Diverse Plants for Ponds or Stormwater Wetlands

Common reed (Phragmites australis) is our tallest native grass and the tallest of the wetland plants listed here, able to reach 3.5m in height. It can form dense cover over wetlands, often pushing through weaker species to colonise large areas of the system. The cylindrical, straw-coloured stems persist throughout the winter, providing oxygen to the roots all through the dormant season. The flowering head of the grass is dark purple and visible in beautiful plumes from August to October.

The Department of the Environment’s Integrated Constructed Wetland Guidance Document advises against excessive use of Phragmites due to the potential to out-compete other species at the expense of diversity and due to the potential for opening up of preferential flow pathways within clay lined systems. The strong root growth can also puncture all but the heaviest of plastic liners. Despite this cautionary note, it is one of the most common wetland plants due to exactly this vigorous growth habit.

Bulrush (Typha latifolia) must be one of the most distinctive of marsh plants, with its tall (up to 2m) statuesque appearance and cigar-like flowering head. It is a very vigorous plant that spreads steadily into ponds and lakes from the margins, thus beginning the process of forming a fen. The spreading nature is advantageous in constructed wetland systems where the dense stems and large quantity of annual leaf litter contribute to the formation of the wetland filter, slowing water and allowing sediments to settle out of suspension. Outside of the flowering period Typha may be distinguished from Sparganium and Iris by the cross section of the leaf, which resembles a long thin crescent moon.

Yellow flag (Iris pseudacorus) is the showy native with sword-like leaves which can be seen on damp pasture, pushing up in extensive clumps in the spring. The leaves often persist over the winter, making it one of the few emergent wetland plants to remain green throughout the year. The showy yellow flowers are slightly larger than daffodils and appear from May to August.

Branched burr reed (Sparganium erectum) can grow to 1.5m or more, and has leaves that resemble the yellow flag, but are roughly triangular in cross section with a distinctive keel on its lower side. Pale globe-shaped flowers appear on branched spikes from June to August and, although beautiful, are not very noticeable unless you are close up. The seeds form distinctive prickly green burrs, c.2cm in diameter. Sparganium can die back considerably in winter, sometimes appearing only as dark mass of decaying leaf litter, or even dying back below water surface level for the dormant season.

Water mint (Mentha aquatica) is a low growing plant with sprawling shoots that can spread laterally to colonise the muddy surface of river banks and pond margins. The leaves are broad and oval to heart-shaped, and have a strong pepperminty scent when crushed. Pale purple flowers appear from August to September which are attractive to bees and butterflies. This plant is listed as edible on the Plants for a Future database (pfaf.org), but check specific warnings before use. Also see the note about edibles below.

Fool’s cress (Apium nodiflorum) is a low growing, mainly submerged plant that is often seen in profusion in open field drains. It looks similar to water cress but is not generally considered edible,^[1] hence the name. White flowers are present from June to September. This is a good plant in early wetland establishment because it quickly occupies the shallow water. As the system develops the plant species composition will change from an initial proliferation of lower growing plants to tall dominant ones.

Water cress (Nasturtium officinale) isn’t a plant that I usually list in reed bed and constructed wetland designs because it is easily recognised as an edible. However, given Patrick Whitefield’s permaculture principle of multiple outputs, the inclusion of edibles is important in the permaculture context. If you are going to eat your wetland plants or use them as fodder for hens etc. it is important that you only plant them in a ‘clean’ grey water or stormwater system and not in a sewage treatment reed bed.

Generally speaking, if you buy your cosmetics, cleaners etc. in your local health food shop, then their ingredients will be more likely to be healthy for you to eat via your wetland plants than if they come from the supermarket. If you want to use edibles, I’d recommend setting up your system as a separate basin with a single feed from the house, i.e. the kitchen sink or bath, and route all washing machine grey water and black water to a separate system.

Pond Plants

There are also plants that are particularly well suited to ponds – be they garden ponds or part of your overall sewage treatment system. I generally tend to omit ponds from my domestic scale constructed wetland and reed bed designs, but where a pond exists then the following species are beautiful and/or functional additions.

White water lily (Nymphaea alba) is a large broad-leaved native lily with beautiful white flowers appearing from June to September. If dissolved oxygen levels in the effluent are too low, these won’t survive.

Pondweeds (Potamogeton sp.) are a large family of pond plants, some of which are ideal for the still water of wetland ponds. Floating leaves vary in shape and size from small thin curled leaves to large oval ones. Flowers are relatively inconspicuous green spikes rising from the water.

