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How to Handle Rain Part Two: Rain Gardens

Posted By Ashleigh Bethea on May 28, 2014 | 0 comments


One of the numerous negative ecological effects of urban development is a higher rate of soil erosion. Forests naturally hold on to soil with their roots. Trees slow the fall of raindrops to keep them from disrupting the soil. The natural bumps and hillocks in the landscape break up the flow of water, giving it more opportunity to be absorbed by plant roots and filtered through the soil before it winds its way into creeks, streams, and rivers.

These natural soil-defense mechanisms do not exist in developed land, where rain falls on rooftops, asphalt, and flat lawns covered in relatively sparse, shallow-rooted plants.

All this means that on developed land, wind and rain carries off much more top soil, dumping it into storm drains and into the water table. This not only degrades the soil quality, but also dumps soil into the local water supply, along with oils and often-toxic pollutants.

For the sustained health of your garden and your community, you should try and minimize erosion and runoff as much as possible with careful garden design. Where downspouts empty onto your yard or where storm waters flow through it, you should take every effort to absorb and filter this water. A well-designed garden will capture water effectively, keeping plant roots moist much longer while also holding on to the soil’s nutrients and keeping pollution out of the local water table.

Double Link Copper Rain Chain

Double Link Copper Rain Chain.

Rain chains, barrels, and gardens are all good ways to capture rainwater before it can cause erosion and pollution.

Depending on how much rainfall you get and how much surface area your roof covers, you might need to employ several of these methods. Rain barrels and chains are simpler and easier to install, so let’s look at them first.

Rain chains are the traditional Japanese way to drain water from your roof.

As opposed to conventional downspouts, which pour water onto the lawn at full force and tend to have a rather utilitarian look, rain chains slow down rainwater as it dances over the chain in a beautiful display. A bowl at the bottom of the rain chain increases its erosion-controlling power even more. Yet rain chains have their limitations—they can not handle very high volumes of stormwater as well as conventional downspouts.

Impressions Bark 90 Gallon Rain Saver

The Impressions Bark 90 Gallon Rain Saver.

Rain barrels are an ingenious way to turn a negative into a positive. A rain barrel not only stops rushing stormwater from eroding your soil. But it also provides you with a healthy, free source of water for your plants.

Rain water does not contain chlorine (like city water) or salt (like softened water), so it is less likely to harm your plants than water from the hose. And for most of us gardeners, watering lawns and gardens accounts for at least 50% of our water usage. A rain barrel can greatly reduce that amount, especially if you combine this natural water source with an efficient irrigation system.

Before you install a rain barrel, however, you should check your local laws and regulations–believe it or not, places like Colorado have outlawed all types of cachement systems for capturing runoff or snow melt, the “logic” being that this rainwater actually belongs to citizens further down-stream that rely on river water to irrigate their arid farms. That’s right, Coloradans–Arizona owns the rain.

The limitation of a rain barrel is, of course, its capacity. The Texas A&M Agrilife Extension Program offers this equation to figure out how much rainwater you will capture: Harvested Water (Gal) =Roof Area (Square Feet) X Rainfall Depth (Inches) X 0.623 (Conversion Factor). The average house has a roof with an area of about 1,500 square feet, which means that during a 1-inch rain event that roof will yield over 900 gallons of water! Obviously your average rain barrel will not hold that much. You could expand your water storage with multiple barrels, but if you live in a very rainy area the amount of storage you would need becomes highly impractical. Some rain is still going to need to drain onto your land. Enter the rain garden.

Rain gardens are designed to have a symbiotic relationship with storm water.

They are ideally sourced right in the path of storm waters or in low areas where storm water pools. Where a normal patch of lawn would become a bog under this deluge, a rain garden will thrive. With plants from 3 different zones absorbing and filtering the water before it ultimately drains away.

A well-designed rain garden will drain well, infiltrating all the water from a 1” rain event in 4 hours or less (this means 1” of rain as measured by a rain gauge, though much more water may accumulate underneath a downspout). Sometimes this is achieved by having an underdrain system that sends water to storm drains after it has passed through the rain garden. At minimum the soil must be well draining in that area to allow storm water to infiltrate and be absorbed by plants. Good drainage can be achieved by digging down 1 to 3 feet and incorporating material like sand or fine gravel to the soil to allow water to pass through.

Rain Garden Graphic

Rain Garden Plan from the Washington State University Extension Program

Once decent drainage has been achieved and you are putting the soil back into place. Create a basin shape that directs water to the center of the rain garden.

The outer edge of the garden should be a berm that rises above the soil line. To help hold in the water your rain garden captures.

This will create a rain garden with three distinct zones: Zone 1, the wet zone, where standing water is common, Zone 2, the moist zone, where standing water happens occasionally, and Zone 3, the dry zone, where standing water rarely happens. Each zone calls for its own particular type of plant.

Black Truffle Cardinal Flower

Black Truffle Cardinal Flower

For Zone 1

you want to install plants that love moisture and don’t mind “wet feet” even for long periods of time. These plants are often referred to as bog plants. Some good choices here were mentioned in Part 1 of this series, like Rodgersia, Astilbe, Lobelia, Japanese Iris, Joe Pye Weed (Eupatorium dubium), Astilbe, and Calla Lily. A few other great water-loving plants are Ferns, Sedge (Carex), and Cardinal Flower. These plants will constantly filter and absorb rain water until it becomes fully infiltrated.

Summerific Cherry Cheesecake Rose Mallow

Summerific Cherry Cheesecake Rose Mallow

For Zone 2

you want to install flexible plants that can tolerate short periods of standing water but prefer moist, well-drained spots. Often the best choices for this zone are plants that have evolved in riparian zones. The areas at the edge of riverbeds, because these plants know how to tolerate standing water during floods and later. As water recedes, they also know how to search it out with their deep roots. Thirsty plants that do well in Zone 2 are Rose Mallow (Hibiscus Moscheutos), Holly (Ilex Verticillata), Hydrangea paniculata, White Fringetree, Cannas, and Daylilies.

Adagio Miscanthus

Adagio Miscanthus

The best plants for zone 3

are often Xeric, meaning they are adapted to tolerate long periods of drought. Many of these plants evolved in deserts or prairies where rainfall is infrequent. It may seem illogical to choose plants that evolved in deserts for rain gardens, but the reason is because if Zones 1 and 2 are doing their jobs, Zone 3 will be so well-drained as to become quite dry.

Xeric plants are quite useful in the rain garden, though, because of their ability to absorb water very quickly.

In the desert, when rain comes, the plants have to suck up all of that water they can get. Xeric plants are ideal for reducing the volume of storm waters. The plants that do well in this zone are: Monarda, Butterfly Weed, Heucherella, Miscanthus, Switchgrass (Panicum virgatum), Coneflower (Echinacea purpurea), Black-Eyed Susan (Rudbeckia), and Aster.

When choosing plants for your Rain Garden, as with any garden, you will get the best results if you choose plants that are hardy in your USDA zone. Prefer the amount of sun or shade the area will receive, and plants that are native to your locale.

One more consideration is when the plant’s growing season is. When plants are actively growing they tend to “drink” a lot more, but when they are dormant, they tend to “drink” very little. Include a lot of plants that start growing early in the season to soak up those spring squalls. As well as some plants that keep growing through summer and fall, to soak up the rest of the year’s downpours.

If you live in a dry climate or you just have problem areas in your yard that seem to dry out more than the rest, check out the next blog in the series. Where we discuss Xeriscaping and how to build Swales.

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