A rain garden is a temporary, water holding basin that captures water runoff from walkways, roofs, lawns or other impervious surfaces. It is a sunken area filled with water-absorbing plants and well-drained soils.
The main function of a rain garden is to manage inconsistent rainfall. A rain garden can soak up to 30% more water than a lawn. It helps to recharge groundwater supply, supports wildlife habitats and removes pollutants from water.
Planning and site selection
Create your rain garden on a slight slope so that it receives water easily. Keep it at least 3 m away from buildings. Try not to position the garden over septic systems and under trees as these places are easily damaged by high soil moisture. Avoid filled areas, steep slopes or sites with very sandy soils.
Design the garden with its longest end perpendicular to the slope to maximise the border that intercepts water runoff. The length of the garden should be twice its width to provide enough space for plants to absorb water. Make sure the outlet of the garden leads into an area that can handle large amounts of water at a time.
The size of the garden depends on the size of the impervious surface draining into it. It should be 20% to 30% the size of the impervious surface. The deeper the basin and the greater the draining capacity of the soil, the greater the volume of water that a rain garden can accommodate.
Digging
A rain garden is usually 10-20 cm deep, depending on the soil type, slope and size of the garden. Make sure the garden bed is level to avoid water pooling on one side. A well-designed garden will drain within 12 to 48 hours. This ensures the health of the plants and prevents mosquito breeding.
Replace heavy soil with a fast-draining mixture that consists of one-half sand, one-quarter compost and one-quarter topsoil.
Pile stones and extra soil on the downhill side of the garden to act as a berm (raised edge) and create a bowl where water can pool to a depth of about 15 cm.
If water does not naturally flow to your rain garden, dig a trench, 7-10cm deep, from your downpipe to the garden. Line the trench with landscape fabric and cover it with stones to create a streambed effect.
Planting
Interestingly, a rain garden is not meant to be a place for wetland plants. Instead, you should grow plants that tolerate temporary flooding and drought, since water will come and go. Choose some hardy, preferably native, plants that require little maintenance.
Try to have a variety of plants with different forms and sizes. Trees and large shrubs deflect and slow rainwater flow. Tall grasses filter out impurities, suck up water and prevent silt from entering ponds or rivers. Short, deep-rooted plants maintain soil structure and direct water into the ground.
Generally, rain gardens have three planting zones that are characterised by soil moisture. Select plants for each zone according to their water needs.
Bottom: plants that tolerate wet conditions
Slope: plants that tolerate occasional standing water
Edge: plants that prefer drier soils
Maintenance
Mulch the garden with compost or shredded bark. Mulching helps to prevent erosion, conserve moisture and suppress weeds. If the water that flows into the garden washes out the mulch, place one or two rocks to break up the flow.
Water new plants regularly, including the drought-tolerant ones. This will encourage deep roots that absorb water more efficiently.
Weed the garden to allow the selected plants to get established. Prune plants that overgrow to keep the inlet and outlet clear of obstructions. Introduce groundcovers and decorative rocks to prevent soil erosion.
References:
Myers, M. (2013, December 12). Plant a Rain Garden. Retrieved from Birds and Blooms Web site: https://www.birdsandblooms.com/gardening/flower-gardening/plant-rain-garden/
Pitcher, J. C. (2008). Rain Garden. Seattle. Retrieved from https://s3.wp.wsu.edu/uploads/sites/2102/2016/03/Rain-Garden_LowRes_Page_2.jpg
Sweetser, R. (2020, July 26). Create a Rain Garden: Two Designs and Plant List . Retrieved from The Old Farmer’s Almanac Web site: https://www.almanac.com/content/rain-gardens-two-designs-and-plant-list
Urban Biodiversity Initiative (UBI) 
