Stormwater plant subcategories
Greenroof (sunny, <200 mm substrate depth), Greenroof (shady, <200 mm substrate depth)
Only a narrow range of plants can survive on greenroofs with substrate 50-200 mm deep and low organic content in full sun with minimal irrigation. The primary factor in plant selection is therefore survival rather than aesthetics. Plants suitable for extensive roofs generally have the following characteristics:
- Wind-tolerant through having permeable fine leaves (e.g. tussocks, lilies, and sedges) or, forming dense, low mounds of interlocking branches (<200 - 300 mm, generally <100 mm), e.g. prostrate Coprosma and Pimelea species, or ground-hugging and anchored at internodes, (e.g. Selliera, Dichondra and Leptinella). This also reduces water demand and plant weight.
- Resilient to cyclic wet and dry conditions, in particular extended drought periods, able to resprout.
- Roots tolerant of high temperatures, as most large roofs receive full sun and substrates over concrete can have high thermal mass, radiating heat into the substrate.
- Shallow, lateral root system; there is not space for large, below-ground storage organs such as tubers.
- Low fertiliser and maintenance needs (resistance to insects and disease).
- Use a variety of species, including plants that spread or seed into gaps, plants of different heights, and long-lived plants. Avoid large (>2 m2) blocks of just one or two species.
- Use a minimum 100 mm depth of substrate (preferably with some localised areas of 150 - 200 mm) unless sedums will be the dominant plant cover, or the site receives afternoon shade, or irrigation is installed.
- Include an irrigation system to enable roofs to be watered in droughts and plant the roof in autumn or winter immediately after medium is placed.
- Have a clearly-defined maintenance contract, preferably with the living roof supplier, for at least 18 months.
Roof garden (>300 mm substrate depth) (any herb or shrub from region)
Many low (<1.0 m height) herbs and shrubs can be planted on roof gardens, with wind tolerance a key requirement. As for green roofs, plants should be tolerant of drought, and backup irrigation included to protect the investment in plants.
Swales are structures designed to transport water from one place to another. Swales receiving road runoff may have substantial sediment load that can smother short groundcovers and carry in weed species. In these cases the swale plants must be robust and dense enough to exclude weed growth. In contrast, plants in swales receiving roof runoff generally have very low sediment loads, allowing shorter turf species to be used. Wide swales, for example overland flow paths for larger storms, can include larger vegetation without unduly blocking flow, for example Astelia grandis and Phormium tenax. Narrow swales in tight spaces that receive frequent flow need vegetation that filters stormwater – standard swale design is based on grasses mown to no closer than 100 mm height. Trees and upright shrubs that will not smother the groundcover with shade or fallen leaves can be included, but should be located on the sides of the swale (not the bed) to avoid disrupting water flows and capacity.
Swale, road edge
Plants within 0.3 to 0.5 m of a swale edge bordering a footpath or road should have relatively upright and/or short growth form, depending on anticipated sediment load. This ensures the edges of the swale remain visible. Visible edges are particularly important where there is no barrier (e.g. no kerb) and adjacent to inlets. Plants that flop onto adjacent footpaths and roads may impede traffic and pedestrian flows and create trip hazards. Some plants, such as creeping herbs, may be suitable if they are brittle or succulent, as traffic ‘prunes’ the growth.
Swale, wet bed
Plants in the bed of a wet swale are almost always in standing water. Such swales are uncommon in New Zealand to date, but common along highways in the United States where nitrogen is a contaminant (because they create anaerobic conditions). Plants suited to wet swales are mostly herbaceous swamp species, i.e. moderate to high fertility and structurally-robust. Plants are generally short, from 0.3 to 1.0 m height with thin, narrow leaves. Some small groundcovers that tolerate being associated with taller plants are included, and these could be mown as a short turf.
Swale, dry bed
Most swales are dry swales, and flow intermittently during and shortly after rainfall. A wide range of plants can be included where the rooting medium is 0.3 m or more thick, and adequately aerated (well to imperfectly-drained). The bulk of the planting are filters, for example, grasses, sedges, ferns (e.g. Blechnum nova zealandiae), and low groundcover shrubs (e.g. Coprosma acerosa) can be included. Some smaller groundcovers that tolerate being associated with taller plants are included, for example, Sellieria radicans under Apodasmia similis; Leptinella diocia and Pratia angulata under Carex secta and Eleocharis acuta.
