Guidelines for Successful Wetland Planting Part 1 – Constructed Wetlands
TYPES OF WETLANDS
There are 4 general categories of wetland that involve planting with sedges and water plants. These include:
- Constructed wetlands which are designed to replicate natural systems
- Degraded naturally occurring wetlands
- Wetlands associated with Sewage Treatment Plants (STPs) and domestic greywater systems
- Rain gardens and detention basins
Planting sedges and water plants in constructed wetlands will be discussed in this article (Part 1). Rehabilitation of degraded natural wetlands will be presented in Part 2-Rehabilitation of Natural Wetlands. Planting wetlands in STP’s and domestic greywater systems are discussed in Part 3-Greywater Disposal Systems. Rain gardens and detention basins will be described in the final article of the series Part 4 – Rain Gardens.
THE ENGINEERING OF CONSTRUCTED WETLANDS – A CRASH COURSE
Constructed wetlands are generally designed by specialist engineers. Purposes of constructed wetlands may include one or a combination of the following: flood protection and flow control; water quality improvement; landscape/recreational amenity; and provision of wildlife habitat.
It is vital that the design purpose of a constructed wetland be considered during preparation of a planting plan. For example creating habitat for a specific bird species will likely require different plant species than a wetland installed for flood protection.
There are many engineering components that make up a constructed wetland. As with the wetland design purpose, a cursory knowledge of such components will assist in planting design. Wetland components together with appropriate species are listed below:
- Open Water Inlet Zone. This is the water entry point and is generally deep, retaining comparatively large volumes of water. Here water ponding promotes physical settlement of sediment suspended – the first phase of treatment. Ideally the open water inlet zone will include a high-flow bypass system to protect the downstream macrophyte zones from damaging floodwaters. Due to its depth, vegetation only grows on the edges of the inlet zone. Species grown here must be able to withstand sedimentation, damage by floodwaters and long periods of inundation. Tall rhizomatus water plants such as Schoenoplectus validus (River Club Rush) and Eleocharis spacelata (Tall Spike Rush) are well suited to this situation. Phragmites australis (Common Reed) also suits the open water inlet zone as it recovers rapidly from major flood events via both seeding and rhizomes.
- Macrophyte Zone. This can include any number of ‘cells’ oriented perpendicular to water flow, each with differing depths. A variety of terminology is used to describe these cells such as shallow marsh, marsh and deep marsh. These have
been described well by Wong et al (1999). Each plays a different stormwater treatment role in the wetland system. - Shallow Marsh (0-500mm). This area has only shallow inundation that regularly dries out. The ongoing wetting and drying cycle significantly influences the organic content and nutrient cycling in sediments. The plants themselves provide sites for algal epiphytes and biofilms to enhance pollutant uptake. Plants that naturally occur in ephemeral swamps/creek do well in the shallow marsh zone. Examples include Baumea rubiginosa (Soft Twig Rush), Eleocharis acuta (Common Spike Rush), Bulboscheonus fluviatilis (Marsh Clubrush), Cyperus exaltatus (Giant Sedge), Philydrum lanuginosum (Frogsmouth), and Schoenoplectus mucronatus.
- Marsh (500mm-1m). This area occasionally dries out. Its role is to maximise plant surface area and associated adhesion of particles to plant stems. Dense swards of foliage are suitable for this area, such as Baumea articulata (Jointed Twig Rush), Lepironia articulata and Schoenoplectus validus (River Club Rush).
- Deep Marsh (permanent inundation). As for the open water inlet zone, the deep marsh zone is permanently inundated and promotes sedimentation of particles. Unlike the water inlet zone however, it is usually less affected by flood damage. This zone is frequently integrated into open space due to its aesthetic appeal. The deep marsh zone supports similar plants to the open water inlet zone. Vigorous growers such as Phragmites australis (Common Reed) and Typha orientalis (Cumbungi) are best avoided in this zone as they can dominate if left unchecked.
- Littoral Zone (frequently dry). This zone surrounds the wetland edge and is the driest of all zones. It undergoes regular water level fluctuations and can be the zone of highest weed invasion. Dense plantings (6 per square metre) of sedges such as Carex appressa (Tall Sedge), Juncus usitatus (Common Rush), Juncus krausii (Sea Rush), Ficinea nodosa (Knobby Club-rush) and Gahnia species are required here.
- Habitat Island(s). An optional feature, the habitat island has areas that remain dry. They often support tall grasses, shrubs and trees.
HOW TO IMPROVE YOUR WETLAND PLANTING SUCCESS
When preparing a wetland planting plan, design should mimic natural plant occurrences. Native plants are preferable and with sufficient lead time, plants of local provenance plants can be supplied. It is notable that most sedges and water plants grow in large swards of the same species. In many cases, this is likely the result of expansion from a rhizome, competition and suitability to the water level. It follows that it is preferable to plant sedges and wetlands plants in clumps of same species for a good result. It is also notable that after planting, species frequently ‘move’ to other more suitable niches with the constructed wetland and do not continue growing where they were first planted. Wherever they end up, the water plants will continue to perform their designed function.
Bluedale recommends 8 steps to planting constructed wetlands:
(1) ORDER YOUR PLANTS IN ADVANCE Give yourself at least 16 weeks lead time, more if you require local provenance. The longer the lead time, the more mature and taller your plants will be. This will make a huge difference in planting success.
(2) Mark the planting zones. It is vital to know final water depths so that each wetland zone will have the correct plant species. If the constructed wetland is already full of water, this will be a relatively simple task and zones can be marked prior to water removal using stakes. Generally, constructed wetlands have structures such as riser outlets that provide for manual control of water level and duration of inundation. If already empty or partially so, consult the wetland design engineer or similar for advice. A detailed drawing depicting water levels may be available. For larger wetlands a survey may be required.
(3) Remove weeds. This should be done prior to planting using herbicide and/or and hand removal techniques.
(4) Monitor bird activity. While undertaking preliminary site inspections, marking out and weed removal it is a good idea to monitor bird activity. Water birds are attracted to the fresh new growth of nursery stock. They can pull plants up, roots and all and cause significant losses. If there are many birds present a staged planting approach may help avoid losses, and exclusion netting should be considered. Planting larger pot sizes of 140mm will also negate damage by bird predation. Advanced ordering will be required if 140mm pots are to be used as this is not a size regularly grown by Bluedale.
(5) Planting. With preliminary work now completed, planting can begin. Set out the plants according to their planting zones. Littoral Zone plants should be planted between the high and low level water marks. When planting, anchor the root ball of the plant by planting it firmly into the substrate of the basin.
(6) Set Water Level. If the wetland has a manual water level control device, set the water level no deeper that 2/3 of the foliage height of plants in the deepest zone. The correct level on inundation is vital. As the plants increase in height, the water level can be progressively raised until full.
(7) Watering. This will be necessary for any plants not inundated during the initial water height setting (as described above). If water has not been returned to the basin, irrigation of all the plants will be necessary until the basin refills.
(8) Monitoring and maintenance. Monitor planting areas over coming weeks for signs of bird predation, irrigation needs and plant dislodgement. Protect plants and replace as necessary. A maintenance program will be required involving weed control and plant replacement. The Littoral Zone usually requires the most attention as weed control techniques such as mulching are not applicable. If manual control of the water level is possible, weeds can be controlled by inundation.
Reference
Wong, T.H.F., Breen, P.F., Somes, N.L.G. and Lloyd S.D. (1999) ‘Managing Urban Stormwater Using Constructed Wetlands’ Industry Report. Cooperative Research Centre for Catchment Hydrology. Second Edition. September 1999
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