It hasn’t rained here for weeks, but when it does, it’s likely to pour, as we have lots of storm event rain, where long dry spells are followed by a real drenching. According to climate change predictions, this episodic rainfall pattern is likely to become even more pronounced in the future.
I’ve got rainwater tanks harvesting as much water as I can find room to store. There’s nearly 10,000 litres (2600 gallons) in 4 tanks of assorted sizes and shapes hidden under the back deck, beside the house and next to the carport which all feed into each other. I’ll explain more about how to do that in another post.
But I’ve still got a few downpipes that I can’t connect to tanks that send roof water coursing into the stormwater drains. We’re on sandstone bedrock so finding a way to capture, hold and slow down that water before it ends up flooding into the local creek wasn’t easy. We also have a sunken basement that has a drainage sump and pump behind one wall. When the pump cuts in, it sends a surge of water into the system too. I found one suitable rain garden spot down the back near the swimming pool, and several years on, it seems to be working well and the Iris pseudoacorus I planted in it is growing well. After heavy rain it’s full to the brim but drains within about 2 hours.
Stormwater solutions, mostly designed by engineers whose brief was to protect built environments, used to be to get all the water away as quickly as possible, and be glad that the rain had washed everything clean for us. Over the years, however, we’ve learned that accelerated runoff from hard surface areas of houses, roads, driveways and paving, is scouring and eroding our river systems. Our love of paving the world also means that rain water isn’t percolating down into the water table as it should, denying larger trees the deep groundwater on which they rely.
Stormwater run-off picks up lots of contaminants from roads, rooftops and gardens and sweeps them all into our water ways, causing significant river pollution from petroleum based products, brake and tyre dust, street litter, pesticides, animal droppings and garden fertilisers. In cooler climates, rainwater heated by warm road surfaces in summer can cause a warm-water influx into local rivers, damaging fragile eco-systems.
How rain gardens work
Rain gardens hold stormwater and filter it down through a sandy and organic soil layer, filled with plant roots. While sand filters out larger particles, the micro-organisms that live in a symbiotic relationship with organic matter and plant roots break-down petroleum-based products and high nitrogen levels from excessive fertilising, cleaning the water before it gets into the stormwater system.
Small amounts of clay in the system also adsorb pollutants like heavy metals and hydrocarbons. Many water authorities throughout the world now use Water Sensitive Urban Design (WSUD) for management of street stormwater, putting in rain gardens (or bio-retention systems) in median strips, kerbsides and parks.
Although you might think your own home garden can’t make much of a difference, if enough people try, collectively even a few square metres each can be effective in cleaning pollutants out of our stormwater, and slowing the water down to it does less damage as it enters local rivers.
And you don’t need to worry about ending up with a damp garden that harbours mosquitos. Rain gardens are designed to drain within about a day so that micro-organisms (which need oxygen) can begin their work as the soil dries out, so the garden doesn’t stay wet and encourage mosquitoes.
Choosing the right place
Home rain gardens either catch surface water running off driveways, paving and lawns or receive stormwater from roof downpipes. A rain garden is not the same as a bog garden. The stormwater has to soak quickly into the soil, so you can’t use a poorly draining part of your garden. Rain gardens next to your downpipes can collect and treat your roof water (or the overflow from your water tank), or be sited further away and receive the water by swale (an open grassed channel), a gravel filled channel or underground pipe.
Rain gardens for hard surface areas such as paving or driveways collect runoff that would normally drain across an adjacent lawn but make sure that the water can’t ever overflow into your neighbour’s garden. As you’ll need to dig out some soil, avoid tree root zones, underground service lines and slopes steeper than about 8% (1:12).
Make it part of your garden
Integrate your rain garden into your existing garden layout by using the same curves or lines as your original garden. It will integrate even better if you
repeat some of the plants or pebble mulch you choose for it in other garden beds. On the surface, the garden needs to be about 1-2 square metres (10 square feet) per downpipe and can be any shape you want, although it does need to be physically separate from other garden beds. While you won’t want it right up against the house, it can be a good way to grow plants under wide house eaves where they would not normally get rainwater.
