Part 9 – Landscaping Engineered Stormwater Devices
9.3 Planting
9.3.1 Site Screening
Site vegetative screening is to comply with Council requirements.
9.3.2 Planting Grades
Planting grades are to be of a suitable size to ensure that vegetation establishes rapidly with minimum mortality rates and/or replacement requirements. Refer to Table 9.2 for the minimum plant grades. Trees are to be a minimum grade of 1.5 m high.
9.3.3 Plant Spacing and Selection
(a) Species Selection
Species are to be selected with regard to good conformation, healthy robust root systems, and low maintenance.
Planting species are to be selected according to the planting list indicated in Table 9.2 and corresponding site topography and ecology unless there are more suitable plants according to site conditions and/or local ecology. Where trees, shrubs and groundcovers are to be planted within a road reserve, the provisions of Part 7 – Street Landscaping design shall also apply.
Species selection considerations include:
- Compliance with Part 3 of this Manual in regard to sight distances where the PSD is within or near the road reserve
- Engineering requirements, including improving post-treatment stormwater water quality
- Ensure that intended plants are not classified as regionally noxious weed or pest species.
- Longevity and corresponding maintenance requirements
- Minimal leaf fall in autumn (which can reduce PSD efficiency)
- Pest and disease resistance
- Services, including overhead cables
- Shading consistent with location and adjacent landowners
- Suitability to environmental conditions, for example, modified groundwater table, exposure to wind and frost, vehicular and cycle traffic.
- Ensure that no species that drop branches, debris, or may in any other way cause damming and/or unplanned flooding in and adjacent to watercourses (such as streams and spillways) are planted within 5.0 m of watercourses
(b) Plant Selection for Specific Landscaped Engineering Stormwater Devices
Raingardens
Plants selected shall be a mix of groundcovers, shrubs and/or small trees (up to 4 m high) that are able to withstand periods of soil waterlogging according to the Marginal and Lower Bank Planting Zone plant species.
Where PSDs occur in the road reserve species indicated in Table 9.2 may be used.
Ensure that no large trees are selected that may impede maintenance requirements and/or require a resource consent for removal should this be required in the future. Should Council approved biodegradable matting be used for mulch, this shall not be visible once plants are fully established.
Stormwater Ponds
Stormwater ponds are to be planted up as soon as possible after civil construction is completed according to the Planting Zones indicated in 9.2.3. All stormwater ponds with an inner batter slope ratio of 1 (vertical) to 4 (horizontal) or steeper must be landscaped as the slope is too great for safe maintenance.
Where site conditions such as unstable soil structures require a more rapid groundcover than shrubs and trees provide, exposed surfaces above the Upper Bank and Lower Bank Planting Zones shall be stabilised with grassing first prior to landscape planting.
Planting within the Marginal Zone shall be installed at the same time that the upper slopes receive grassing to minimise slope toe erosion. The Wet Zone shall be planted up once the normal standing water level has been achieved. Note: Council opinion is that “wet pond” type detention dams should be avoided if possible. Refer to Figure 9.1, Stormwater Pond Staged Landscape Planting.
Pond plant species shall be a mix of Council approved groundcovers, shrubs and trees from Table 9.2 as site conditions and engineering requirements permit.
Vegetated Filters
Species shall be a mix of Council approved groundcovers, shrubs and trees from Table 9.2 according to the Planting Zone criteria, as site conditions and engineering requirements permit.
Swales
Swales may be turfed to ensure rapid establishment and mitigate channel surface scouring. Generally, grass needs to be maintained at heights between 50 and 150 mm, depending on engineering design parameters.
Where engineering requirements permit, Carex virgata or Carex germinate may be planted in the wet and marginal zones. No other ground cover, shrub or tree species are permitted in these zones. These need to be planted with mulch rounds.
(c) Plant Spacing
Plants are to be planted according to the following spacing allocations:
- Trees, shrubs and groundcovers, as per Table 7.2 required spacings.
- Within the Marginal Zone, Carex shall be evenly staggered at 1.0 m intervals.
- Where plantings are to include approved partially submerged species, these are to be irregularly clumped in groups of 3 to 7 plants along the circumference of the stormwater pond.
- For permanent stormwater ponds, plant 0.4 m below the designed normal standing waterline, approved sedges and rushes.
- Amenity plantings of tussocks are to be clumped in groups of 3 to 10 plants.
- Trees shall be spaced at minimum 2.5 m intervals from other trees and underplanted with 4 equidistant same-species groundcovers, installed 0.75 m from the plant stem. The groundcover species shall provide a weed suppression canopy while the tree is establishing, and as such will have no more than 1 m mature height and minimum 0.75 m spread. Ensure that the groundcover species does not compete with the tree establishment requirements. Depending on the zone planting locations, possible plants would be Phormium “Green Dwarf”, various Carex such as Carex Virgata, and Coprosma groundcovers such as Coprosma kirkii “Minogue”.
- In respect to the pond maintenance access track:
- No shrub or groundcover centres are to be located within 1.0 m of the track.
- No trees centres are to be located within 2.5 m of the track.
- Plantings within 2.0 m either side of the access track are to have species that are able to recover quickly should they become damaged during pond maintenance.
- In subdivision and shopping precincts, planting design either side of the access track should also ensure that the track may be used for pedestrian amenity purposes.
9.3.4 Planting Definitions
The following definitions are applicable when implementing PSD planting:
“Established”
Plants are established when they:
- Are healthy and free of pests, disease, spray and weed-trimmer damage; and
- Are grown to the approximate species mature height; and
- Have obtained a shape and form generally consistent with the species type; and
- Are producing seeds/propagating naturally.
“Establishing”
Plants are establishing when they:
- Are healthy and free of pests, disease, spray and weed-trimmer damage; and
- Are growing generally consistent with the species type shape and form.
“Failed” or “Failure”
Plants have failed when they have one or more of the following:
- Stunted growth (up to 5 years post installation). This requires further investigation to determine the cause and who or what is responsible.
- Been more than irreparably damaged by pests and/or disease and/or weed cover suppression.
- Been severely spray damaged.
- Been ring barked or severely damaged by a weed-trimmer or manual tool.
- Died.
“Installed”
Installed plants are those that have been planted intentionally according to the PSD planting plan.