Swale, veg edge
Plants along an edge with other vegetation can be larger and broader than those adjacent to a road or footpath or other space designed for flows of people and vehicles. However, the plants should not smother those in the swale bed that filter water-flows. They can include trees and shrubs. Edge plants usually have more favourable growing conditions than those in the swale bed as their roots can access both wet and dry areas.
Rain garden, road edge
Plants within 0.3 to 0.5 m of a rain garden edge with a footpath or road should have relatively upright and/or short growth form. This ensures the edges of the rain garden are visible. Visibility of edges is particularly important where there is no barrier (no kerb) and near inlets (so maintenance staff can see if they are blocked). Plants that flop onto adjacent footpaths and roads may impede traffic and pedestrian flows and create trip hazards. Some plants, such as creeping herbs, may be suitable if they are brittle or succulent, as traffic will ‘auto-prune’ the growth.
Rain garden, CiPTED bed
CiPTED is Crime Prevention Through Environmental Design. It seeks to reduce opportunity for criminal behaviour and enhance positive social interaction for people in public places. Seven qualities are identified (see www.justice.govt.nz/):
- landscaping that promotes safe access, movement and connections
- planting heights and densities that retain sight lines, allowing people to see and be seen
- clear and logical layout
- landscape that encourage people to use public spaces, ‘passive surveillance’
- an attractive space with cues that show the place is cared for
- design that has reduced ongoing maintenance needs
- active security; using landscaping to limit access to sensitive environments, private or dangerous areas
Rain garden, dry bed
Rain garden plants need to match the characteristics of the rain garden growing medium, substrate depth, and the amount of runoff entering the rain garden. Specifications for rain garden media vary from very sandy, rapidly-draining mixes with 5 to 15% organic matter by volume, to slow-draining mixes using natural soils and/or a high proportion of organic matter. The latter medium suit plants that tolerate of being in ponding water and low aerated soils for 2 to 3 days. Rain gardens generally receive moderate levels of nutrients, because they are designed to remove nutrients (which can be a stormwater pollutant) so they should not be fertilised once installed. Plants naturally found in pakihi or other low-fertility ecosystems have not been included.
Plants with hairy leaves are generally unsuitable as silt in stormwater will coat and block the stomata in all but the cleanest sites. Trees, particularly evergreen trees, enhance rain garden performance by boosting the volume of water removed during rain events. Deciduous trees should only be selected (a) if inlets and overflows are designed to self-clear or are maintained frequently during autumn and (b) the leaf fall is thin and fine enough to avoid supressing groundcover plants. The groundcover plants are important for maintaining infiltration rates and reducing impacts of clogging and compaction. Ground cover plants should not die back in winter and most should be vigorous, to allow development of full groundcover within 12 to 18 months. Some trees and shrubs that are tolerant of dry conditions have not been included, as they are not found naturally in depressions in the landscape. This includes Pseudopanax ferox, Kunzea ericoides and Pimelea prostrata.
Rain garden, veg edge
Plants along an edge with other vegetation can be larger and broader than those adjacent to a road or footpath or other space designed for flows of people and vehicles. However, the plants should not smother those in the rain garden bed, as this vegetation is important to maintain infiltration into the rain garden medium. A very broad range of plants are suitable. Suitability is constrained only by the depth of the rain garden medium (usually 0.6 to over 1 m depth) and available moisture. Plants along edges near inlets are likely to receive more water, so are less susceptible to drought. Plants that are distant from inlets may become drought stressed. This can be mitigated to some extent by selecting plants with deep root systems (these will be at least 0.3 m tall).