Downpipe rain garden
These rain gardens start as a pit about 40-80cm deep, dug down next to the existing subsurface stormwater pipe connection. The bottom of the pit slopes slightly away from the house and the uphill side closer to the house is water-proofed to protect house foundations. A perforated pipe laid in sand or gravel in the bottom of the trench is connected back to the stormwater to pick up the cleaned free water as it slowly percolates down and fills the bottom of the pit.
A vertical overflow pipe from the stormwater, to just below the pit surface, picks up any water that exceeds the capacity of the rain garden, so there’s no flooding. A sand and organic soil mix provides the planting and filtering medium, which is then covered with a pebble mulch. This helps conserve moisture during dry periods, prevent organic matter from floating upwards and avoid scouring from the water pouring out of the downpipe during heavy rain. Connecting a spreader pipe to split the water flow across the width of the garden is also a good idea. The finished surface level is about 10-15cm below the surrounding area.
You could even used a raised garden bed under a downpipe for similar effect if you’re unable to dig one in. It would need to be at least 500mm deep, with room left at the top to take water flooding in, and a drainage system connected to the base, to take water back to the stormwater system.
Dry river bed rain garden
Some of the most beautiful rain gardens I’ve seen look like a dry river bed when it’s not raining, with artfully placed boulders, large pebbles and gravel mimicking nature’s own exemplary design style. This works well when you’ve got large amounts of hard surface run-off that you need to concentrate into one run-off channel before it gets to the rain garden itself, like in this show-garden design (left), where the creek bed flows during storm events, fed by the open downpipe. They also double as a great children’s play area.
Surface drainage rain garden
Position your rain garden alongside the lowest point of the hard surface area. For paving, an appropriate size is about 5% of your area of hard surface. For example 30m2 (330 sqft) of paving needs about 1.5m2 (16.5sqft) of rain garden. If the soil already drains well, all you need is a shallow, saucer-shaped depression about 15cm (6in) deep for the water to pond in and a 15cm (6in) high berm, or raised lip, of hard packed soil on the low side to hold it. Poor draining soil requires a deeper excavation and soil replacement with a sandy, organic mix. You can mulch with pebbles, although an organic mulch may encourage micro-organisms to better filter petroleum based products. Plant out with grasses and small shrubs. If you have poor soil drainage, connect an underground drainage pipe on a fall from the bottom of the rain garden to carry any free water back into the stormwater system. This is called an under-drain system.
Plants need to withstand both temporary inundation and dry periods, like those that grow naturally along river banks. A surprising variety can do just that, and there will be suitable plants that are native to your local area, whatever your home country, although they may need supplementary water in the first couple of months. Many rushes, sedges, reeds, grasses and grass-like plants like iris can thrive in these difficult conditions (although you need to check the suitability of each individual species).
In Australia, those genera include Juncus, some Dianella and Lomandra, Gahnia and Eragrostis and you can use groundcovers like Hardenbergia, Goodenia and Myoporum.
In larger and deeper rain gardens, a layered effect of low, medium and taller plants will look more interesting and assist water percolation. Australian native shrubs that cope with temporary inundation are Callistemon, Melalueca, Lomatia and Kunzea.
In the USA, there are many states where rain gardens are common and there are suitable local plant lists, such as Michigan, Oregon, Kansas, Virginia and California. There’s a very comprehensive design and construction guide by the University of Wisconsin.
In the UK, this handy guide includes design ideas and planting suggestions.
Establishment and maintenance
Give your new plants some water if there’s not much rain while they’re establishing, which may take a a couple of months. Weeding is a necessary evil, as weed seeds will be washed down from other parts of the garden. After a few years, accumulated silts will gradually fill up your raingarden, so you will need to periodically scrape some off with an earth rake and redistribute it elsewhere in the garden. By this time, your rain garden should have become a unique mini ecosystem providing good habitat for insects and reptiles.
Getting your town council involved
Kerbside rain gardens hold road run-off and trap pollutants, as well as beautifying your street. Many enlightened cities in the world, such as Portland, Oregon with its Green Steets program, try to retrofit them into existing streets where possible, training local residents in routine maintenance. A simple opening in the kerb directs road stormwater into the rain garden, with overflow redirected to the storm drains. Some drain to groundwater, and others combine this with slow drainage to existing drains.
Singapore, with its amazing tropical downpours, is creating huge raingardens to capture the runoff from apartment developments, making places of great beauty for the residents at the same time.