground water reserves in a clean state after the pollution has been removed from it,” she said.


Better rain garden substrates 16


Kraus said when a rain garden is being constructed the existing native soil is removed creating a depression or pit in the landscape. Te depression, which is typically about 4 feet deep, is filled with an engineered substrate that is designed to have quick water infiltration.


“During a rain event, large volumes, literally hundreds of thousands of gallons of water, can enter the substrate,” she said. “Te substrate is designed to slow the rate of percolation or movement down through the rain garden.”


Kraus said the substrate should be biologically- rich containing a wide array of beneficial fungi, bacteria and other micro-organisms that break down the pollutants in the water. She said some rain garden substrates contain pine bark as their organic matter component.


“Pine bark does not bring a lot of this microflora to the substrate,” she said. “Te water slowly inoculates the substrate with microflora over time.”


Kraus said the nursery industry needs pine bark as a growing media component. Her research seeks to identify a better source of organic matter to replace pine bark.


“Compost, such as yard waste, naturally brings the microflora with it,” she said. “Microflora develop during the composting process. Compost is a better source of organic matter for the chemical reactions that have to occur in order to break down storm water pollutants.


“Every municipality creates yard waste and many compost it. Yard waste compost could be a more effective substrate component and it will help to diminish the demand for pine bark that currently exists.”


Expanding rain garden plant choices


For the HRI-funded rain garden research project Kraus is studying four plant species: Panicum virgatum ‘Shenadoah’, Monarda fistulosa, Itea virginica and Betula nigra ‘Dura Heat’.


“Over a combination of research projects I have looked at about 25 species,” Kraus said. “We are


Spring2016


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