Railway ballast, trackside
This area is the 2 to 5 m strip immediately adjacent to the railway ballast, and often includes steep railway batters. Plants in this zone must tolerate buffeting turbulence from trains without shedding leaves, flower spikes, or branches, and must be low, so as not to encroach on the visibility of the railway tracks. Plants in this zone are often in thin soils with minimal rooting depth, so need to be tolerant of water stress (droughty conditions). Plants with a tight, dense growth form are often ideal, as this helps reduce maintenance by supressing establishment from unwanted plants such as trees, blackberry and gorse. The natural equivalent to this area includes coastal gravels, cliffs, scree slopes and alluvial gravels.
Most plants are less than 0.6 m in height and have a spreading or prostrate growth form, for example, Phormium cookianum, Coprosma acerosa, Muehlenbeckia astonii, locally prostrate Leptospermum scoparium and Cassinia leptophylla. Ground-cover herbs, tussocks and sedges are included for areas of railway ballast with very high plant stress where such plants can be competitive, for example, Acaena microphylla, Muehlenbeckia axillaris, Pimelea prostrata in dry areas, and Blechnum nova-zealandiae and Asplenium oblongifolium for moist shady banks.
Railway ballast, outer rail
This area is the strip immediately adjacent to the fence that excludes people and stock from the railway corridor in rural and urban areas. The primary roles of vegetation in this area are to prevent access from the fence to the railway, and buffer adjacent land. Taller shrubs (2 to 5 m) and upright, narrow small trees are suitable. Vegetation should not shed branches or flower heads. In the narrow railway corridor large trees are likely to need maintenance and present a risk to the railway should they fall onto the tracks.
The planting list includes vines and scramblers that will drape over fences, for example Muehlenbeckia complexa.
All road vegetation must contribute to safe journeys, so the maximum vegetation height and frangibility (brittleness and fragility) in relation to road edges, sight–lines for drivers and pedestrians, runoff zones and safety-setbacks for road structures (barriers, reflectors, signs) are tightly prescribed. Plants are a major road asset. Selection can improve road predictability, encourage safer driver behaviour, and protect pedestrians and cyclists. Plants also help mitigate the impacts of road runoff, noise, dust, heat and biosecurity (weed spread). Plant selection also helps way-finding, and can reinforce local sense of place and cultural identity (whakapapa).
The New Zealand Highway Landscaping Guidelines are being rewritten and should be available late 2014 on the NZ Transport Agency website. It is essential reading as poor plant selection and location, particularly combined with inadequate soil quality, has resulted in dangerous, ugly and expensive roadside environments. Appendices include NZTA standard specifications, a design review template and typical designation conditions.
Road verge/traffic island, water table
The water table may also be a swale and is usually directly adjacent to the sealed road surface and/or gravel road shoulder. The road edge is usually sprayed and/or mown frequently, and driven over by traffic that leaves the road. All plants must therefore be frangible (unless protected by a safety barrier) and maintained within the height tolerances specified by NZTA. Frangible plants have a stem less than 100 mm diameter at maturity measured 400 mm above the ground. The growing medium is usually compacted, shallow and infertile, and usually droughty. Most plants in this zone are herbs, sedges, tussocks and groundcovers.
Road verge/traffic island, sight line
All or part of this area is called the ‘clear zone’, which extends 9 m horizontally from the road edge along highways (open road) where adjacent land is gently sloping. The zone also extends vertically, and is narrower where slopes are steeper. Vegetation in this zone must also be frangible, unless protected by a barrier. Plants in this zone should be securely rooted, with flexible stems (not brittle) so traffic does not suck plant material onto the road. This includes flower stalks and heads.
Road verge/traffic island, frangible edge
As for sight line, but vegetation can be >0.3 m where rooted from the ground, as this area lies outside and beyond the sight line.
Road verge/traffic island, mid
Vegetation in this zone includes trees and other non-frangible plants, unless it lies within the clear zone.
Road verge/traffic island, back/fence
This includes plants that will scramble up fences and the tallest trees.
Road verge/traffic island, cut/fill
Plant lists for cut-and-fill slopes must take into account aspect and shade. These slopes often have shallow or no topsoil, shallow root zones, and are dislocated from natural drainage. Depending on the slope, aspect, use of mulches, and quality and depth of topsoil replaced, these sites may be excessively wet (low slopes with compacted finer soils) or very droughty. A limited root depth will prevent trees from developing a stable root system.