Once a year the Virginia Home Grown Team is at the RVA Big Market in Richmond’s Bryan Park. We have a wonderful time interacting with everyone who stops by. I particularly enjoy answering gardening questions, and this year a common question was about rain gardens.
Many gardeners take the name literally thinking the garden is like a pond, while others don’t understand how it works. Another person didn’t realize that specific plants grew in a rain garden.
I do understand the confusion so I will start with a simple definition: a rain garden is a shallow depression in the landscape designed to capture water from hard surfaces such as roofs, sidewalks and driveways.
Once captured, the water stills and the silt settles out of the runoff, plant roots and soil absorb pollutants, and within 48 hours, the cleaner water filters down to replenish the groundwater. Between rain events, the rain garden is dry.
Plants are superheroes here because their roots absorb the heavy metals, phosphorus and nitrogen found in rainwater, or picked up flowing along the hard surfaces. Their teammate is the soil as it traps and prevents pesticides, oils, bacteria and other heavy metals from polluting our waterways. Not every plant can withstand having their roots submerged for a day or two and then be subjected to periods of dry conditions between rainfall. Equating these growing conditions to the fluctuating water levels in a swamp, wetland or stream provides some understanding. Realizing the center of the rain garden will stay saturated longer than the edge guides plants choices, too.
Using native plants found in wetter environments will ensure success and support our native pollinators at the same time. Of course, I want my rain garden to be beautiful and pollinator friendly, too! A medium to small rain garden plant list includes shrubs, grasses and perennials and trees if larger in size. The Plant Virginia Natives website has a thorough list of Virginia native plants suitable for rain gardens.
When making plant choices think about the flower color, the plant height, and when they bloom for season long beauty. Choose a handful of plants then create drifts of each type. Groupings of a plant are aesthetically pleasing, simpler to maintain and easier for pollinators to find.
What size to make a rain garden can only be answered by calculating the runoff area that will flow into it. The Rain Garden Alliance has simplified the calculations with a fillable form using the measurements of your roof or hardscape. Don’t get too concerned about digging a deep hole because this depression is only 6”–8” deep, while the length and width depends on the results of the calculations of your hard surfaces. A rain garden “bowl” can be as small as 7’ x 8’ and 6” deep.
The key is the drainage. To test the drainage, dig a 12” deep hole about 12” wide where you want the rain garden and fill it with water. After 24 hours check the hole and if the water has percolated into the soil, then move forward with your plans. If not, then the top 12-36” of soil of the rain garden needs to be amended for proper drainage. To do so, amend the soil using the ratio of two parts sand to one part compost and one part existing soil. The depth of the amended soil depends on the percentage of water remaining in the hole; the more water, the closer to 36” — the depth of the amended soil needs to be. Remember, standing water is a pond which leads to mosquitoes, so amending the soil so it percolates is the key to success.
The last touch to finish the garden is to apply a 2” layer of mulch to reduce weed growth and to retain soil moisture between rain events for the plants. However, rain gardens are not without maintenance. While plants need tending and mulch needs refreshing, a rain garden depression will slowly fill if the sediment collecting on the surface of the mulch is not removed annually. Given the many benefits a rain garden provides the environment, this is a small task in comparison.
Happy Gardening!
Peggy Singlemann