Duckweed (Lemna sp.) are the smallest flowering plants in Europe. The tiny leaves float on the surface with a thread-like root about 2cm long that trails in the water. Some species of duckweed are edible (search the Plants for a Future^[2] database for more details).

Due to the danger of vegetation hiding deep water, ponds should have plenty of open water and not be completely covered in duckweed or other plants. Wind can blow duckweed into a corner, making it appear like a firm green carpet, so if you do have plant cover over your pond be sure to rake it out regularly (taking care to work hygienically). The pond should be fenced from small children anyway.

A Note on Diversity

Other plants for constructed wetlands and ponds are listed in the Irish Integrated Constructed Wetland Guidance document, as shown in Appendix V. For ICW systems, a broad diversity of species is specifically sought, both for habitat enhancement and for the diversity of treatment roles that different plant species play.

In your permaculture design, the ideal plant for your reed bed or constructed wetland is the one that is easily available locally and has good robust growth and a myriad of other uses as well. Thus if you have two or three tall wetland plant species growing locally, use these rather than buying in plants. Throw in a handful of local stream sediment too, to introduce seeds and aquatic macroinvertebrate species (insects and other creepy -crawlies) at the same time. However, don’t source material from outside your stream or river catchment, to avoid spreading zebra mussel or other invasive aquatic species.

The stronger the growth, the greater the biomass generation – which you can use as a source of compost material. For safety reasons though, I prefer to harvest comfrey or willow planted over a percolation area rather than harvesting reeds from the reed bed system. The risk of getting cuts and scratches from reeds and the risk of pathogen contamination during such work is too high. In contrast, willow and comfrey is planted into clean soil over a covered percolation area, which makes for safer and more pleasant work.

If you are harvesting plants from your local area, be sure to get land owner permission, observe any regulatory limitations that may apply to the area and take due safety precautions while working in wetland habitats. Wetlands are often valuable wildlife habitats, so the area may have special protections that you should observe. If in doubt, check with a local wildlife ranger before taking to the water with waders, gloves and spade.

For gravel reed bed systems I usually stick to Phragmites, Iris and Mentha as the three species of choice. Other species won’t provide the same dominance that you’ll want from a well functioning system.

After planting, it is important that the constructed wetland or reed bed remains sufficiently wet throughout the early growing seasons to promote plant growth. In the first season of planting, some species may not show shoots until as late as mid June, so don’t be overly anxious if the system is slow to establish. Plant density increases significantly in the second season, after the first season’s plants have had time to settle in and to send out lateral runners and rhizomes.

 What to Plant Where?

In either horizontal or vertical flow reed beds the best plant to use is common reed (Phragmites australis). This should form 90% of the total planted area. At the outlet end of horizontal flow reed beds, or at the most visible edge of vertical flow reed beds, you may wish to add yellow flag (Iris pseudacorus) and water mint (Mentha aquatica) for colour and some wildlife diversity.

Plant Phragmites at 0.6m spacing in a large block over most of the bed; Iris at 0.3m spacing in a tight block running across the full width of the system at the outlet end. Mentha may be planted at 0.2 to 0.6m spacing along the very outer perimeter.

For vertical flow reed beds, plant mainly with Phragmites at 0.6m spacing. Iris and Mentha may also be added around the outer perimeter.

For constructed wetlands there is a much greater diversity of plants that will grow well and provide good treatment properties. Table 9 overleaf shows which plants to use and the location within your constructed wetland system. The list draws on the plants described in this chapter only, so if you have plants growing locally that seem to have similar characteristics, these may also be perfectly adequate.

Phragmites is highlighted in the ICW Guidance Document as a potentially invasive species of ICW systems that can take over a large area and dominate to the expense of other species, and also as a species that can open the ground excessively, causing undesirable preferential flow paths.

However, it is a very efficient species for use in the early section of a wetland. If wastewater treatment is a higher priority than ecological diversity within the system then Phragmites is well worth including. Where a heavy clay subsoil exists or a robust plastic liner is used, then leakage is unlikely to be a problem. Phragmites is typically used along with species such as Iris, Typha and Sparganium as the main plants within the wetland. Lower growing species also provide diversity in the early years of establishment. Alternatively where a number of wetland basins are being used, omit the more dominant species from the final basin(s) and plant instead with lower, weaker species.

