By Todd Layt
Summary Guide to Successful planting of Bio-retention swales and basins:
- Use Evergreen Plants, as they reduce nutrient all year. Eg; Lomandra varieties that handle wet feet. Avoid plants that go dormant such as Juncus, and Carex.
- If using turf, use Zoysia, as it needs far less maintenance than other turf. Eg; Nara™ native turf in swales can be mown as little as twice per year.
- Plant so that when mature there are few gaps for weeds to invade.
- If temporary erosion matting is used, make sure it is a type of jute mesh with holes that allows water to penetrate. Thick jute mats become hydrophobic harming the growth of plants, and reducing the swales ability to drain.
Here we have a more in depth and detailed look at green life for bio-rentention swales and basins.
In recent years the lack of space for wetlands to clean water prior to outflow to stormwater, and the increase in hard surfaces, has seen the use of bio-rention swales and basins increase. The vegetation of bio-rention swales basins often fails in the short and long term due to poor species selection and erosion. To be successful you need to protect the soil in the short term, yet have long term success with robust plants that not only survive the harsh condition of wet and dry, but also help prevent erosion. So what are the best techniques and the most appropriate plants? Turf has been well documented to be the best method to not only reduce erosion but also the best method to clean the water.1 However turf is not the only solution, as new breeding has developed a number of native plant species that perform this task well.
For bio-retention swales native plants or turf are generally preferred compared to exotics. Although exotic Empire™ Zoysia japonica ‘SS500’ does this extremely well, the Native turf Nara™ Zoysia macrantha ‘MAC03’ is becoming extremely popular for this use because it is native and it performs well. The advantages of this instant native turf provide compelling arguments for its use. Firstly research shows instant turf is far better than plants and geo textiles at preventing erosion. 1 In other research Nara™ Native Turf was shown to increase the strength of soil more than other turf types including couch and Kikuyu. 2 Nara needs far less mowing than those types, and can be left natural without mowing and still remain over time under 30cm high. Nara™ has shown great ability in trials to handle both dry conditions and wet feet in Bio-retention swales.3 In other research, Nara™ has been shown to be far stronger when pegged in the ground and will not dislodge as easy as couch. 4 Turf in general has been presented in research to clean water extremely well in both removing nutrients and heavy metals. 1 Finally, it is far more cost effective than geo textiles or plants. 5
For aesthetic and bio diversity reasons plants can be a desirable component of bio-retention swales. However a lot of the native plants that have been used brown off badly in drought and winter, leaving aesthetically unappealing landscapes, and leaving long periods of dormant plants that have little effect in nutrient reduction in dormant periods. The public prefer evergreen plants. Recent breeding in wet feet tolerant Lomandra has not only provided plants that can handle wet and dry7 and remain evergreen, but an array of visually pleasing plants with different heights, textures, and even colours. Generally till recently only Lomandra hystrix types were available in the industry for this use. The problem with the common forms is that they seeded and were of varying heights and varied in their ability to handle the wet. These new uniform sterile forms change all that. Katie Belles™ Lomandra hystrix ‘LHBYF’ was the first new variety that showed its natural ability to handle both wet and dry. It is a larger sterile long lived plant that has excellent erosion control capabilities, and as with all Lomandra suited to this situation it is good at reducing nutrient and heavy metal out flow. 6 Lomandra is better suited to reducing nutrient and heavy metals in the long term compared Carex and Juncus, which have been the traditional plants to use. As Carex and Juncus go dormant when it is dry, or in winter, they do the job for as little as 6 months of the year. Carex and Juncus initially for the first 6 months strip more nutrients and metals than Lomandra, but in the long run Lomandra with its evergreen nature out performs plants that go dormant and brown. A lower growing alternative, Tropic Cascade™ Lomandra hystrix ‘LHWP’ is now being used and is performing well at this task. It is half the height and has beautiful weeping dark green foliage. A new plant called Lucky Stripe™ Lomandra hystrix ‘LMV200’ is destined to become popular for this use, also being remarkable at staying evergreen in wet and dry, this plant will be a boon to Landscape architects wanting to add colour to swales. With its vivid white and green variegated foliage, it contrasts remarkably with green plants, and turns a bio-retentions swale into a design statement.
Fine textured more compact Lomandra that handle wet feet7 are finally available. Shara™ Lomandra fluviatilis ‘ABU7’ has been widely used in Bio-retention swales and basins in Queensland with great success, where humidity and wet feet usually kill fine leaf Lomandra. It is a mid-sized to smaller sized Lomandra, with great cold tolerance for climates such as Canberra and high humidity tolerance for climates such as Darwin. A new plant called Evergreen Baby™ Lomandra labill. ‘LM600’ has also been shown to work in the same conditions as Shara™ Lomandra, resulting in a very low growing plant for Bio-retention swales and basins. Nafray® Pennisetum alopecuroides ‘PA300’ has been used in Swales and Basins for many years, proving to be an effective option for wet and dry and erosion control, but it will brown off in winter.
If a shrub type plant or ground cover plant is needed, both Mundi™ Westringia fruticosa ‘WES05’ and Grey Box™ Westringia fruticosa ‘WES04’ handle dry conditions and wet feet. It would be best not to plant them at the bottom of the swale as the erosion capabilities are less than Lomandra, but planted in amongst the Lomandra in drifts, mainly on the banks they will provide a beautiful Landscape feature. These are the only two Westringia type plants known to handle periodic wet feet, as most types are known not too cope. Grey Box™ Westringia won plant of the year in 2015, and its wet feet abilities helped that win greatly.7
If the swales is generally dry and only periodically wet many more plant types can be used. Liriope make a good choice particularly Just Right® Liriope muscari ‘LIRJ’ which is very wet feet tolerant. Little Jess™ Dianella caerulea ‘DCMP01’ works well particularly further up the slope, as does Lucia™ Dianella caerulea ‘DC101’ which spreads very well and has very good erosion control capabilities.
With plants unlike turf extra temporary erosion control is required. Avoid solid biodegradable mats such as Jute Mats as they quickly become hydrophobic and prevent water seeping into the ground which not only defeats the purpose of the water seeping into the swale or basin and cleaning the water or replenishing the ground water supply, but also weaken the plants and causing many fatalities. Use woven material that has some small holes in it that allow water to penetrate, such as jute mesh. Plant the plants close enough that when mature there will be few gaps for weeds to penetrate. If stronger flow protection is needed geocell type protection can be used in combination with these type of plants, but will greatly increase the cost.
Breeding of new plants that better handle wet feet, and dry, has given a new lease of life to Bio-retention swales basins. They now not only function well but stay evergreen and look great. These same plants can be used by the home gardener in rain gardens, providing an ornamental garden that reduces pollution by removing contaminates prior to water flowing into the stormwater system.
- Phytoaccumulation of trace metals at the Sunny Corner mine, New South Wales, with suggestions for a possible remediation strategy, Australasian Journal of Ecotoxicology 01/2003; 9:69-82