Amy Tsuneyoshi
Hawai‘i Herald Columnist
Rain gardens are a constructed vegetated depression that is usually located where water collects or flows off your property. It’s a great way to briefly capture stormwater runoff and allow water to seep into the ground. It reduces flooding and can filter about 80% of the pollutants such as dirt, fertilizer and other contaminants picked up as the water flows over surfaces before entering storm drains and streams, and empty into the ocean. Here is a peek into creating a rain garden:
Planning
Assess the area and identify where the water enters and exits the property. Identify the lowest part of the property where the water will drain as this will likely be the best location for the rain garden.
Do a soil infiltration test for this area to see if the soil will be able to drain at a reasonable rate. Dig a test hole about one foot deep and fill with water. Note how long it takes to drain and repeat two more times. If the hole drains within a couple hours each time it is filled, the soil is suitable for a rain garden. If the hole still has water in it six hours after filling it the third time, the soil does not drain adequately. A different location should be considered or an alternative stormwater retention practice should be used.
The main source of storm water will likely come from the roof. Other impermeable surfaces, like a concrete driveway, also contribute to storm water runoff. Observe where the water flows. Does it all drain into one area, or does the water flow off the property in multiple spots? Estimate the appropriate size of the rain garden by measuring the area of impermeable surfaces (i.e., roof, driveway) that drains into the area. The rain garden should be around one-tenth of that area to be able to accommodate the water flow during storm conditions. For more details on how to size your rain garden, please check the resources listed below.
Construction
Check with your county agency to determine if permits are needed and if there are any underground utilities where the proposed rain garden will be located at least one week before starting.
Planting
Choose native plants that are specific for that elevation and rainfall zone. Native grasses and sedges and other groundcovers will form matted root systems to help stabilize the soil. Water the plants well when they are first planted and then water weekly for a few months or so. Once established, they can survive with minimal water and tolerate brief periods of being inundated by storm water.
Maintenance
Once your plants are well established and thriving, you can reduce watering to only as needed. Regular maintenance like weeding and pruning is needed to keep the garden looking nice. Depending on how much sediment the rain garden is capturing, you may need to periodically clear the sediment to keep the rain garden functioning properly.
The information here is just a brief introduction to rain gardens. Below are great resources for more details on the placement, construction, and maintenance of rain gardens.
Resources:
Rain Gardens – Hui o Ko‘olaupoko: huihawaii.org/rain-gardens.html
Rain Gardens – Storm Water Hawaii: stormwaterhawaii.com/tips/rain-gardens/Hawaii One Call Center at 811: digsafelyhawaii.com/ or
866-423-7287
Soak Up the Rain: Rain Gardens (US EPA): epa.gov/soakuptherain/soak-rain-rain-gardens
