Saving stormwater

You can do a lot around your house to prevent stormwater runoff, simply by opting for permeable surfaces when landscaping.

Traditionally, city flood reduction strategies involved ensuring rainfall was channelled quickly away from city streets and into stormwater drains using impermeable surfaces like paved roads, footpaths and carparks. More recently, we’ve come to appreciate the value of retaining this water in the urban environment, to keep our street trees and public and private gardens alive and cities’ ecological assets thriving.

These days, a higher proportion of water-permeable paving or no paving at all is the desired approach, and we can all help with this wider-scale stormwater management by avoiding or dramatically reducing impermeable (hard) surfaces around our homes.

What are impermeable surfaces?
As much as 90 per cent of our urban environment is covered in impermeable surfaces that don’t allow water through: materials such as concrete, stone paving, asphalt, compacted gravel, and highly compacted soil.

On the other hand, surfaces that are permeable to water include non-compacted soil, vegetable and ornamental gardens, mulched paths and garden beds, and lawns – and outside cities, grasslands, forest floors and riparian zones.

Better paving
The single best thing you can do at home to reduce stormwater runoff is to grow a garden or a ground cover lawn instead of paving the outdoors. But if hard paving is a must, then consider using materials and designs that let water through.

Some of the simplest designs for more water-permeable driveways and garden paths use traditional materials such as stone, brick or concrete, but in very limited amounts. For example, two thin strips of a hard material can support the weight of a car in a driveway while avoiding larger expanses of concrete, and a narrow line of pavers can be enough to lead you to the front door.

Alternatively, where a larger area of hard landscaping is needed, you can create a grid using those same traditional materials, with soil-filled gaps between each paver to support grass or other groundcover plants and let water through.

Gravels and stone toppings without fines make a great permeable surface too. Fines are finely crushed particles of stone or gravel that compact to create impermeability. Pea gravel (with rounded stones) is highly permeable, as the shape prevents the pebbles from interlocking and compacting, allowing water to drain freely.

There are also higher-tech options like permeable pavers made from a composite of asphalt, stone, recycled tyres and a binder, and plastic grid pavers designed to be infilled with soil or gravel that provide stability and strength as well as letting water through. (I’m a bit concerned about future microplastic waste with some of these products, though.) Councils are using some of these products to replace old roads and carparks and to create new walking and cycling paths, and they can be useful for home landscaping too.

Ground cover plants combined with paving
If you’re keen to combine paving with plants, be sure to leave considerable gaps between pavers to allow water infiltration and to give plants space to grow. Choose the right species and they will grow beautifully in the gaps and soften the paved area.

Good options include a prostrate form of chalk sticks (Senecio talinoides) with their blue-green cylindrical succulent foliage, or woolly thyme (Thymus pseudolanuginosus) that flowers pink in summer to create texture and beauty. Alternatively, plant green blob (Sedum spurium) with its bright evergreen rosettes, short-leaved stonecrop (Sedum brevifolium) with its showy white flowers, or seaside daisy (Rigeron karvinskianus). While these plants are all considered ground covers, some can grow to 15 centimetres or even taller, so give them a trim every now and then to keep them low to the ground. Whatever you choose, all plants will need regular watering for the first three weeks to help them establish.

Ground cover plants combined with gravel or mulch
If you decide to do away with hard paving altogether, then you can use gravel or mulch (for example woodchips, sawdust or straw) combined with ground cover plants to create verdant paths, sitting areas and driveways. Keep in mind that these materials work best on flat sites and are not so well suited for slopes, where rain can wash them away.

Your mulched and gravelled areas can be planted with a range of ground cover plants to increase your site’s rainfall-absorbing capacity and to green up your property. There are many wonderful plants to choose from, such as the thickly-growing creeping boobialla (Myoporum parvifolium) or pink-flowered pigface (Carpobrotus glaucescens). Lamb’s ear (Stachys lanata) is particularly popular for its soft velvety leaves. Yellow button (Chrysocephalum apiculatum) is a pretty Australian native flower that provides nectar and pollen for native bees. Creeping rosemary (Salvia rosmarinus prostratus) provides evergreen aromatic foliage that can be used to flavour a stew or potatoes, while the spiralling blue-green leaves of myrtle spurge (Euphorbia myrsinites) add architectural form to any landscape. All these plants have low water needs and are easy to maintain.

Ground cover plants to walk on
While some ground cover plants like woolly thyme are said to tolerate moderate foot traffic, it’s best to keep them on the edges of your paths rather than making paths out of them. The exception of course is lawn grasses. Couch grass, buffalo grass including popular variety Sir Walter, and Japanese lawn grass (Zoysia japonica) are some common choices. They do tend to take over garden beds though, so you may like to protect your beds with deeply set steel edging. If you do decide on grass for your pathways, use a sandy loam soil substrate to reduce compaction from foot traffic.

In a drying climate it might seem counterproductive to increase garden areas and to establish lawns and ground covers. The temptation to pave is high. But lawns and gardens are our most permeable materials; they soak up rainfall, reduce stormwater runoff, and produce evapotranspiration that regulates important local water cycles and cools down our homes. If you have the opportunity, start planting.