The dominant plants of wetlands are monocotyledons. This big group of flowering plants includes grasses, reeds, rushes, sedges, New Zealand flax and cabbage trees. Typically they have long, narrow leaves with parallel veins, and flower parts in groups of three. Those that grow to the size of a tree do not form wood or bark.
Growing throughout New Zealand in fertile wetlands, raupō or bulrush (Typha orientalis) is a tall reed that forms dense leafy stands up to 3 metres high in still water. Raupō has thick, starchy rhizomes that expand through soft silt or form a mat in watery places. The tall, strap-like leaves are spongy, with air spaces that help aerate the underwater parts. All foliage dies down each winter. The flowers are held in a brown spike.
Māori baked cakes called pungapunga from the abundant pollen gathered from the cylindrical flower stem. Raupō seeds are borne on fluffy masses which are dispersed by the wind.
New Zealand flax
Like raupō, New Zealand flax (harakeke, Phormium tenax) also forms dense, leafy stands up to 3 metres tall in lowland swamps throughout New Zealand. Red, tubular flowers are held on tall (5–6-metre) spikes. Unlike raupō, it does not die down in winter months. Nor does it tolerate as much immersion, growing on slightly higher ground if the two occur together at a site.
Rushes are typical wetland plants. Some are leafy and grass-like, while others, known to Māori as wīwī, are leafless and consist of clumps of stems.
New Zealand has 15 native and 32 introduced species of Juncus – the most common type of rush. They range from the 2-metre-tall J. pallidus of coastal swamps to tiny 2-centimetre J. pusillu s, found in wetland turfs.
The wire rush Empodisma minus and cane rush Sporodanthus ferrugineus belong to the Restoniaceae family. They are significant peat-formers. Wire rush consists of thin, many-branched stems. Its hairy roots form a dense carpet that acts like a sponge – it can hold up to 15 times its weight in water.
Most bogs that formed with cane rush have been drained for conversion to pasture. The species is now confined to a few sites in the Waikato region, although there are large bogs with the related rush S. traversii on the Chatham Islands.
New Zealand has many sedges – 170 native and 43 introduced species. Common wetland species include various cutty grasses (Gahnia, Cyperus and Carex species), bastard or hook grasses (Uncinia), the comb sedge (Oreobolus pectinatus), and various spike sedges (Eleocharis).
One of the most widespread in swamps is Carex secta, a large tussock-like sedge that grows above 1–2-metre trunks.
Cushion fields of low-growing shrubs and herbaceous plants form in alpine and subantarctic bogs. The dominant plants have such closely set branches and leaves that they form little mounds resembling cushions. Commonly seen are comb sedge, and species of Donatia, Phyllacne, Gaimardia and Centrolepis.
Turf communities are made up of plants that are prostrate (growing flat along the ground) and tiny, often less than 3 centimetres tall. They grow close together, forming a dense carpet at the margin of wetlands. These are typically species-rich communities – 416 plants (one-fifth of the native flowering species) have been found in wetland turfs. Of these, 62 are listed among New Zealand’s threatened or most uncommon plants.
At least nine species of sphagnum moss are known from New Zealand wetlands. Sphagnum cristatum is found in infertile swamps throughout the country, from sea level up to about 2,000 metres. It forms dense, spongy clumps made of thousands of leafy stems, about 15–20 centimetres long. The entire mass can absorb up to 25 times its weight in water. Other common wetland mosses include Drepanocladus species, Polytrichum commune and Campylopus introflexus.