Stormwater Wetlands

Stormwater is the runoff from roof, road and yard surfaces around the house. This can be contaminated with droppings from birds and pets, drips of oil or petrol, detergents from car washing and other pollutants. Anything going into the stormwater gullies may reach groundwater or rivers with little or no settlement or filtration, depending on the design of your stormwater disposal system – often just a pipe into a drain, or a deep soak pit.

If this is the case for your property, you can protect the receiving water by building a small stormwater wetland to filter the runoff first. Any form of pond or marsh area will help to slow the flow of rain water from paved surfaces, filtering the water and providing a wildlife habitat. Careful design maximises the efficiency of the stormwater system, so follow the same general design principles as per soil based constructed wetlands detailed in this book.

Sizing is somewhat arbitrary, unless you have been instructed to ameliorate stormwater flow volumes as part of your planning permission, in which case you’ll need a site specific design. Typically stormwater wetlands need between 3 and 10% of the total contributory catchment if the yard is fairly clean. Designed correctly, this will provide flow balancing as well as filtration. By contrast, farmyard runoff typically requires a wetland of c.200% of the yard and roof surface area to cater for the higher nutrient inputs.

If you live in an area with combined sewers – where sewage and stormwater both drain into the same pipe network – then you can actively help to reduce the overall load on the municipal sewer by taking out your roof water and routing it through a garden pond or wetland instead of letting it feed into the main sewers. When combined sewers are used, high volumes generated during storm events will often bypass the treatment system completely (by design) and be routed directly to the local river. If you divert the stormwater from your roof for reuse in the garden, a pond, or even an unlined wetland area or bog garden, then the overall municipal treatment system will perform better. When a storm occurs, the storm surge will be reduced and therefore less raw sewage will be displaced directly into your local river.

Some towns and villages already use constructed wetlands for storm surge overflow to catch and clean the effluent and keep the watercourses healthy and happy. Such wetlands are often used in conjunction with urban runoff and tertiary polishing of treated sewage effluent. The low cost, robust nature and effectiveness makes them ideal for this combination of functions, helping to keep downstream rivers and wetlands clean and healthy.

In terms of designs, stormwater wetlands differ from sewage treatment wetlands in some key ways, as follows:

Typical sewage treatment wetlands generally have a fixed outlet flow control weir that maintains the water depth at a consistent level, regardless of inputs. By contrast, stormwater wetlands are best designed to allow variable depths. This allows water to accumulate in times of heavy rainfall and then drop slowly and steadily over the coming hours and days. This helps to maintain the natural flow patterns within the wider river catchment, helping to even out the flood/drought cycles associated with runoff from extensive paved surface areas.

Stormwater wetlands do not necessarily need to be well lined. They need to hold enough water to remain sufficiently moist for wetland plants to thrive, but due to the relatively clean nature compared to sewage, it is not so crucial that they hold water. A very similar system that permits full drawdown of water between storms is called a bioretention area: also a legitimate SUDS component in its own right, but with different design requirements.

They are generally a lot cleaner than sewage treatment wetlands. This attribute allows them to be used for other applications in the garden, such as forming part of a garden pond or an edible wetland area. In a permaculture context, you can make more use of clean stormwater than runoff from roads or dirty yards. Therefore, if you want to grow wetland edibles such as bulrush, watercress and water mint for example, it may be worth having two distinct stormwater wetland areas – one for roof runoff for a productive wetland area, and a separate one for filtering yard and road runoff.

Given that stormwater wetlands are cleaner, they will have a lower nutrient input than sewage treatment wetlands. As a result the planting can be more diverse. In sewage treatment wetlands the species diversity tends to become limited by large vigorous plants that dominate over time, thriving on the abundance of nutrient inputs. Stormwater systems, by contrast, provide opportunities for a wider selection of plants to be introduced. Throw in a handful of riverbank mud from a local stream – but be sure to stay within the same catchment area to avoid introducing aquatic invasive species.

For millennia natural riparian (river side) wetlands have filtered water, trapped sediments, controlled floods, buffered droughts and helped to keep rivers clean and healthy. As these habitats come under ever increasing pressure the ecosystems services that they offer become notably compromised. By creating stormwater wetlands in our gardens, farms and towns we can help to reinstate these habitats, the benefits they offer us and the wildlife they support in a way that is easy, low impact and beautiful. As Bill Mollison famously put it, “though the problems of the world are increasingly complex, the solutions remain embarrassingly simple”.