Replenishing Rainwater Gardens
Groundwater Part 3
By Andrea Peck
If you're not careful, it will sneak up on you. Just simply mosey along any old sidewalk and sooner or later you'll walk past one. It will probably be difficult to spot this clandestine garden feature since it is camouflaged as an ordinary plant bed. But, don't be fooled – you may be looking at the super hero of water harvesting: the rainwater garden. Rainwater gardens may be hidey, but they address a multitude of water sins just by existing.
Rainwater gardens are slight depressions in the garden that are planted with a special blend of drought tolerant and water tolerant flowers or grasses that allow water to pool and soak into the ground. Often native species are well-suited for the job because they tolerate local rainwater vagaries. They also tend to grow long roots that help filter out impurities that accumulate as the water runs into the system. Generally, rainwater gardens are placed in an area that naturally has a water flow towards it. This natural element is then tweaked to pull in more water, allowing it to percolate into the soil. Rainwater gardens intercept the bad boy of the water world: runoff. Runoff can start out as pure as the driven snow, but inevitably turns into a bad apple by collecting pollution as it rolls towards streams, oceans and rivers. Runoff causes general upheaval wherever it runs, often in the form of flooding.
Creating your own rainwater garden is a possibility for the do-it-yourselfer who is willing to do a bit of planning and research. (I'll give an outline here, but for a thorough understanding it is best to research fully before taking on this project.)
You will first need to determine the size of your roof or other impermeable surface that will be feeding into the rainwater garden. A general rule of thumb is to build a rainwater garden that is 10-20% the size of the section of roof or surface that will supply the water. The garden should be built 10 to 15 feet from the foundation of your home. It should not be located near a septic tank, utilities or other structures. Make sure that you select a safe spot – water can wreak damage. Once you find an empty spot that is the right size, you must test your soil for permeability. One way to do this is to dig a hole 1 – 2 feet down and fill it with water. (During dry weather it is suggested that you fill it three times and “measure” the third dousing). Note the water depth and the time. Wait until all the water is drained, note the time again and follow this calculation:
Number of inches of water ÷ the number of hours it took to drain =
Example:
12 inches ÷ 8 hours = 1.5 inches per hour
If your soil drains at .50 inches per hour or better, it is considered well-draining soil.
If you had to bring out the big guns to dig the hole, or if your water is refusing to drain, you may be looking at a spot that has hard-packed soil; you may have to consider another location. Pick another spot and try the drainage test again.
Finding a good spot will be the beginning of your real work. You will have to measure off the area and dig down 8-12 inches. Also, rainwater gardens need an access point, so you will need to build that in. Many gardeners choose to create a rocky swale, reminiscent of a natural dry stream to bring the water from its source (such as the roof downspout) into the rainwater garden. The rainwater garden must also have an overflow in the event of extreme rains.
Before planting, make sure that you have good soil. Generally, you will have to amend the soil, so be prepared to dig a bit more. For well-draining soil, a mixture of 65% native soil and 35% compost is a good mix. For poor soil, 60% screened sand and 40% yard-waste compost is a good combination. Mix the soil well and maintain the 8 to 12- inch depth. Make sure that the basin of the rainwater garden is level. This will prevent pooling in an uneven fashion.
Many plants, including groundcovers, grasses, shrubs, even trees, are appropriate in the rainwater garden. Some examples include: yarrow, manzanita, hair grass, Indian paintbrush, California poppy, sage, manzanita, toyon, coffeeberry and trees, such as California bay laurel, willow and California sycamore.
When planting, leave room for ample growth and finish by mulching. Weed as necessary, water to establish plants and enjoy when rain finally drops.
For more ideas and a video of how to build a rain garden click the link below and enjoy!